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8.3 Creative Projects

Regardless of the path you’ve taken through our written material, at some point you need to start doing something with this information. We’ve looked at many concepts separately but in a real project you will apply several of these concepts all at the same time. Linked from this section are some suggestions for projects you can do that allow you the opportunity to apply what you’ve learned.We invite you to exercise your creativity in your chosen field of study or application, synthesizing your knowledge and imagination in the complex, fascinating, ubiquitous world of sound.

 

8.2.4 Live Sound

8.2.4.1 Designing a Sound Delivery System

Theatre and concert performances introduce unique challenges in pre-production not present in sound for CD, DVD, film, or video due to the fact that the sound is delivered live.  One of the most important parts of the process in this context is the design of a sound delivery system.  The purpose of the design is to ensure clarity of sound and a uniform experience among audience members.

In a live performance, it’s quite possible that when the performers on the stage create their sound, that sound does not arrive at the audience loudly or clearly enough to be intelligible. A sound designer or sound engineer is hired to design a sound reinforcement system to address this problem. The basic process is to use microphones near the performers to pick up whatever sound they’re making and then play that sound out of strategically-located loudspeakers.

There are several things to consider when designing and operating a sound reinforcement system:

  • The loudspeakers must faithfully generate a loud enough sound.
  • The microphones must pick up the source sound as faithfully as possible without getting in the way.
  • The loudspeakers must be positioned in a way that will direct the sound to the listeners without sending too much sound to the walls or back to the microphones. This is because reflections and reverberations affect intelligibility and gain.
  • Ideally, the sound system will deliver a similar listening experience to all the listeners regardless of where they sit.

Many of these considerations can be analyzed before you purchase the sound equipment so that you can spend your money wisely. Also, once the equipment is installed, the system can be tested and adjusted for better performance. These adjustments include repositioning microphones and loudspeakers to improve gain and frequency response, replacing equipment with something else that performs better, and adjusting the settings on equalizers, compressors, crossovers, and power amplifiers.

Most loudspeakers have a certain amount of directivity. Loudspeaker directivity is described in terms of the 6 dB down point – a horizontal and vertical angle off-axis corresponding to the location where the sound is reduced by 6 dB.  The 6 dB down point is significant because, as a rule of thumb, you want the loudness at any two points in the audience to differ by no more than 6 dB. In other words, the seat on the end of the aisle shouldn’t sound more than 6 dB quieter or louder than the seat in the middle of the row, or anywhere else in the audience.

The issue of loudspeaker directivity is complicated by the fact that loudspeakers naturally have a different directivity for each frequency. A single circular loudspeaker driver is more directional as the frequency increases because the loudspeaker diameter gets larger relative to the wavelength of the frequency. This high-frequency directivity effect is illustrated in Figure 8.21. Each of the six plots in the figure represents a different frequency produced by the same circular loudspeaker driver. In the figures,  is the wavelength of the sound.  (Recall that the higher the frequency, the smaller the wavelength.  See Chapter 2 for the definition of wavelength, and see Chapter 1 for an explanation of how to read a polar plot.)

Going from top to bottom, left to right in Figure 8.21, the frequencies being depicted get smaller.  Notice that frequencies having a wavelength that is longer than the diameter of the loudspeaker are dispersed very widely, as shown in the first two polar plots. Once the frequency has a wavelength that is equal to the diameter of the loudspeaker, the loudspeaker begins to exercise some directional control over the sound. This directivity gets narrower as the frequency increases and the wavelength decreases.

Figure 8.21 Directivity of circular radiators. Diagrams created from actual measured sound

Figure 8.21 Directivity of circular radiators. Diagrams created from actual measured sound

This varying directivity per frequency for a single loudspeaker driver partially explains why most full-range loudspeakers have multiple drivers. The problem is not that a single loudspeaker can’t produce the entire audible spectrum. Any set of headphones uses a single driver for the entire spectrum. The problem with using one loudspeaker driver for the entire spectrum is that you can’t distribute all the frequencies uniformly across the listening area. The listeners sitting right in front of the loudspeaker will hear everything fine, but for the listeners sitting to the side of the loudspeaker, the low frequencies will be much louder than the high ones. To distribute frequencies more uniformly, a second loudspeaker driver can be added, considerably smaller than the first.  Then an electronic unit called a crossover directs the high frequencies to the small driver and the low frequencies to the large driver.  We two different-size drivers, you can achieve a much more uniform directional dispersion, as shown in Figure 8.22. In this case, the larger driver is 5" in diameter and the smaller one is 1" in diameter. Wavelengths corresponding to frequencies of 500 Hz and 1000 Hz have larger wavelengths than 5", so they are fairly omnidirectional. The reason that frequencies of 2000 Hz and above have consistent directivity is that the frequencies are distributed to the two loudspeaker drivers in a way that keeps the relationship consistent between the wavelength and the diameter of the driver. The 2000 Hz and 4000 Hz frequencies would be directed through the 5” diameter driver because their wavelengths are between 6” and3”. The 8000 Hz and 16,000 Hz frequencies would be distributed to the 1” diameter driver because their wavelengths are between 2” and1”. This way the two different size drivers are able to exercise directional control over the frequencies that are radiating.

Figure 8.22 Directivity of 2-way loudspeaker system with 5" and 1" diameter drivers

Figure 8.22 Directivity of 2-way loudspeaker system with 5" and 1" diameter drivers

There are many other strategies used by loudspeaker designers to get consistent pattern control, but all must take into account the size of the loudspeaker drivers and way in which they affect frequencies. You can simply look at any loudspeaker and easily determine the lowest possible directional frequency based on the loudspeaker’s size.

Understanding how a loudspeaker exercises directional control over the sound it radiates can also help you decide where to install and aim a loudspeaker to provide consistent sound levels across the area of your audience. Using the inverse square law in conjunction with the loudspeaker directivity information, you can find a solution that provides even sound coverage over a large audience area using a single loudspeaker. (The inverse square law is introduced in Chapter 4.)

Consider the example 1000 Hz vertical polar plot for a loudspeaker shown in Figure 8.23. If you’re going to use that loudspeaker in the theatre shown in Figure 8.24, where do you aim the loudspeaker?

Figure 8.23 Vertical 1000 Hz polar plot for a loudspeaker

Figure 8.23 Vertical 1000 Hz polar plot for a loudspeaker

Figure 8.24 Section view of audience area with distances and angles for a loudspeaker

Figure 8.24 Section view of audience area with distances and angles for a loudspeaker

Most beginning sound system designers will choose to aim the loudspeaker at seat B thinking that it will keep the entire audience as close as possible to the on-axis point of the loudspeaker. To test the idea, we can calculate the dB loss over distance using the inverse square law for each seat and then subtract any additional dB loss incurred by going off-axis from the loudspeaker. Seat B is directly on axis with the loudspeaker, and according to the polar plot there is a loss of approximately 2 dB at 0 degrees. Seat A is 33 degrees down from the on-axis point of the loudspeaker, corresponding to 327 degrees on the polar plot, which shows an approximate loss of 3 dB. Seat C is 14 degrees off axis from the loudspeaker, resulting in a loss of 6 dB according to the polar plot. Assuming that the loudspeaker is outputting 100 dBSPL at 1 meter (3.28 feet), we can calculate the dBSPL level for each seat as shown in Table 8.1.

  • A
    • Seat\: A\: dBSPL = 100 dB + \left ( 20\log_{10}\frac{3.28'}{33.17'} \right )-3 dB
    • Seat\: A\: dBSPL = 100 dB + \left ( 20\log_{10}0.1 \right )-3 dB
    • Seat\: A\: dBSPL = 100 dB + \left ( 20\ast -1 \right )-3 dB
    • Seat\: A\: dBSPL = 100 dB + \left ( -20\right )-3 dB
    • Seat\: A\: dBSPL = 77\, dBSPL
  • B
    • Seat\: B\: dBSPL = 100 dB + \left ( 20\log_{10}\frac{3.28'}{50.53'} \right )-2 dB
    • Seat\: B\: dBSPL = 100 dB + \left ( 20\log_{10}0.06 \right )-2 dB
    • Seat\: B\: dBSPL = 100 dB + \left ( 20\ast -1.19 \right )-2 dB
    • Seat\: B\: dBSPL = 100 dB + \left ( -23.75\right )-2 dB
    • Seat\: B\: dBSPL = 74.25\, dBSPL
  • C
    • Seat\: C\: dBSPL = 100 dB + \left ( 20\log_{10}\frac{3.28'}{77.31'} \right )-6 dB
    • Seat\: C\: dBSPL = 100 dB + \left ( 20\log_{10}0.04 \right )-6 dB
    • Seat\: C\: dBSPL = 100 dB + \left ( 20\ast -1.37 \right )-6 dB
    • Seat\: C\: dBSPL = 100 dB + \left ( -27.45\right )-6 dB
    • Seat\: C\: dBSPL = 66.55\, dBSPL

Table 8.1 Calculating dBSPL of a given loudspeaker aimed on-axis with seat B

In this case the loudest seat is seat A at 77 dBSPL, and seat C is the quietest at 66.55 dBSPL, with a 10.45 dB difference. As discussed, we want all the audience locations to be within a 6 dB range. But before we throw this loudspeaker away and try to find one that works better, let’s take a moment to examine the reasons why we have such a poor result. The reason seat C is so much quieter than the other seats is that it is the farthest away from the loudspeaker and is receiving the largest reduction due to directivity. By comparison, A is the closest to the loudspeaker, resulting in the lowest loss over distance and only a 3 dB reduction due to directivity. To even this out let’s try having the farthest seat away be the seat with the least directivity loss, and the closest seat to the loudspeaker have the most directivity loss.

The angle with the least directivity loss is around 350 degrees, so if we aim the loudspeaker so that seat C lines up with that 350 degree point, that seat will have no directivity loss. With that aim point, seat B will then have a directivity loss of 3 dB, and seat A will have a directivity loss of 10 dB. Now we can recalculate the dBSPL for each seat as shown in Table 8.2.

  • A
    • Seat\: A\: dBSPL = 100 dB + \left ( 20\log_{10}\frac{3.28'}{33.17'} \right )-10 dB
    • Seat\: A\: dBSPL = 100 dB + \left ( 20\log_{10}0.1 \right )-10 dB
    • Seat\: A\: dBSPL = 100 dB + \left ( 20\ast -1 \right )-10 dB
    • Seat\: A\: dBSPL = 100 dB + \left ( -20\right )-10 dB
    • Seat\: A\: dBSPL = 70\, dBSPL
  • B
    • Seat\: B\: dBSPL = 100 dB + \left ( 20\log_{10}\frac{3.28'}{50.53'} \right )-3 dB
    • Seat\: B\: dBSPL = 100 dB + \left ( 20\log_{10}0.06 \right )-3 dB
    • Seat\: B\: dBSPL = 100 dB + \left ( 20\ast -1.19 \right )-3 dB
    • Seat\: B\: dBSPL = 100 dB + \left ( -23.75\right )-3 dB
    • Seat\: B\: dBSPL = 73.25\, dBSPL
  • C
    • Seat\: C\: dBSPL = 100 dB + \left ( 20\log_{10}\frac{3.28'}{77.31'} \right )-0 dB
    • Seat\: C\: dBSPL = 100 dB + \left ( 20\log_{10}0.04 \right )-0 dB
    • Seat\: C\: dBSPL = 100 dB + \left ( 20\ast -1.37 \right )-0 dB
    • Seat\: C\: dBSPL = 100 dB + \left ( -27.45\right )-0 dB
    • Seat\: C\: dBSPL = 72.55\, dBSPL

Table 8.2 Calculating dBSPL of a given loudspeaker aimed on-axis with seat C

In this case our loudest seat is seat B at 73.25 dB SPL, and our quietest seat is seat A at 70 dBSPL, for a difference of 3.25 dB. Compared with the previous difference of 10.55 dB, we now have a much more even distribution of sound to the point where most listeners will hardly notice the difference. Before we fully commit to this plan, we have to test these angles at several different frequencies, but this example serves to illustrate an important rule of thumb when aiming loudspeakers. In most cases, the best course of action is to aim the loudspeaker at the farthest seat, and have the closest seat be the farthest off-axis to the loudspeaker. This way, as you move from the closest seat to the farthest seat, while you're losing dB over the extra distance you're also gaining dB by moving more directly on-axis with the loudspeaker.

Aside:  EASE was developed by German engineers ADA (Acoustic Design Ahnert) in 1990 and introduced at the 88th AES Convention.  That's also the same year that Microsoft announced Windows 3.0.

Fortunately there are software tools that can help you determine the best loudspeakers to use and the best way to deploy them in your space. These tools range in price from free solutions such as MAPP Online Pro from Meyer Sound shown in Figure 8.25 to relatively expensive commercial products like EASE from the Ahnert Feistel Media Group, shown in Figure 8.26. These programs allow you to create a 2D or 3D drawing of the room and place virtual loudspeakers in the drawing to see how they disperse the sound. The virtual loudspeaker files come in several formats. The most common is the EASE format. EASE is the most expensive and comprehensive solution out there, and fortunately most other programs have the ability to import EASE loudspeaker files. Another format is the Common Loudspeaker Format (CLF). CLF files use an open format, and many manufacturers are starting to publish their loudspeaker data in CLF. Information on loudspeaker modeling software that uses CLF can be found at the website for the Common Loudspeaker Format Group http://www.clfgroup.org.

Figure 8.25 MAPP Online Pro software from Meyer Sound

Figure 8.25 MAPP Online Pro software from Meyer Sound

Figure 8.26 EASE software

Figure 8.26 EASE software

8.2.4.2 System Documentation

Once you've decided on a loudspeaker system that distributes the sound the way you want, you need to begin the process of designing the systems that capture the sound of the performance and feed it into the loudspeaker system. Typically this involves creating a set of drawings that give you the opportunity to think through the entire sound system and explain to others – installers, contractors, or operators, for example – how the system will function.

Aside:  You can read the entire USITT document on System Diagram guidelines by visiting the USITT website.

The first diagram to create is the System Diagram. This is similar in function to an electrical circuit diagram, showing you which parts are used and how they're wired up.  The sound system diagram shows how all the components of a sound system connect together in the audio signal chain, starting from the microphones and other input devices all the way through to the loudspeakers that reproduce that sound. These diagrams can be created digitally with vector drawing programs such as AutoCAD and VectorWorks or diagramming programs such as Visio and OmniGraffle.

The United States Institute for Theatre Technology has published some guidelines for creating system diagrams. The most common symbol or block used in system diagrams is the generic device block shown in Figure 8.27. The EQUIPMENT TYPE label should be replaced with a descriptive term such a CD PLAYER or MIXING CONSOLE. You can also specify the exact make and model of the equipment in the label above the block.

Figure 8.27 A generic device block for system diagrams

Figure 8.27 A generic device block for system diagrams

There are also symbols to represent microphones, power amplifiers, and loudspeakers. You can connect all the various symbols to represent an entire sound system. Figure 8.28 shows a very small sound system, and Figure 8.29 shows a full system diagram for a small musical theatre production.

Figure 8.28 A small system diagram

Figure 8.28 A small system diagram

Figure 8.29 System diagram for a full sound system

Figure 8.29 System diagram for a full sound system

While the system diagram shows the basic signal flow for the entire sound system, there is a lot of detail missing about the specific interconnections between devices. This is where a patch plot can be helpful. A patch plot is essentially a spreadsheet that shows every connection point in the sound system. You should be able to use the patch plot to determine which and how many cables you’ll need for the sound system.  It can also be a useful tool in troubleshooting a sound system that isn’t behaving properly. The majority of the time when things go wrong with your sound system or something isn’t working, it's because it isn’t connected properly or one of the cables has been damaged. A good patch plot can help you find the problem by showing you where all the connections are located in the signal path. There is no industry standard for creating a patch plot, but the rule of thumb is to err on the side of too much information. You want every possible detail about every audio connection made in the sound system. Sometimes color coding can help make the patch plot easier to understand. Figure 8.30 shows an example patch plot for the sound system in Figure 8.28.

Figure 8.30 Patch plot for a simple sound system

Figure 8.30 Patch plot for a simple sound system

8.2.4.3 Sound Analysis Systems

Aside:  Acoustic systems are systems in which the sounds produced depend on the shape and material of the sound-producing instruments. Electroacoustic systems produce sound through electronic technology such as amplifiers and loudspeakers.

Section 8.2.4.1 discussed mathematical methods and tools that help you to determine were loudspeakers should be placed to maximize clarity and minimize the differences in what is heard in different locations in an auditorium.  However, even with good loudspeaker placement, you’ll find there are differences between the original sound signal and how it sounds when it arrives as the listener.  Different frequency components respond differently to their environment, and frequency components interact with each other as sounds from multiple sources combine in the air.  The question is, how are these frequencies heard by the audience once they pass through loudspeakers and travel through space encountering obstructions, varying air temperatures, comb filtering, and so forth? Is each frequency arriving at the audience’s ears at the desired amplitude? Are certain frequencies too loud or too quiet?  If the high frequencies are too quiet, you could sacrifice the brightness or clarity in the sound.  Low frequencies that are too quiet could result in muffled voices.  There are no clear guidelines on what the “right” frequency response is because it usually boils down to personal preference, artistic considerations, performance styles, and so forth.  In any case, before you can decide if you have a problem, the first step is to analyze the frequency response in your environment. With practice you can hear and identify frequencies, but sometimes being able to see the frequencies can help you to diagnose and solve problems. This is especially true when you’re setting up the sound system for a live performance in a theatre.

A sound analysis system is one of the fundamental tools for ensuring that frequencies are being received at proper levels. The system consists of a computer running the analysis software, an audio interface with inputs and outputs, and a special analysis microphone.  An analysis microphone is different from a traditional recording microphone. Most recording microphones have a varying response or sensitivity at different frequencies across the spectrum. This is often a desired result of their manufacturing and design, and part of what gives each microphone its unique sound. For analyzing acoustic or electroacoustic systems, you need a microphone that measures all frequencies equally.  This is often referred to as having a flat response.  In addition, most microphones are directional. They pick up sound better in the front than in the back. A good analysis microphone should be omnidirectional so it can pick up the sound coming at it from all directions. Figure 8.31 shows a popular analysis microphone from Earthworks.

Figure 8.31 Earthworks M30 analysis microphone

Figure 8.31 Earthworks M30 analysis microphone

There are many choices for analysis software, but they all fall into two main categories: signal dependent and signal independent.  Signal dependent sound analysis systems rely on a known stimulus signal that the software generates – e.g., a sine wave sweep.  A sine wave sweep is a sound that begins at a low frequency sine wave and smoothly moves up in frequency to some given high frequency limit.  The sweep, lasting a few seconds or less, is sent by a direct cable connection to the loudspeaker. You then place your analysis microphone at the listening location you want to analyze. The microphone picks up the sound radiated by the loudspeaker so that you can compare what the microphone picks up with what was actually sent out.

The analysis software records and stores the information in a file called an impulse response.  The impulse response is a graph of the sound wave with time on the x-axis and the amplitude of the sound wave on the y-axis.  This same information can be displayed in a frequency response graph, which has frequencies on the x-axis and the amplitude of each frequency on the y-axis.  (In Chapter 7, we’ll explain the mathematics that transforms the impulse response graph to the frequency response graph, and vice versa.) Figure 8.32 shows an example frequency response graph created by the procedure just described.

Figure 8.32 Frequency response graph created from a signal dependent sound analysis system

Figure 8.32 Frequency response graph created from a signal dependent sound analysis system

Figure 8.33 shows a screenshot from FuzzMeasure Pro, a signal dependent analysis program that runs on the Mac operating system.  The frequency response is on the top, and the impulse response is at the bottom.  As you recall from Chapter 2, the frequency response has frequencies on the horizontal axis and amplitudes of these frequency components on the vertical axis.  It should how the frequencies “responded” to their environment as they moved from the loudspeaker to the microphone.  We know that the sine wave emitted had frequencies distributed evenly across the audible spectrum, so if the sound was not affected in passage, the frequency response graph should be flat.  But notice in the graph that the frequencies between 30 Hz and 500 Hz are 6 to 10 dB louder than the rest, which is their response to the environment.

Figure 8.33 FuzzMeasure Pro sound analysis software

Figure 8.33 FuzzMeasure Pro sound analysis software

When you look at an analysis such as this, it’s up to you to decide if you’ve identified a problem that you want to solve. Keep in mind that the goal isn’t necessarily to make the frequency response graph be a straight line, indicating all frequencies are of equal amplitude. The goal is to make the right kind of sound. Before you can decide what to do, you need to determine why the frequency response sounds like this. There are many possible reasons.  It could be that you’re too far off-axis from the loudspeaker generating the sound. That’s not a problem you can really solve when you’re analyzing a listening space for a large audience, since not everyone can sit in the prime location. You could move the analysis microphone so that you’re on-axis with the loudspeaker, but you can’t fix the off-axis frequency response for the loudspeaker itself.  In the example shown in Figure 8.34 the loudspeaker system that is generating the sound uses two sets of sound radiators. One set of loudspeakers generates the frequencies above 500 Hz. The other set generates the frequencies below 500 Hz. Given that information, you could conclude that the low-frequency loudspeakers are simply louder than the high frequency ones. If this is causing a sound that you don’t want, you could fix it by reducing the level of the low-frequency loudspeakers.

Figure 8.34 Frequency response graph showing a low frequency boost

Figure 8.34 Frequency response graph showing a low frequency boost

Figure 8.35 shows the result of after this correction. The grey line shows the original frequency response and the black line shows the frequency response after reducing the amplitude of the low-frequency loudspeakers by 6 dB.

Figure 8.34 Frequency response graph showing a low frequency boost

Figure 8.34 Frequency response graph showing a low frequency boost

The previous example gives you a sketch of how a sound analysis system might be used. You place yourself in a chosen position in a room where sound is to be performed or played, generate sound that is played through loudspeakers, and then measure the sound as it is received at your chosen position. The frequencies that are actually detected may not be precisely the frequency components of the original sound that was generated or played.   By looking at the difference between what you played and what you are able to measure, you can analyze the frequency response of your loudspeakers, the acoustics of your room, or a combination of the two. The frequencies that are measured by the sound analysis system are dependent not only on the sound originally produced, but also on the loudspeakers’ types and positions, the location of the listener in the room, and the acoustics of the room. Thus, in addition to measuring the frequency response of your loudspeakers, the sound analysis system can help you to determine if different locations in the room vary significantly in their frequency response, leaving it to you to decide if this is a problem and what factor might be the source.

The advantage to a signal dependent system is that it’s easy to use, and with it you can get a good general picture of how frequencies will sound in a given acoustic space with certain loudspeakers. You also can save the frequency response graphs to refer to and analyze later. The disadvantage to a signal dependent analysis system is that it uses only artificially-generated signals like sine sweeps, not real music or performances.

If you want to analyze actual music or performances, you need to use a signal independent analysis system. These systems allow you to analyze the frequency response recorded music, voice, sound effects, or even live performances as they sound in your acoustic space. In contrast to systems like FuzzMeasure, which know the precise sweep of frequencies they’re generating, signal independent systems must be given a direct copy of the sound being played so that the original sound can be compared with the sound that passes through the air and is received by the analysis microphone. This is accomplished by taking the original sound and sending one copy of it to the loudspeakers while a second copy is sent directly, via cable, to the sound analysis software. The software presumably is running on a computer that has a sound card attached with two sound inputs. One of the inputs is the analysis microphone and one is a direct feed from the sound source. The software compares the two signals in real time – as the music or sound is played – and tells you what is different about them.

The advantage of the signal independent system is that it can analyze “real” sound as it is being played or performed. However, real sound has frequency components that constantly change, as we can tell from the constantly changing pitches that we hear. Thus, there isn’t one fixed frequency response graph that gives you a picture of how your loudspeakers and room are dealing with the frequencies of the sound. The graph changes dynamically over the entire time that the sound is played. For this reason, you can’t simply save one graph and carry it off with you for analysis. Instead, your analysis consists of observing the constantly-changing frequency response graph in real time, as the sound is played. If you wanted to save a single frequency response graph, you’d have to do what we did to generate Figure 8.36 – that is, get a “screen capture” of the frequency response graph at a specific moment in time – and the information you have is about only that moment. Another disadvantage of signal independent systems is that they analyze the noise in the environment along with the desired sound.

Figure 8.36 was produced from a popular signal independent analysis program called Smaart Live, which runs on Windows and Mac operating systems. The graph shows the difference, in decibels, between the amplitudes of the frequencies played vs. those received by the analysis microphone. Because this is only a snapshot in time, coupled with the fact that noise is measured as well, it isn’t very informative to look at just one graph like this. Being able to glean useful information from a signal independent sound analysis system comes from experience in working with real sound – learning how to compare what you want, what you see, what you understand is going on mathematically, and – most importantly – what you hear.

Figure 8.36 Smaart Live sound analysis software

Figure 8.36 Smaart Live sound analysis software

8.2.4.4 System Optimization

Once you have the sound system installed and everything is functioning, the system needs to be optimized. System optimization is a process of tuning and adjusting the various components of the sound system so that

  • they're operating at the proper volume levels,
  • the frequency response of the sound system is consistent and desirable,
  • destructive interactions between system components and the acoustical environment have been minimized, and
  • the timing of the various system components has been adjusted so the audience hears the sounds at the right time.

The first optimization you should perform applied to the gain structure of the sound system. When working with sound systems in either a live performance or recording situation, gain structure is a big concern. In a live performance situation, the goal is to amplify sound. In order to achieve the highest potential for loudness, you need to get each device in your system operating at the highest level possible so you don’t lose any volume as the sound travels through the system. In a recording situation, you're primarily concerned with signal-to-noise ratio. In both of these cases, good gain structure is the solution.

In order to understand gain structure, you first need to understand that all sound equipment makes noise. All sound devices also contains amplifiers. What you want to do is amplify the sound without amplifying the noise. In a sound system with good gain structure, every device is receiving and sending sound at the highest level possible without clipping. Lining up the gain for each device involves lining up the clip points. You can do this by starting with the first device in your signal chain – typically a microphone or some sort of playback device. It’s easier to set up gain structure using a playback source because you can control the output volume. Start by playing something on the CD, synthesizer, computer, iPod or whatever your playback device is in a way that outputs the highest volume possible. This is usually done with either normalized pink noise or a normalized sine wave. Turn up the gain preamplifier on the mixing console or sound card input so that the level coming from the playback source clips the input. Then back off the gain until that sound is just below clipping. If you're recording this sound, your gain structure is now complete. Just repeat this process for each input. If it's a live performer on a microphone, ask him to perform at the highest volume they expect to generate and adjust the input gain accordingly.

If you’re in a live situation, the mixing console will likely feed its sound into another device such as a processor or power amplifier. With the normalized audio from your playback source still running, adjust the output level of the mixing console so it's also just below clipping. Then adjust the input level of the next device in the signal chain so that it's receiving this signal at just below its clipping point. Repeat this process until you've adjusted every input and output in your sound system. At this point, everything should clip at the same time. If you increase the level of the playback source or input preamplifier on the mixing console, you should see every meter in your system register a clipped signal. If you’ve done this correctly, you should now have plenty of sound coming from your sound system without any hiss or other noise. If the sound system is too loud, simply turn down the last device in the signal chain. Usually this is the power amplifier.

Setting up proper gain structure in a sound system is fairly simple once you're familiar with the process. The Max demo on gain structure associated with this section gives you an opportunity to practice the technique. Then you should be ready to line up the gain for your own systems.

Once you have the gain structure optimized, the next thing you need to do is try to minimize destructive interactions between loudspeakers. One reason that loudspeaker directivity is important is due to the potential for multiple loudspeakers to interact destructively if their coverage overlaps in physical space. Most loudspeakers can exercise some directional control over frequencies higher than 1 kHz, but frequencies lower than 1 kHz tend to be fairly omnidirectional, which means they will more easily run into each other in the air. The basic strategy to avoid destructive interactions is to adjust the angle between two loudspeakers so their coverage zone intersects at the same dBSPL, and at the point in the coverage pattern where they are 6 dB quieter than the on-axis level, as shown in Figure 8.37. This overlap point is the only place where the two loudspeakers combine at the same level. If you can pull that off, you can then adjust the timing of the loudspeakers so they’re perfectly in phase at that overlap point. Destructive interaction is eliminated because the waves reinforce each other, creating a 6 dB boost that eliminates the dip in sound level at high frequencies.   The result is that there is even sound across the covered area. The small number of listeners who happen to be sitting in an area of overlap between two loudspeakers will effectively be covered by a virtual coherent loudspeaker.

When you move away from that perfect overlap point, one loudspeaker gets louder as you move closer to it, while the other gets quieter as you move farther away. This is handy for two reasons. First, the overall combined level should remain pretty consistent at any angle as you move through the perfect overlap point. Second, for any angle outside of that perfect overlap point, while the timing relationship between the two loudspeaker arrivals begins to differ, the loudspeakers also differ more and more in level. As pure comb filtering requires both of the interacting signals to be at the same amplitude, the level difference greatly reduces the effect of the comb filtering introduced by the shift in timing. The place where the sound from the two loudspeakers arrives at the same amplitude and comb filters the most is at center of the overlap, but this is the place where we aligned the timing perfectly to prevent comb filtering in the first place. With this technique, not only do you get the wider coverage that comes with multiple loudspeakers, but you also get to avoid the comb filtering!

Figure 8.37 Minimizing comb filtering between two loudspeakers

Figure 8.37 Minimizing comb filtering between two loudspeakers

What about the low frequencies in this example? Well, they’re going to run into each other at similar amplitudes all around the room because they’re more omnidirectional than the high frequencies. However, they also have longer wavelengths, which means they require much larger offsets in time to cause destructive interaction. Consequently, they largely reinforce each other, giving an overall low frequency boost. Sometimes this free bass boost sounds good. If not, you can easily fix it with a system EQ adjustment by adding a low shelf filter that reduces the low frequencies by a certain amount to flatten out the frequency response of the system. This process is demonstrated in our video on loudspeaker interaction.

You should work with your loudspeakers in smaller groups, sometimes called systems. A center cluster of loudspeakers being used to cover the entire listening area from a single point source would be considered a system. You need to work with all the loudspeakers in that cluster to ensure they are working well together. A row of front fill loudspeakers at the edge of the stage being used to cover the front few rows will also need to be optimized as an individual system.

Once you have each loudspeaker system optimized, you need to work with all the systems together to ensure they don’t destructively interact with each other. This typically involves manipulating the timing of each system. There are two main strategies for time aligning loudspeaker systems. You can line the system up for coherence, or you can line the system up for precedence imaging. The coherence strategy involves working with each loudspeaker system to ensure that their coverage areas are as isolated as possible. This process is very similar to the process we described above for aligning the splay angles of two loudspeakers. In this case, you're doing the same thing for two loudspeaker systems. If you can line up two different systems so that the 6 dB down point of each system lands in the same point in space, you can then apply delay to the system arriving first so that both systems arrive at the same time, causing a perfect reinforcement. If you can pull this off for the entire sound system and the entire listening area, the listeners will effectively be listening to a single, giant loudspeaker with optimal coherence.

The natural propagation of sound in an acoustic space is inherently not very coherent due to the reflection and absorption of sound, resulting in destructive and constructive interactions that vary across the listening area. This lack of natural coherence is often the reason that a sound reinforcement system is installed in the first place. A sound system that has been optimized for coherence has the characteristic of sounding very clear and very consistent across the listening area. These can be very desirable qualities in a sound system where clarity and intelligibility are important. The downside to this optimization strategy is that it sometimes does not sound very natural. This is because with coherence optimized sound systems, the direct sound from the original source (i.e. a singer/performer on stage) has typically little to no impact on the audience, and so the audience perceives the sound as coming directly from the loudspeakers. If you’re close enough to the stage and the singer, and the loudspeakers are way off to the side or far overhead, it can be strange to see the actual source yet hear the sound come from somewhere else. In an arena or stadium setting, or at a rock concert where you likely wouldn’t hear much direct sound in the first place, this isn’t as big a problem. Sound designers are sometimes willing to accept a slightly unnatural sound if it means that they can solve the clarity and intelligibility problems that occur in the acoustic space.

Aside:  While your loudspeakers might sit still for the whole show, the performers usually don't.  Out Board's TiMax tracker and soundhub delay matrix system use radar technology to track actors and performers around a stage in three dimensions, automating and adjusting the delay times to maintain precedence and deliver natural, realistic sound throughout the performance.

Optimizing the sound system for precedence imaging is completely opposite to the coherence strategy. In this case, the goal is to increase the clarity and loudness of the sound system while maintaining a natural sound as much as possible. In other words, you want the audience to be able to hear and understand everything in the performance but you want them to think that what they are hearing is coming naturally from the performer instead of coming from loudspeakers in a sound system. In a precedence imaging sound system, each loudspeaker system behaves like an early reflection in an acoustic space. For this strategy to work, you want to maximize the overlap between the various loudspeaker systems. Each listener should be able to hear two or three loudspeaker systems from a single seat. The danger here is that these overlapping loudspeaker systems can easily comb filter in a way that will make the sound unpleasant or completely unintelligible. Using the precedence effect described in Chapter 4, you can manipulate the delay of each loudspeaker system so they arrive at the listener at least five milliseconds apart but no more than 30 milliseconds apart. The signals still comb filter, but in a way that our hearing system naturally compensates for. Once all of the loudspeakers are lined up, you’ll also want to delay the entire sound system back to the performer position on stage. As long as the natural sound from the performer arrives first, followed by a succession of similar sounds from the various loudspeaker systems each within this precedence timing window, you can get an increased volume and clarity as perceived by the listener while still maintaining the effect of a natural acoustic sound. If that natural sound is a priority, you can achieve acceptable results with this method, but you will sacrifice some of the additional clarity and intelligibility that comes with a coherent sound system.

Both of these optimization strategies are valid, and you'll need to evaluate your situation in each case to decide which kind of optimized system best addresses the priorities of your situation. In either case, you need some sort of system processor to perform the EQ and delay functions for the loudspeaker systems. These processors usually take the form of a dedicated digital signal-processing unit with multiple audio inputs and outputs. These system processors typically require a separate computer for programming, but once the system has been programmed, the units perform quite reliably without any external control. Figure 8.38 shows an example of a programming interface for a system processor.

Figure 8.38 Programming interface for a digital system processor

Figure 8.38 Programming interface for a digital system processor

8.2.4.5 Multi-Channel Playback

Mid-Side can also be effective as a playback technique for delivering stereo sound to a large listening area. One of the limitations to stereo sound is that the effect relies on having the listener perfectly centered between the two loudspeakers. This is usually not a problem for a single person listening in a small living room. If you have more than one listener, such as in a public performance space, it can be difficult if not impossible to get all the listeners perfectly centered between the two loudspeakers. The listeners who are positioned to the left or right of the center line will not hear a stereo effect. Instead they will perceive most of the sound to be coming from whichever loudspeaker they are closest to. A more effective strategy would be to set up three loudspeakers. One would be your Mid loudspeaker and would be positioned in front of the listeners. The other two loudspeakers would be positioned directly on either side of the listeners as shown in Figure 8.39.

Figure 8.39 Mid Side loudspeaker setup

Figure 8.39 Mid Side loudspeaker setup

If you have an existing audio track that has been mixed in stereo, you can create a reverse Mid-Side matrix to convert the stereo information to a Mid-Side format. The Mid loudspeaker gets a L+R audio signal equivalent to summing the two stereo tracks to a single mono signal. The Side+ loudspeaker gets a L-R audio signal, equivalent to inverting the right channel polarity and summing the two channels to a mono signal. This will cancel out anything that is equal in the two channels essentially, removing all the Mid information. The Side- loudspeaker gets a R-L audio signal. Inverting the left channel polarity and summing to mono or simply inverting the Side+ signal can achieve this effect. The listeners in this scenario will all hear something similar to a stereo effect. The right channel stereo audio will cancel out in the air between the Mid and Side+ loudspeakers and the left channel stereo audio will cancel out in the air between the Mid and Side- loudspeakers. Because the Side+/- loudspeakers are directly to the side of the listeners, they will all hear this stereo effect regardless of whether they are directly in front of the MID loudspeaker. Just like Mid Side recording, the stereo image can be widened or narrowed as the balance between the Mid loudspeaker and Side loudspeakers is adjusted.

You don’t need to stop at just three loudspeakers. As long as you have more outputs on your playback system you can continue to add loudspeakers to your system to help you create more interesting soundscapes. The concept of Mid-Side playback illustrates an important concept. Having multiple loudspeakers doesn’t mean you have surround sound. If you play the same sound out of each loudspeaker, the precedence effect takes over and each listener will source the sound to the closest loudspeaker. To create surround sound effects, you need to have different sounds in each loudspeaker. The concept of Mid-Side playback demonstrates how you can modify a single sound to have different properties in three loudspeakers, but you could also have completely different sounds playing from each loudspeaker. For example, instead of having a single track of raindrops playing out of ten loudspeakers, you could have ten different recordings of water dripping onto various surfaces. This will create a much more realistic and immersive rain effect. You can also mimic acoustic effects using multiple loudspeakers. You could have the dry sound of a recorded musical instrument playing out of the loudspeakers closest to the stage and then play various reverberant or wet versions of the recording out of the loudspeakers near the walls. With multiple playback channels and multiple loudspeakers you can also create the effect of a sound moving around the room by automating volume changes over time.

8.2.4.6 Playback and Control

Sound playback has evolved greatly in the past decades, and it’s safe to say tape decks with multiple operators and reel changes are a thing of history.  While some small productions may still use CD players, MiniDiscs, or even MP3 players to playback their sound, it’s also safe to say that computer-based playback is the system of choice, especially in any professional production.  Already an integral part of the digital audio workflow, computers offer flexibility, scalability, predictability, and unprecedented control over audio playback.  Being able to consistently run a performance and reduce operator error is a huge advantage that computer playback provides.  Yet as simple as it may be to operate on the surface, the potential complexity behind a single click of a button can be enormous.

Popular computer sound playback software systems include SFX by Stage Research for Windows operating systems, and QLab by Figure 56 on a Mac.  These playback tools allow for many methods of control and automation, including sending and receiving MIDI commands, scripting, telnet, and more, allowing them to communicate with almost any other application or device.  These playback systems also allow you to use multiple audio outputs, sending sound out anywhere you want, be it a few specific locations, or the entire sound system. This is essential for creating immersive and dynamic surround effects. You’ll need a separate physical output channel from your computer audio interface for each loudspeaker location (or group of loudspeakers, depending on your routing) in your system that you want to control individually.

Controlling these systems can be as simple as using the mouse pointer on your computer to click a GO button.  Yet that single click could trigger layers and layers of sound and control cues, with specifically timed sequences that execute an entire automated scene change or special effect.  Theme parks use these kind of playback systems to automatically control an entire show or environment, including sound playback, lighting effects, mechanical automation, and any other special effects.   In these cases, sometimes the simple GO isn’t even triggered by a human operator, but by a timed script, making the entire playback and control a consistent and self-reliant process.  Using MIDI or Open Sound Control you can get into very complex control systems.  Other possible examples include using sensors built into scenery or costumes for actor control, as well as synchronizing sound, lighting, and projection systems to keep precisely timed sequences operating together and exactly on cue, such as a simulated lighting strike.  Outside of an actual performance, these control systems can benefit you as a designer by providing a means of wireless remote control from a laptop or tablet, allowing you to make changes to cues while listening from various locations in the theatre.

Using tools such as Max or PD, you can capture input from all kinds of sources such as cameras, mobile devices, or even video game controllers, and use that control data to generate MIDI commands to control sound playback.  You’ll always learn more actually doing it than simply reading about it, so included in this section are several exercises to get you going making your own custom control and sound playback systems.

8.2.3 Post-production

8.2.3.1 Overdubbing

Post-production for film and video often requires a process of overdubbing.  Overdubbing production audio is referred to as ADR, which stands for Automated Dialog Replacement or Additional Dialogue Recording (depending on who you ask).   During this process, an actor is brought in to a recording studio, looks at the scene that was filmed during the production process, and listens to the performance she gave. The actor then attempts to recreate that performance vocally. Overdubbing is typically done in small chunks in a loop so the actor has multiple attempts to get it right. She's trying to recreate not only the sound but also the speed of the original performance so that the new recording is synchronized with the movement of the lips on the screen. System clicks and streamers can be used to help the actor. Clicks (sometimes called beeps) are a rhythmic sound, like a metronome, that counts down to a certain point when the actor needs to start or hit a particular word. Streamers are a visual reference that follows the same speed of the clicks. The streamer is a solid line across the screen that moves in time with the clicks so you can see when important synchronization events occur. Clicks and streamers are also used in other post-production audio tasks for synchronizing sound effects and music during recording sessions. A click refers to a metronome that the conductor and musicians listen to do keep the music in time with the picture. A streamer is a colored vertical line that moves across the screen over a period of 2 to 4 seconds. When the streamer reaches the end of the screen the music is meant to reach a certain point. For example, the beginning of each measure might need to synchronize with the switch in the camera shot.

Figure 8.18 A blue streamer used to help a musicians time out their performance with the picture

Figure 8.18 A blue streamer used to help a musicians time out their performance with the picture

8.2.3.2 Mixing

Mixing is the process multiple sounds recorded on different tracks in a DAW are combined, with adjustments made to their relative levels, frequency components, dynamics, and special effects.  Then the resulting mix is channeled to different speakers appropriately.  The mixing process, hardware, and software were covered in detail in Chapter 7.  Thus, we focus here on practical and design considerations that direct the mixing process.

When you sit down to mix a recording you go through a process of trying to balance how you want the recording to sound against the quality of the recording. Often you will be limited in what you’re able to achieve with the mix because the source recording does not allow you sufficient manipulation. For example, if your recording of a band playing a song was recorded using a single overhead microphone, your ability to mix that recording is severely limited because all the instruments, room acoustic, and background noise are on the same track. You can turn the whole thing up or down, EQ and compress the overall recording, and add some reverb, but you have no control over the balance between the different instruments. On the other end of the spectrum you could have a recording with each instrument, voice, and other elements on separate tracks recording with separate microphones that were well-isolated from each other. In this scenario you have quite a bit of control over the mix, but mixing down 48 or more tracks is very time consuming. If you don’t have the time or expertise to harness all of that data, you may be forced to settle for something less than what you envision for the mix. Ultimately, you could work on a mix for the rest of your life and never be completely satisfied. So make sure you have clear goals and priorities for what you want to achieve with the mix and work through each priority until you run out of time or run out of parameters to manipulate.

Mixing sound for film or video can be particularly challenging because there are often quite a few different sounds happening at once. One way of taming the mix is to use surround sound. Mixing the various elements to different loudspeakers separates the sound such that each can be heard in the mix. Voices are typically mixed to the center channel, while music and sound effects are mixed to the four different surround channels. Loudness and dynamics are also an issue that gets close attention in the mixing process. In some cases you may need to meet a specific average loudness level over the course of the entire video. In other cases, you might need to compress the voices but leave the rest of the mix unchanged. The mix engineer will typically create stems (similar to busses or groups) to help with the process, such as a vocal stem, a music stem, and a sound effects stem. These stems can then be manipulated for various delivery mediums. You can see the usefulness of stems in situations where the sound being mixed is destined for one medium – for television broadcast as well as DVD distribution, for example.  The audio needs of a television broadcast are very different from the needs of a DVD. If the mix is ultimately going to be adjusted for both of these media, it is much easier to use stems rather returning to the original multitrack source, which may involve several hundred tracks.

 

8.2.3.3 Mastering

When you've completed the mixing process for a recording project, the next step is mastering the mixed-down audio. Mastering is the process of adjusting the dynamics and frequency response of a mix in order to optimize it for listening in various environments and prepare it for storage on the chosen medium. The term mastering comes from the idea of a master copy from which all other copies are made. Mastering is a particularly important step for music destined for CD or DVD, ensuring consistent levels and dynamics from one song to the next in an album.

In some ways you could describe the mastering process as making the mix sound louder. When mixing a multitrack recording, one thing you watch for is clipped signals. Once the mix is completed, you may have a well-balanced mix, but overall the mix sounds quieter than other mixes you hear on commercial recordings. What is typically happening is that you have one instrument in your mix that is a bit more dynamic than the others, and in order to keep the mix from clipping, you have to turn everything down because of this one instrument. One step in the mastering process is to use a multi-band compressor to address this problem.

A multi-band compressor is a set of compressors each of which operates on a limited frequency band without affecting others. A traditional compressor attenuates the entire mix when one frequency exceeds the threshold. A multi-band compressor, on the other hand, attenuates an instrument that is dynamic in one frequency band without attenuating other bands. This is often much more effective than using a simple EQ because the processing is only applied when needed, whereas an EQ will boost or cut a certain range of frequencies all the time. This allows you to let the less-dynamic frequencies take a more prominent role in the mix, resulting in the entire mix sounding louder.

Figure 8.19 shows an example of a multi-band compressor with five separate bands of compression centered on the frequency indicated by the colored dots. You can set a separate threshold, gain, and compression range. In this case, a range is taking place of a ratio. The idea is that you want to compress the frequency band to stay within a given range. As you adjust the gain for each band, that range gets shifted up or down. This has the effect of manipulating the overall frequency response of the track in a way that is responsive to the changing amplitudes of the various frequencies. For example, if the frequency band centered at 1.5 kHz gets suddenly very loud, it can be attenuated for that period of time but then restored when the sound in that band drops back down to a more regular level.

Figure 8.19 A multi-band compressor used for mastering

Figure 8.19 A multi-band compressor used for mastering

You may also choose to apply some overall EQ in the mastering process to suit your taste. In some cases you may also manipulate the stereo image a bit to widen or narrow the overall stereo effect. You may also want to add a multi-band limiter at the end of the processing chain to catch any stray clipped signals that may have resulted from your other processes. If you're converting to a lower bit depth, you should also apply a dither process to the mix to account for any quantization errors. For example, CDs require 16-bit samples, but most recording systems use 24 bits. Even if you are not converting bit depth you may still want to use dither since most DAW programs process the audio internally at 32 or 64 bits before returning back the original 24 bits. A 24-bit dither could help you avoid any quantization errors that would occur in that process. Figure 8.20 shows an example of a multi-band limiter that includes a dither processor.

Figure 8.20 A multi-band limiter with dither

Figure 8.20 A multi-band limiter with dither

8.2.2 Production

8.2.2.1 Capturing the Four-Dimensional Sound Field

When listening to sound in an acoustic space, such as at a live orchestral concert, you hear different sounds arriving from many directions. The various instruments are spread out on a stage, and their sound arrives at your ears somewhat spread out in time and direction according to the physical location of the instruments. You also hear subtly nuanced copies of the instrument sounds as they’re reflected from the room surfaces at even more varying times and directions. The audience makes their own sound in applause, conversation, shuffling in seats, cell phones going off, etc. These sounds arrive from different directions as well. Our ability to perceive this four-dimensional effect is the result of the physical characteristics of our hearing system. With two ears, the differences in arrival time and intensity between them allow us to perceive sounds coming from many different directions. Capturing this effect with audio equipment and then either reinforcing the live audio or recreating the effect upon playback is quite challenging.

The biggest obstacle is the microphone. A traditional microphone records the sound pressure amplitude at a single point in space. All the various sound waves arriving from different directions at different times are merged into a single electrical voltage wave on a wire. With all the data merged into a single audio signal, much of the four-dimensional acoustic information is lost. When you play that recorded sound out of a loudspeaker, all the reproduced sounds are now coming from a single direction as well. Adding more loudspeakers doesn’t solve the problem because then you just have every sound repeated identically from every direction, and the precedence effect will simply kick in and tell our brain that the sound is only coming from the lone source that hits our ears first.

Aside:  If the instruments are all acoustically isolated, the musicians may have a hard time hearing themselves and each other. This poses a significant obstacle, as they will have a difficult time trying to play together. To address this problem, you have to set up a complicated monitoring system.  Typically each musician has a set of headphones that feeds him or her a custom mix of the sounds from each mic/instrument.

The first step in addressing some of these problems is to start using more than one microphone. Stereo is the most common recording and playback technique. Stereo is an entirely man-made effect, but produces a more dynamic effect upon playback of the recorded material with only one additional loudspeaker. The basic idea is that since we have two ears, two loudspeakers should be sufficient to reproduce some of the four-dimensional effects of acoustic sound. It's important to understand that there is no such thing as stereo sound in an acoustic space. You can’t make a stereo recording of a natural sound. When recording sound that will be played back in stereo, the most common strategy is recording each sound source with a dedicated microphone that is as acoustically isolated as possible from the other sound sources and microphones. For example, if you were trying to record a simple rock band, you would put a microphone on each drum in the drum kit as close to the drum as possible. For the electric bass, you would put a microphone as close as possible to the amplifier and probably use a hardwired cable from the instrument itself. This gives you two signals to work with for that instrument. You would do the same for the guitar. If possible, you might even isolate the bass amplifier and the guitar amplifier inside acoustically sealed boxes or rooms to keep their sound from bleeding into the other microphones. The singer would also be isolated in a separate room with a dedicated microphone.

During the recording process, the signal from each microphone is recorded on a separate track in the DAW software and written to a separate audio file on the hard drive. With an isolated recording of each instrument, a mix can be created that distributes the sound of each instrument between two channels of audio that are routed to the left and right stereo loudspeaker. To the listener sitting between the two loudspeakers, a sound that is found only on the left channel sounds like it comes from the left of the listener and vice versa for the right channel. A sound mixed equally into both channels appears to the listener as though the sound is coming from an invisible loudspeaker directly in the middle. This is called the phantom center channel. By adjusting the balance between the two channels, you can place sounds at various locations in the phantom image between the two loudspeakers. This flexibility in mixing is possible only because each instrument was recorded in isolation. This stereo mixing effect is very popular and produces acceptable results for most listeners.

When recording in a situation where it's not practical to use multiple microphones in isolation – such as for a live performance or a location recording where you're capturing an environmental sound – it's still possible to capture the sound in a way that creates a stereo-like effect. This is typically done using two microphones and manipulating the way the pickup patterns of the microphones overlap. Figure 8.5 shows a polar plot for a cardioid microphone. Recall that a cardioid microphone is a directional microphone that picks up the sound very well on-axis with the front of the microphone but doesn't pick up the sound as well off-axis. This polar plot shows only one plotted line, representing the pickup pattern for a specific frequency (usually 1 kHz), but keep in mind that the directivity of the microphone changes slightly for different frequencies. Lower frequencies are less directional and higher frequencies are more directional than what is shown in Figure 8.5. With that in mind, we can examine the plot for this frequency to get an idea of how the microphone responds to sounds from varying directions. Our reference level is taken at 0° (directly on-axis). The dark black line representing the relative pickup level of the microphone intersects with the 0 dB line at 0°. As you move off-axis, the sensitivity of the microphone changes. At around 75°, the line intersects with the -5 dB point on the graph, meaning that at that angle, the microphone picks up the sound 5 dB quieter than it does on-axis. As you move to around 120°, the microphone now picks up the sound 15 dB quieter than the on-axis level. At 180° the level is null.

Figure 8.5 Polar pattern for a cardioid microphone

Figure 8.5 Polar pattern for a cardioid microphone

One strategy for recording sound with a stereo effect is to use an XY cross pair. The technique works by taking two matched cardioid microphones and positioning them so the microphone capsules line up horizontally at 45° angles that cross over the on-axis point of the opposite microphone. Getting the capsules to line up horizontally is very important because you want the sound from every direction to arrive at both microphones at the same time and therefore in the same phase.

Figure 8.6 A portable recording device using integrated microphones in an XY cross pair

Figure 8.6 A portable recording device using integrated microphones in an XY cross pair

Aside:  At 0° on-axis to the XY pair, the individual microphone elements are still tilted 45°, making the microphone's pickup a few dB quieter than its own on-axis level would be.  Yet because the sound arrives at both microphones at the same level and the same phase, the sound is perfectly reinforced, causing a boost in amplitude.  In this case the result is actually slightly louder than the on-axis level of either individual microphone.

Figure 8.6 shows a recording device with integrated XY cross pair microphones, and Figure 8.7 shows the polar patterns of both microphones when used in this configuration. The signals of these two microphones are recorded onto separate tracks and then routed to separate loudspeakers for playback. The stereo effect happens when these two signals combine in the air from the loudspeakers. Let’s first examine the audio signals that are unique to the left and right channels. For a sound that arrives at the microphones 90° off-axis, there is approximately a 15 dB difference in level for that sound captured between the two microphones. As a rule of thumb, whenever you have a level difference that is 10 dB or greater between two similar sounds, the louder sound takes precedence. Consequently, when that sound is played back through the two loudspeakers, it is perceived as though it's entirely located at the right loudspeaker. Likewise, a sound arriving 270° off-axis sounds as though it's located entirely at the left loudspeaker. At 0°, the sound arrives at both microphones at the same level. Because the sound is at an equal level in both microphones, and therefore is played back equally loud through both loudspeakers, it sounds to the listener as if it's coming from the phantom center image of the stereo field. At 45°, the polar plots tell us that the sound arrives at the right microphone approximately 7 dB louder than at the left. Since this is within the 10 dB range for perception, the level in the left channel causes the stereo image of the sound to be pulled slightly over from the right channel, now seeming to come from somewhere between the right speaker and the phantom center location. If the microphones are placed appropriately relative to the sound being recorded, this technique can provide a fairly effective stereo image without requiring any additional mixing or panning.

Figure 8.7 Polar patterns for two cardioid microphones in an XY cross pair

Figure 8.7 Polar patterns for two cardioid microphones in an XY cross pair

Another technique for recording a live sound for a stereo effect is called mid-side. Mid-side also uses two microphones, but unlike XY, one microphone is a cardioid microphone and the other is a bidirectional or figure-eight microphone. The cardioid microphone is called the mid microphone and is pointed forward (on-axis), and the figure-eight microphone is called the side microphone and is pointed perpendicular to the mid microphone. Figure 8.8 shows the polar patterns of these two microphones in a mid-side configuration.

Figure 8.8 Polar patterns for two microphones in a mid-side setup

Figure 8.8 Polar patterns for two microphones in a mid-side setup

The side microphone has a single diaphragm that responds to pressure changes on either side of the microphone. The important thing to understand here is that because of the single diaphragm, the sounds on either side of the microphone are captured in opposite polarity.  That is, a sound that causes a positive impulse on the right of the microphone causes a negative impulse on the left of the microphone. It is this polarity effect of the figure-eight microphone that allows the mid-side technique to work. After you’ve recorded the signal from these two microphones onto separate channels, you have to set up a mid-side matrix decoder in your mixing console or DAW software in order to create the stereo mix. To create a mid-side matrix, you take the audio from the mid microphone and route it to both left and right output channels (pan center). The audio from the side microphone gets split two ways. First it gets sent to the left channel (pan left).  Then it gets sent also to the right channel (pan right) with the polarity inverted.  Figure 8.9 shows a mid-side matrix setup in Logic. The “Gain” plugin inserted on the “Side -” track is being used only to invert the polarity (erroneously labeled “Phase Invert” in the plug-in interface).

Figure 8.9 Mid-side matrix in Logic

Figure 8.9 Mid-side matrix in Logic

Through the constructive and destructive combinations of the mid and side signals at varying angles, this matrix creates a stereo effect at its output. The center image is essentially derived from the on-axis response of the mid microphone, which by design happens also to be the off-axis point of the side microphone.  Any sound that arrives at 0° to the mid microphone is added to both the left and right channels without any interaction from the signal from the side microphone, since at 0° to the mid-side setup the side microphone pickup is null. If you look at the polar plot, you can see that the mid microphone picks up every sound within a 120° spread with only 6 dB or so of variation in level.  Aside from this slight level difference, the mid microphone doesn't contain any information that can alone be used to determine a sound’s placement in the stereo field.  However, approaching the 300° point (arriving more from the left of the mid-side setup), you can see that the sound arriving at the mid microphone is also picked up by the side microphone at the same level and the same polarity. Similarly, a sound that arrives at 60° also arrives at the side microphone at the same level as the mid, but this time it is inverted in polarity from the signal at the mid microphone.  If you look at how these two signals combine, you can see that the mid sound at 300° mixes together with the “Side +” track and, because it is the same polarity, it reinforces in level. That same sound mixes together with the “Side -” track and cancels out because of the polarity inversion.  The sound that arrives from the left of the mid-side setup therefore is louder on the left channel and accordingly appears to come from the left side of the stereo field upon playback.  Conversely, a sound coming from the right side at 60° reinforces when mixed with the “Side-“ track but cancels out when mixed with the “Side+” track, and the matrixed result is louder in the right channel and accordingly appears to come from the right of the stereo field.  Sounds that arrive between 0° and 300° or 0° and 60°have a more moderate reinforcing and canceling effect, and the resulting sound appears at some varying degree between left, right, and center depending on the specific angle. This creates the perception of sound that is spread between the two channels in the stereo image.

The result here is quite similar to the XY cross pair technique with one significant difference. Adjusting the relative level of the “Mid” track alters the spread of the stereo image. Figure 8.10 shows a mid-side polar pattern with the mid microphone attenuated -10 dB. Notice that the angle where the two microphones pick up the sound at equal levels has narrowed to 45° and 315°. This means that when they are mixed together in the mid-side matrix, a smaller range of sounds are mixed equally into both left and right channels. This effectively widens the stereo image. Conversely, increasing the level of the mid microphone relative to the side microphone causes more sounds to be mixed into the left and right channels equally, thereby narrowing the stereo image. Unlike the XY cross pair, with mid-side the stereo image can be easily manipulated after the recording has already been made.

Figure 8.10 Polar patterns for two microphones in mid-side setup with the mid microphone attenuated −10 dB (wide mode)

Figure 8.10 Polar patterns for two microphones in mid-side setup with the mid microphone attenuated −10 dB (wide mode)

The concept behind mid-side recording can be expanded in a number of ways to allow recordings to capture sound in many directions while still maintaining the ability to recreate the desired directional information on playback. One example is shown in Figure 8.11.  This microphone from the Soundfield Company has four microphone capsules in a tetrahedral arrangement, each pointing a different direction. Using proprietary matrix processing, the four audio signals captured from this microphone can be combined to generate a mono, stereo, mid-side, four-channel surround, five-channel surround, or even a seven-channel surround signal.

Figure 8.11 Soundfield microphone

Figure 8.11 Soundfield microphone

The counterpart to setting up microphones for stereo recording is setting up loudspeakers for stereo listening.  Thus, we touch on the mid-side technique in the context of loudspeakers in Section 8.2.4.5.

The most simplistic (and arguably the most effective) method for capturing four-dimensional sound is binaural recording. It’s quite phenomenal that despite having only two transducers in our hearing system (our ears), we are somehow able to hear and perceive sounds from all directions. So instead of using complicated setups with multiple microphones, just by putting two microphones inside the ears of a real human, you can capture exactly what the two ears are hearing.  This method of capture inherently includes all of the complex inter-aural time and intensity difference information caused by the physical location of the ears and the human head that allows the brain to decode and perceive the direction of the sound. If this recorded sound is then played back through headphones, the listener perceives the sound almost exactly as it was perceived by the listener in the original recording. While wearable headphone-style binaural microphone setups exist, sticking small microphones inside the ears of a real human is not always practical, and an acceptable compromise is to use a binaural dummy head microphone. A dummy head microphone is essentially the plastic head of a mannequin with molds of a real human ear on either side of the head. Inside each of these prosthetic ears is a small microphone, the two together capturing a binaural recording. Figure 8.12 shows a commercially available dummy head microphone from Neumann.

With binaural recording, the results are quite effective. All the level, phase, and frequency response information of the sound arriving at both ears individually that allows us to perceive sound is maintained in the recording. The real limitation here is that the effect is largely lost when the sound is played through loudspeakers. The inter-aural isolation provided by headphones is required when listening to binaural recordings in order to get the full effect.

Figure 8.12 A binaural recording dummy head with built-in microphones

Figure 8.12 A binaural recording dummy head with built-in microphones

A few algorithms have been developed that mimic the binaural localization effect. These algorithms have been implemented into binaural panning plug-ins that are available for use in many DAW software programs, allowing you to artificially create binaural effects without requiring the dummy head recordings. An example of a binaural panning plug-in is shown in Figure 8.13. One algorithm is called the Cetera algorithm and is owned by the Starkey hearing aid company. They use the algorithm in their hearing aids to help the reinforced sound from a hearing aid sound more like the natural response of the ear. Starkey created a demo of their algorithm called the Starkey Virtual Barbershop. Although this recording sounds like it was captured with a binaural recording system, the binaural localization effects are actually rendered on a computer using the Cetera algorithm.

Figure 8.13 The binaural panning interface in Logic

Figure 8.13 The binaural panning interface in Logic

8.2.2.2 Setting Levels for Recording

Before you actually hit the “record” button you’ll want to verify that all your input levels are correct. The goal is to adjust the microphone preamplifiers so the signal from each microphone is coming into the digital converters at the highest voltage possible without clipping. Start by record-enabling the track in your software and then have the performer do the loudest part of his or her performance. Adjust the preamplifier so that this level is at least 6 dB below clipping. This way when the performer sings or speaks louder, you can avoid a clip. While recording, keep an eye on the input meters for each track and make sure nothing clips. If you do get a clip, at the very least you need to reduce the input gain on the preamplifier. You may also need to redo that part of the recording if the clip was bad enough to cause audible distortion.

8.2.2.3 Multitrack Recording

As you learned in previous chapters, today’s recording studios are equipped with powerful multitrack recording software that allows you to record different voices and instruments on different tracks. This environment requires that you make choices about what elements should be recorded at the same. For example, if you’re recording music using real instruments, you need to decide whether to record everything at the same time or record each instrument separately. The advantage to recording everything at the same time is that the musicians can play together and feel their way through the song. Musicians usually prefer this, and you almost always get a better performance when they play together. The downside to this method is that with all the musicians playing in the same room, it’s difficult to get good isolation between the instruments in the recording. Unless you’re very careful, you’ll pick up the sound of the drums on the vocalist’s microphone. This is hard to fix in post-production, because if you want the vocals louder in the mix, the drums get louder, also.

If you decide to record each track separately, your biggest problem is going to be synchronization. If all the various voices and instruments are not playing together, they will not be very well synchronized. They will start and end their notes at slightly different times, they may not blend very well, and they may not all play at the same tempo. The first thing you need to do to combat this problem is to make sure the performer can hear the other tracks that have already been recorded. Usually this can be accomplished by giving the performer a set of headphones to wear that are connected to your audio interface. The first track you record will set the standard for tempo, duration, etc. If the subsequent tracks can be recorded while the performer listens to the previous ones, the original track can set the tempo. Depending on what you are recording, you may also be able to provide a metronome or click track for the performers to hear while they perform their track. Most recording software includes a metronome or click track feature. Even if you use good monitoring and have the performers follow a metronome, there will still be synchronization issues. You may need to have them do certain parts several times until you get one that times out correctly. You may also have to manipulate the timing after the fact in the editing process.

Figure 8.14 shows a view from the mixing console during a recording session for a film score. You can see through the window into the stage where the orchestra is playing. Notice that the conductor and other musicians are wearing headphones to allow them to hear each other and possibly even the metronome associated with the beat map indicated by the timing view on the overhead screen. These are all strategies for avoiding synchronization issues with large multi-track recordings.

Figure 8.14 A view from the mixing console during a recording session for a film score

Figure 8.14 A view from the mixing console during a recording session for a film score

Another issue you will encounter in nearly every recording is that performers will make mistakes. Often the performer will need to make several attempts at a given performance before an acceptable performance is recorded. These multiple attempts are called takes, and each take represents a separate recording of the performer attempting the same performance. In some cases a given take may be mostly acceptable, but there was one small mistake. Instead of doing an entire new take, you can just re-record that short period of the performance that contained the mistake. This is called punch-in recording.

To make a punch-in recording you set up a start and stop time marker and start playback on the session. The performer hears the preceding several seconds and starts singing along with the recording. When the timeline encounters the start marker, the software starts writing the recording to the track and then reverts back to playback mode when the stop marker is reached. In the end you will have several takes and punch-ins folded into a single track. This is called a composite track, usually abbreviated to comp. A comp track allows you to unfold the track to see all the various takes. You can then go through them and select all the parts you want to keep from each take. A composite single version is created from all the takes you select. Figure 8.15 shows a multi-track recording session in Logic that uses a composite track that has been unfolded to show each take. The comp track is at the top and is color-coded to indicate which take was used for each period of time on the track.

Figure 8.15 A multi-track recording session in Logic with a composite track made up from multiple takes

Figure 8.15 A multi-track recording session in Logic with a composite track made up from multiple takes

8.2.2.4 Recording Sound for Film or Video

Production audio for film or video refers to the sound captured during the production process – when the film or video is actually being shot. In production, there may be various sounds you're trying to capture – the voices of the actors, environmental sounds, sounds of props, or other scenic elements. When recording in a controlled sound stage studio, you generally can capture the production audio with reasonably good quality, but when recording on location you constantly have to battle background noise. The challenge in either situation is to capture the sounds you need without capturing the sounds you don’t need.

All the same rules apply in this situation as in other recording situations. You need good quality microphones, and you need to get them as close as possible to the thing you're trying to record. Microphone placement can be challenging in a production environment where high definition cameras are close up on actors, capturing a lot of detail. Typically the microphone needs to be invisible or out of the camera shot. Actors can wear wireless lavaliere microphones as long as they can be hidden under some clothing. This placement affects the quality of the sound being picked up by the microphone, but in the production environment compromises are a necessity. The primary emphasis is of course on capturing the things that would be impossible or expensive to change in post-production, like performances or action sequences. For example, if you don’t get the perfect performance from the actors, or the scenery falls down, it's very difficult to fix the problem without completely repeating the entire production process. On the other hand, if the production audio is not captured with a high enough quality, the actor can be brought back in alone to re-record the audio without having to reshoot the video. The bottom line is that in the production environment, the picture and the performance are the most important things. Capturing the production audio is ultimately of less importance because it's easier to fix in post-production.

With this in mind, most production audio engineers are willing to make some compromises by putting microphones under the clothing. The other option is to mount a small directional microphone to a long pole called a boom pole. Someone outside the camera shot holds the pole, and he or she can get the microphone fairly close to actors or objects without getting the microphone in the shot. Because reshooting is so expensive, the most important job of the boom pole operator is to keep the microphone out of the shot. Picking up usable production audio is secondary.

Musical scores are another major element in film and video. Composers typically work first on developing a few musical themes to be used throughout the film while the editing process is still happening. Once a full edit is completed, the composer takes the themes and creates versions of various lengths to fit with the timing of the edited scenes. Sometimes this is done entirely with electronic instruments directly in the DAW with the video imported to the project file for a visual reference. Other times, a recording session is conducted where an orchestra is brought into a recording studio called a scoring stage. The film is projected in the studio and the composer along with a conductor performs the musical passages for the film using clicks and streamers to help synchronize the important moments.

A final challenge to be mentioned in the context of film and video is the synchronization of sound and picture. The audio is typically captured on a completely different recording medium than the video or film, and the two are put back together in post-production. In the analog domain this can be a very tricky process. The early attempt at facilitating synchronization was the clapboard slate. The slate has an area where you can write the name of the show, the scene number, and the take number. There is also a block of wood connected to the slate with a hinge. This block of wood can be raised and lowered quickly onto the slate to make a loud clap sound. The person holding the slate reads out loud the information written on the slate while holding the slate in front of the camera and then drops the clapper. In post-production the slate information can be seen on the film and heard on the audio recording. This way you know that you have the correct audio recording with the correct video. The clap sound can easily be heard on the audio recording, and on the video you can easily see the moment that the clapper closes. The editor can line up the sound of the clap with the image of the clapper closing, and then everything after that is in sync. This simple and low-tech solution has proven to be quite effective and is still used in modern filmmaking along with other improvements in synchronization technology.

A time code format called SMPTE has been developed to address the issue of synchronization. The format of SMPTE time code is described in Chapter 6. The idea behind time code synchronization is that the film has a built-in measuring system. There are 24 frames or still pictures every second in traditional motion picture film, with each frame being easily identified. The problem is that on an audiotape there is no inherent way to know which part of audio goes with which frame of video. Part of the SMPTE time code specification includes a method of encoding the time code into an audio signal that can be recorded on a separate track of audio on the tape recorder. This way, the entire audio recording is linked to each frame of the video. In the digital domain, this time code can be encoded into the video signal as well as the audio signal, and the computer can keep everything in sync. The slate clapper has even been updated to display the current time code value to facilitate synchronization in post-production.

8.2.2.5 Sound Effects

Sound effects are important components in film, video, and theatre productions. Sound effects are sometimes referred to as Foley sound, named after Jack Foley, who did the original work on sound effect techniques in the early days of silent films. Foley artists are a special breed of filmmaker who create all the sound effects for a film manually in a recording session. Foley stages are recording studios with all kinds of toys, floor surfaces, and other gadgets that make various sounds. The Foley artists go into the stage and watch the film while performing all the sounds required, the sounds ranging from footsteps and turning doorknobs to guns, rain, and other environmental sounds. The process is a lot of fun, and some people build entire careers as Foley artists.

The first step in the creation of sound effects is acquiring some source material to work with. Commercial sound effect libraries are available for purchase, and there are some online sources for free sound effects, but the free sources are often of inconsistent quality. Sometimes you may need to go out and record your own source material. The goal here is not necessarily to find the exact sound you are looking for. Instead, you can try to find source material that has some of the characteristics of the sound. Then you can edit, mix and process the source material to achieve a more exact result. There are a few common tools used to transform your source sound. One of the most useful is pitch shifting. Spaghetti noodles breaking can sound like a tree falling when pitched down an octave or two. When using pitch shift, you will get the most dramatic transformative results when using a pitch shifter that does not attempt to maintain the original timing. In other words, when a sound is pitched up, it should also speed up. Pitch shifting in sound effect creation is demonstrated in the video associated with this section.

Another strategy is to mix several sounds together in a way that creates an entirely new sound. If you can break down the sound you are looking for into descriptive layers, you can then find source material for each of those layers and mix them all together. For example, you would never be able to record the roar of a Tyrannosaurus Rex, but if you can describe the sound you’re looking for, perhaps as something between an elephant trumpeting and a lion roaring, you’re well on your way to finding that source material and creating that new, hybrid sound.

Sometimes reverb or EQ can help achieve the sound you are looking for. If you have a vocal recording that you want to sound like it is coming from an old answering machine, using an EQ to filter out the low and high frequencies but enhance the mid frequencies can mimic the sound of the small loudspeakers in those machines. Making something sound farther away can be accomplished by reducing the high frequencies with an EQ to mimic the effect of the high frequency loss over distance, and some reverb can mimic the effect of the sound reflecting from several surfaces during its long trip.

8.2.2.6 MIDI and Other Digital Tools for Sound and Music Composition

In this section, we introduce some of the possibilities for sound music creation that today’s software tools offer, even to someone not formally educated as a composer. We include this section not to suggest that MIDI-generated music is a perfectly good substitute for music composed by classically trained musicians. However, computer-generated music can sometimes be appropriate for a project for reasons having to do with time, budget, resources, or style. Sound designers for film or theatre not infrequently take advantage of MIDI synthesizers and samplers as they create their soundscapes. Sometimes, a segment of music can be “sketched in” by the sound designer with MIDI sequencers, and this “rough draft” can serve as scratch or temp music in a production or film. In this section, we’ll demonstrate the functionality of digital music tools and show you how to get the most out of them for music composition.

Traditionally, composing is a process of conceiving of melodies and harmonies and putting them down on paper, called a musical score. Many new MIDI and other digital tools have been created to help streamline and improve the scoring process – Finale, Sibelius, or the open source MuseScore, to name a few. Written musical scores can be played by live musicians either as part of a performance or during a recording session. But today’s scoring software gives you even more options, allowing you to play back the score directly from the computer, interpreting the score as MIDI messages and generating the music through samplers and synthesizers. Scoring software also allows you to create audio data from the MIDI score and save it to a permanent file. Most software also lets you export your musical score as individual MIDI tracks, which can be then imported, arranged, and mixed in your DAW.

The musical quality of the audio that is generated from scoring software is dependent not only on the quality samplers and synthesizers used, but also on the scoring performance data that you include – marking for dynamics, articulation, and so forth. Although the amount of musical control and intuitiveness within scoring software continues to improve, we can’t really expect the software to interpret allegro con moto, or even a crescendo or fermata, the way an actual musician would. If not handled carefully, computer-generated music can sound very “canned” and mechanical. Still, it’s possible to use your digital tools to create scratch or temp music for a production or film, providing a pretty good sense of the composition to collaborators, musicians, and audiences until the final score is ready to be produced. There are also a number of ways to tweak and improve computer-generated music. If you learn to use your tools well, you can achieve surprisingly good results.

As mentioned, the quality of the sound you generate depends in large part on your sample playback system. The system could be the firmware on a hardware device, a standalone software application, or a software plug-in that runs within your DAW. A basic sampler plays back a specific sound according to a received trigger – e.g., a key pressed on the MIDI keyboard or note data received from a music scoring application. Basic samplers may not even play a unique sound for each different key, but instead they mathematically shape one sample file to produce multiple notes (as described in Chapter 7), where multiple variations of a sample or note are played back depending, for example, on how hard/loud a note is played. These samplers may also utilize other performance parameters and data to manipulate the sound for a more dynamic, realistic feel. For instance, the mod wheel can be linked to an LFO that imparts a controllable vibrato characteristic to the sound sample. While these sampler features greatly improve the realism of sample playback, the most powerful samplers go far beyond this.

Another potent feature provided by some samplers is the round robin. Suppose you play two of the same notes on a real instrument. Although both notes may have the same instrument and pitch and essentially the same force, in “real life” the two notes never sound exactly the same. With round robin, more than one version of a note is available for each instrument and velocity. The sampler automatically cycles playback through this a set of similar samples so that no two consecutive “identical” notes sound exactly the same. In order to have round robin capability, the sampler has to have multiple takes of the audio samples for every note, and for multisampled sounds this means multiple takes for every velocity as well. The number of round robin sound samples included varies from two to five, depending on the system. This duplication of samples obviously increases the memory requirements for the sampler. Some sampler systems instead use subtle processing to vary the way a sample sounds each time it is played back, simulating the round robin effect without the need for additional samples, but this may not achieve quite the same realism.

Another feature of high-end sampler instruments and sampler systems is multiple articulations. Consider the sound of a guitar, which has a different timbre depending if it’s played with a pick, plucked with a finger, palm-muted, or hammered on. Classical stringed instruments have even more articulations than do guitars. Rather than having a separate sampler instrument for each articulation, all of these articulations can be layered into one instrument, yet with the ability for you to maintain individual control. Typically the sampler has a number of keyswitches that switch between the articulations. These keyswitches are keys on the keyboard that are perhaps above or below the instrument’s musical range. Pressing the key sets the articulation for notes to be played subsequently.

An example of keyswitches on a sampler can be seen in Figure 8.16. Some sampler systems even have intelligent playback that switch articulations automatically depending on how notes overlap or are played in relation to each other. The more articulations the system has, the better for getting realistic sound and range from an instrument,. However, knowing how and when to employ those articulations is often limited by your familiarity with the particular instrument, so musical experience plays an important role here.

Figure 8.16 Sampler with keyswitches shown in blue

Figure 8.16 Sampler with keyswitches shown in blue

Aside:  In addition to the round-robin and multiple articulation samples, some instruments also include release samples, such as the subtle sound of a string being released or muted, which are played back when a note is released to help give it a natural and realistic ending.  Even these release samples could have round-robin variations of their own!

As you can see, realistic computer-generated music depends not only on the quality, but also the quantity and diversity of the content to play back. Realistic virtual instruments demand powerful and intelligent sampler playback systems, not to mention the computer hardware specs to support them. One company’s upright bass instrument alone contains over 21,000 samples. For optimum performance, some of these larger sampler systems even allow networked playback, out-sourcing the CPU and memory load of samples to other dedicated computers.

With the power and scope of the virtual instruments emerging today, it’s possible to produce full orchestral scores from your DAW. As nice as these virtual instruments are, they will only get you so far without additional production skills that help you get the most out of your digital compositions.

With the complexity of and diversity of the instruments that you’ll attempt to perform on a simple MIDI keyboard, it may take several passes at individual parts and sections to capture a performance you’re happy with. With MIDI, of course, merging performances and editing out mistakes is a simple task. It doesn’t require dozens of layers of audio files, messy crossfades, and the inherent issues of recorded audio. You can do as many takes as necessary and break down the parts in whatever way that helps to improve the performance.

Aside:  In addition to the round-robin and multiple articulation samples, some instruments also include release samples, such as the subtle sound of a string being released or muted, which are played back when a note is released to help give it a natural and realistic ending.  Even these release samples could have round-robin variations of their own!

If your timing is inconsistent between performances, you can always quantize the notes. Quantization in MIDI refers to adjusting the performance data to the nearest selectable time value, be it whole note, half note, quarter note, etc. While quantizing can help tighten up your performance, it is also a main contributor to your composition sounding mechanical and unnatural. While you don’t want to be off by a whole beat, tiny imperfections in timing is natural with human performance and can help your music sound more convincing. Some DAW sequencers in addition to a unit of timing will let you choose a degree of quantization, in other words how forceful do you want to be when pushing your note data toward that fixed value. This will let you maintain some of the feel of your actual performance.

Most sequencers also have a collection of other MIDI processing functions – e.g., randomization. You can select a group of notes that you’ve quantized, and tell it to randomize the starting position and duration of the note (as well as other parameters such as note velocity) by some small amount, introducing some of these timing imperfections back into the performance. These processes are ironically sometimes known as humanization functions, which nevertheless can come in handy when polishing up a digital music composition.

When actual musicians play their instruments, particularly longer sustained notes, they typically don’t play them at a constant level and timbre. They could be doing this in response to a marking of dynamics in the score (such as a crescendo), or they may simply be expressing their own style and interpretation of the music. Differences in the way notes are played also result from simple human factors. One trumpet player pushes the air a little harder than another to get a note started or to end a long note when he’s running out of breath. The bowing motion on a stringed instrument varies slightly as the bow moves back and forth, and differs from one musician to another. Only a machine can produce a note in exactly the same way every time. These nuances aren’t captured in timing or note velocity information, but some samplers can respond to MIDI control data in order to achieve these more dynamic performances. As the name might imply, an important one of these is MIDI controller 11, expression. Manipulating the expression value is a great way to add a natural, continuous arc to longer sustained notes, and it’s a great way to simulate dynamic swells as well as variations in string bowing. In many cases, controller 11 simply affects the volume of the playback, although some samplers are programmed to respond to it in a more dynamic way. Many MIDI keyboards have an expression pedal input, allowing you to control the expression with a variable foot pedal. If your keyboard has knobs or faders, you can also set these to write expression data, or you can always draw it in to your sequencer software by hand with a mouse or track pad.

An example of expression data captured into a MIDI sequencer region can be seen in Figure 8.17. MIDI controller 1, the modulation wheel, is also often linked to a variable element of the sampler instrument. In many cases, it will default as a vibrato control for the sound, either by applying a simple LFO to the pitch, or even dynamically crossfading with a recorded vibrato sample. When long sustained notes are played or sung live, a note pitch may start out normally, but, as the note goes on, an increasing amount of vibrato may be applied, giving it a nice fullness. Taking another pass over your music performance with the modulation wheel in hand, you can bring in varying levels of vibrato at appropriate times, enhancing the character and natural feel of your composition.

Figure 8.17 Expression data captured inside a MIDI sequencer region

Figure 8.17 Expression data captured inside a MIDI sequencer region

As you can see, getting a great piece of music out of your digital composition isn’t always as simple as playing a few notes on your MIDI keyboard.  With all of the nuanced control available in today’s sampler systems, crafting even a single instrument part can require careful attention to detail and is often an exercise in patience.  Perhaps hiring a live musician is sounding much better at the moment?  Certainly, when you have access to the real deal, it is often a simpler and more effective way to create a piece of music.  Yet if you enjoy total control over their production and are deeply familiar and experienced with your digital music production toolset, virtual instruments can be a fast and powerful means of bringing your music to life.

8.2.1 Pre-production

8.1.1.1 Making a Schedule

A logical place to begin as you initiative a project is by making a schedule for the creation of the different audio artifacts that make up the entire system.  Your project could be comprised of many different types of sounds, including live recordings (often on different tracks at different moments), existing recordings, Foley sound effects, MIDI-synthesized music or sound, or algorithmically-generated music or sound.  Some sounds may need to be recorded in the presence of others that have already been created, and these interdependencies determine your scheduling.  For example, if you’re going to record singers against a MIDI-sequenced backing track, the backing track will have to be created ahead of time – at least enough of the backing track so that that the singers are able to sing on pitch and in time with the music.

You also need to consider where and how the artifacts will be made.  Recording sessions may need to be scheduled and musicians hired.  Also, you’ll need to get the proper equipment ready for on-location or outdoor recording sessions.

To some extent you can’t plan out the entire process ahead of time. The creative process is messy and somewhat unpredictable. But the last thing you want is to run into some technical roadblock when the creative juices start flowing. So plan ahead as best you can to make sure that you have all the necessary elements in the right place at the right time as you work through the project.

8.1.1.2 Setting up for Recording

In Chapter 1, we introduced a wide variety of audio hardware and software.  We’ll now give an overview of different recording situations you might encounter and the equipment appropriate for each.

The ideal recording situation would be a professional recording studio. These studios typically have a large stage area where the performance can be recorded. Ideally, the stage has very meticulously controlled acoustics along with separate isolation booths, high-quality microphones, a control room that is isolated acoustically from the stage, and a professional recording engineer to conduct the recording. If you have enough money, you can rent time in these studios and get very good results. If you have a lot of money, you can build one yourself! Figure 8.1 shows an example of a world-class professional recording studio.

Figure 8.1 Manifold recording studio near Chapel Hill, NC

Figure 8.1 Manifold recording studio near Chapel Hill, NC

While you can get some really amazing recordings in a professional recording studio, you can still get respectable results by setting up your own recording environment using off-the-shelf tools. The first thing to do is find a place to record. This needs to be a quiet room without a lot of noise from the air handling and a fairly neutral acoustic environment. In other words, you usually don’t want to hear the room in your recordings. Examples of isolation rooms and techniques can be found in Chapter 4.

Once you have a room in which to make your recording you need a way to get some cables run into the room for your microphones as well as any headphones or other monitors required for the performer. These cables ultimately run back to wherever you are setting up your computer and recording interface. One thing to keep in mind is that, depending on how well your recording room is isolated from your control room, you may need to have a talkback microphone at your computer routed to the performer’s headphones. Some mixing consoles include a talkback microphone. A simple way to set up your own is to just connect a microphone to your audio interface and set up an extra recording track in your recording software. If that track is record-enabled, the performer will hear it along with his or her own microphone . You can also get a dedicated monitor management device that includes a talkback microphone and the ability to control and route the recording signal to all the various headphones and monitors for both you and the performer. An example of a monitor management device is shown in Figure 8.2.

Figure 8.2 The Mackie Big Knob monitor management device

Figure 8.2 The Mackie Big Knob monitor management device

While it’s not technically a recording studio, you might find yourself in the situation where you need to record a live performance happening in a theatre, concert hall, or other live venue. The trick here is to get your recording without impacting the live performance. The simplest solution is to set up a couple area microphones on the stage somewhere and run them back to your recording equipment. If there are already microphones in place for live sound reinforcement, you can sometimes use those to get your recording. One way to do that is to work with the live sound engineer to get you a mix of all the microphones using an auxiliary output of the live mixing console. Assuming extra aux outputs are available, this is typically a simple thing to implement. Just make sure you speak with the engineer ahead of time about it. You won’t make any friends if you show up 15 minutes before the show starts and ask the engineer for a feed from the mixing console. The disadvantage to this is that you’re pretty much stuck with the mix that the live engineer gives you. Everything is already mixed. If you ask nicely you might be able to get two aux feeds with vocals on one and musical instruments on the other. This gives you a little more flexibility, but you are still limited to the mix that is happening on each aux feed. A better solution that requires more planning is to split the microphone signals to go to your recording equipment as well as the live mixing console. It is important to use a transformer-isolated splitter so it doesn’t negatively impact the live sound. These isolated splitters will essentially make your feed invisible to the live mixing console. You can get splitters with multiple channels or just single channel splitters like the one shown in Figure 8.3. With this strategy you can get a separate track for each microphone on the stage allowing you to create your own mix later when the live performance is over.

Figure 8.3 A transformer isolated microphone splitter

Figure 8.3 A transformer isolated microphone splitter

Sometimes you need to record something that is not practical to bring into a controlled environment like a recording studio. For example, if you want to record the sound of an airplane taking off, you’re going to have to go stand next to a runway at the airport with a microphone. This is called recording on-location. In these situations, the goal is to find a balance between fidelity, isolation, cost, and convenience. It's very difficult to get all four of these things in every scenario, so compromises have to be made. The solution that offers the best isolation is not likely to be convenient and may be fairly expensive. For example, if you want to record thunder without picking up the sounds of rain, dog barks, cars, birds, and other environmental sounds, you need to find an outdoor location that is far from roads, houses, and trees but that also offers shelter from the rain for your equipment. Once you find that place, you need to predict when a thunderstorm will happen and get there in time to set up your equipment before the storm begins. Since this is not a very practical plan, you may have to make some compromises and be prepared to spend a long time recording storms in order to get a few moments where you manage to get the sound of thunder when nothing else is happening.  Every recording situation is different, but if you understand the options that are available, you can be better prepared to make a good decision.

There are some unique equipment considerations you need to keep in mind when recording on-location. You may not have access to electricity so all your equipment may need to be battery-powered. We described some handheld, battery-powered recording devices in Chapter 1. Another common problem when recording on-location is wind. Wind blowing across a microphone will destroy your recording. Even a light breeze can be problematic. Most microphones come with a simple windscreen that can go over the microphone but in particularly windy conditions, a more robust windscreen may be necessary such as the one shown in Figure 8.4.

Figure 8.4 A Rycote Windjammer microphone wind screen

Figure 8.4 A Rycote Windjammer microphone wind screen

8.2 Workflow in Sound and Music Production

There are three main stages in the workflow for sound and music creation:  pre-production, when you design your project and choose your basic equipment; production, when you record and edit the sound; and post-production, when you mix and master the sound (for CDs and DVDs) and deliver it, either live or on a permanent storage medium.  In this chapter, we examine these three steps as they apply to sound, regardless of its ultimate purpose – including music destined for CD, DVD, or the web; sound scores for film or video; or music and sound effects for live theatre performances.  The three main stages can be further divided into a number of steps, with some variations depending on the purpose and delivery method of the sound.

  • Pre-production

o   Designing and composing sound and music

o   Analyzing recording needs (choosing microphones, hardware and software, recording environment, etc.), making a schedule, and preparing for recording

  • Production

o   Recording, finding, and/or synthesizing sound and music

o   Creating sound effects

o   Synchronizing

  • Post-production

o   Audio processing individual tracks and mixing tracks (applying EQ, dynamics processing, special effects, stereo separation, etc.)

o   Overdubbing

o   Mastering (for CD and DVD music production)

o   Finishing the synchronization of sound and visual elements (for production of sound scores for film or video)

o   Channeling output

Clearly, all of this work is based on the first important, creative step – sound design and/or music composition.  Design and composition are very big topics in and of themselves and are beyond the scope of this book.  In what follows, we assume that for the most part the sound that is to be created has been designed or the music to be recorded has been composed, and you’re ready to make the designs and compositions come alive.

8.1 Sound for All Occasions

This book is intended to be useful to sound designers and technicians in theatre, film, and music production.  It is also aimed at computer scientists who would like to collaborate with such artists and practitioners or design the next generation of sound hardware and software for them.

We've provided a lot of information in previous chapters moving through the concepts, applications, and underlying science of audio processing.  In the end, we realize that most of our readers want to do something creative, whether designing and setting up the sound backdrop for a theatre performance, creating a soundtrack for film, producing music, or writing programs for innovative sound processing.  It's doubtful and not even necessary that you mastered all the material in the previous chapters, but it serves as a foundation and a reference for your practical and artistic work.  In this chapter, we pull things together by going into more detail on the artists' and practitioners' hands-on work.

7.4 References

Flanagan, J. L., and R. M. Golden.  1966. "Phase Vocoder." Bell System Technical Journal. 45: 1493-1509.

Boulanger, Richard, and Victor Lazzarini, eds.  The Audio Programming Book. MIT Press, 2011.

Ifeachor, Emmanual C., and Barrie W. Jervis. Digital Signal Processsing: A Practical Approach. Addison-Wesley Publishing, 1993.

By |December 15th, 2014|Chapter 7|0 Comments

7.3.11 Real-Time vs. Off-Line Processing

To this point, we’ve primarily considered off-line processing of audio data in the programs that we’ve asked you to write in the exercises.  This makes the concepts easier to grasp, but hides the very important issue of real-time processing, where operations have to keep pace with the rate at which sound is played.

Chapter 2 introduces the idea of audio streams.  In Chapter 2, we give a simple program that evaluates a sine function at the frequency of desire notes and writes the output directly to the audio device so that notes are played when the program runs.  Chapter 5 gives a program that reads a raw audio file and writes that to the audio device to play it as the program runs.  The program from Chapter 5 with a few modifications is given here for review.


/*Use option -lasound on compile line.  Send in number of samples and raw sound file name.*/

#include </usr/include/alsa/asoundlib.h>
#include <math.h>
#include <iostream>
using namespace std;

static char *device = "default";	/*default playback device */
snd_output_t *output = NULL;
#define PI 3.14159

int main(int argc, char *argv[])
{
        int err, numRead;
        snd_pcm_t *handle;
        snd_pcm_sframes_t frames;
        int numSamples = atoi(argv[1]);

        char* buffer = (char*) malloc((size_t) numSamples);
        FILE *inFile = fopen(argv[2], "rb");
        numRead = fread(buffer, 1, numSamples, inFile);
        fclose(inFile);   

        if ((err = snd_pcm_open(&handle, device, SND_PCM_STREAM_PLAYBACK, 0)) < 0){
            printf("Playback open error: %s\n", snd_strerror(err));
            exit(EXIT_FAILURE);
        }
        if ((err = snd_pcm_set_params(handle,
				SND_PCM_FORMAT_U8,
				SND_PCM_ACCESS_RW_INTERLEAVED,
				1,
				44100, 1, 400000) ) < 0 ){
            printf("Playback open error: %s\n", snd_strerror(err));
            exit(EXIT_FAILURE);
        }

        frames = snd_pcm_writei(handle, buffer, numSamples);
        if (frames < 0)
          frames = snd_pcm_recover(handle, frames, 0);
        if (frames < 0) {
          printf("snd_pcm_writei failed: %s\n", snd_strerror(err));
        }  
}

Program 7.1 Reading and writing raw audio data

This program uses the library function send_pcm_writei to send samples to the audio device to be played. The audio samples are read from in input file into a buffer and transmitted to the audio device without modification The variable buffer indicates where the samples are stored, and sizeof(buffer)/8 gives the number of samples given that this is 8-bit audio.

Consider what happens when you have a much larger stream of audio coming in and you want to process it in real time before writing it to the audio device. This entails continuously filling up and emptying the buffer at a rate that keeps up with the sampling rate.

Let’s do some analysis to determine how much time is available for processing based on a given buffer size. For a buffer size of N and a sampling rate of r, then N/r seconds can be passed before additional audio data will be required for playing. For and N=4096 and r=44100, this would be \frac{4096}{44100}=0.0929\: ms. (This scheme implies that there will be latency between the input and output, at most N/r seconds.)

What is you wanted to filter the input audio before sending it to the output? We’ve seen that filtering is more efficient in the frequency domain using the FFT. Assuming the input is in the time domain, our program has to do the following:

  • convert data to the frequency domain with inverse FFT
  • multiply the filter and the audio data
  • convert data back to the time domain with inverse FFT
  • write the data to the audio device

The computational complexity of the FFT and IFFT is 0\left ( N\log N \right ), on the order of 4096\ast 12=49152 operations (times 2). Multiplying the filter and the audio data is 0\left ( N \right ), and writing the data to the audio devices is also 0\left ( N \right ), adding on the order of the order of 2*4096 operations. This yields on the order of 106496 operations to be done in 0.0929\; ms, or about 0.9\; \mu s per operation. Considering that today’s computers can do more than 100,000 MIPS (millions of instructions per second), this is not unreasonable.

We refer the reader to Boulanger and Lazzarini's Audio Programming Book for more examples of real-time audio processing.

7.3.10 Experiments with Filtering: Vocoders and Pitch Glides

 

Vocoders were introduced in Section 7.1.8. The implementation of a vocoder is sketched in Algorithm 7.6 and diagrammed in Figure 7.49. The MATLAB and C++ exercises associated with this section encourage you to try your hand at the implementation.

algorithm vocoder

/*

Input:

c, an array of audio samples constituting the carrier signal

m, n array of audio samples constituting the modulator signal

Output:

v, the carrier wave modulated with the modulator wave */

{

Initialize v with 0s

Divide the carrier into octave-separated frequency bands with bandpass filters

Divide the modulator into the same octave-separated frequency bands with bandpass filters for each band

use the modulator as an amplitude envelope for the carrier

}

Algorithm 7.6 Sketch of an implementation of a vocoder

Figure 7.49 Overview of vocoder implementation

Figure 7.49 Overview of vocoder implementation

Another interesting programming exercise is implementation of a pitch glide. A Risset pitch glide is an audio illusion that sounds like a constantly rising pitch. It is the aural equivalent of the visual image of a stripe on a barber pole that seems to be rising constantly. Implementing the pitch glide is suggested as an exercise for this section.