When deciding whether to use dynamics processing you should keep in mind that a dynamics processor is simply an automatic volume knob. Any time you find yourself constantly adjusting the level of a sound, you may want to consider using some sort of dynamics processor to handle that for you. Most dynamics processors are in the form of downwards compressors. These compressors work by reducing the level of sounds that are too loud but letting quieter sounds pass without any change in level.
[aside]There is some disagreement among audio professionals about the use of compressors. There are some who consider using a compressor as a form of cheating. Their argument is that no compressor can match the level of artistry that can be accomplished by a skilled mixer with their fingers on the faders. In fact, if you ask some audio mix engineers which compressors they use, they will respond by saying that they have ten compressors and will show them to you by holding up both hands and wiggling their fingers![/aside]
One example when compression can be helpful is when mixing multiple sounds together from a multitrack recording. The human voice singing with other instruments is usually a much more dynamic sound than the other instruments. Guitars and basses, for example are not known as particularly dynamic instruments. A singer is constantly changing volume throughout a song. This is one of the tools a singer uses to produce an interesting performance. When mixing a singer along with the instruments from a band, the band essentially creates a fairly stable noise floor. The word noise is not used here in a negative context; rather, it is used to describe a sound that is different from the vocal that has the potential of masking the vocal if there is not enough difference in level between the two. As a rule of thumb, for adequate intelligibility of the human voice, the peaks of the voice signal need to be approximately 25 dB louder than the noise floor, which in this case is the band. It is quite possible for a singer to perform with a 30 dB dynamic range. In other words, the quietest parts of the vocal performance are 30 dB quieter than the loudest parts of the vocal performance.
If the level of the band is more or less static and the voice is moving all around, how are you going to maintain that 25 dB ratio between the peaks of the voice and the level of the band? In this situation you will never find a single level for the vocal fader that will allow it to be heard and understood consistently throughout the song. You could painstakingly draw in a volume automation curve in your DAW software, or you could use a compressor to do it for you. If you can set the threshold somewhere in the middle of the dynamic range of the vocal signal and use a 2:1 or 4:1 compression ratio, can easily turn that 30 dB of dynamic range into a 20 dB range or less. Since the compressor is turning down all the loud parts, the compressed signal will sound much quieter than the uncompressed signal, but if you turn the signal up using either the output gain of the compressor or the channel fader you can bring it back to a better level. With the compressed signal, you can now much more easily find a level for the voice that allows it to sit well in the mix. Depending on how aggressive you are about the compression, you may still need to automate a few volume changes, but the compressor has helped turn a very difficult to solve problem into something more manageable.
Rather than using a compressor to allow a sound to more easily take focus over a background sound, you can also use compression as a tool for getting a sound to sit in the mix in a way that allows other sounds to take focus. This technique is used often in theatre and film for background music and sound effects. The common scenario is when a sound designer or composer tries to put in some underscore music or background sounds into a scene for a play or a film and the director inevitably says, “turn it down, it’s too loud.” You turn it down by 6 dB or so and the director still thinks it’s too loud. By the time you turn it down enough to satisfy the director, you can hardly hear the sound and before long, you’ll be told to simply cut it because it isn’t contributing to the scene in any meaningful way.
The secret to solving this problem is often compression. When the director says the sound is too loud, what he really means is that the sound is too interesting. More interesting than the actor, in fact, and consequently the audience is more likely to pay attention to the music or the background sound than they are to the actor. One common culprit when a sound is distracting is that it is too dynamic. If the music is constantly jumping up and down in level, it will draw your focus. Using a compressor to make the underscore music or background sounds less dynamic allows them to sit in the mix and enhance the scene without distracting from the performance of the actor.
Compression can be a useful tool, but like any good thing, if it’s overused compression can be detrimental to the quality of your sound. Dynamics are one quality of sound and music that makes it exciting, interesting, and evocative. A song with dynamics that have been completely squashed will not be very interesting to listen to and can cause great fatigue on the ears. Also, if you apply compression inappropriately, it may cause audible artifacts in the sound, where you can hear when the sound is being attenuated and released. This is referred to as “pumping” or “breathing,” and it usually means you’ve taken the compression too far or in the wrong direction. So you have to be very strategic about the use of compression and go easy on the compression ratio. Often, a mild compression ratio is enough to tame an overly dynamic sound without completely stripping it of all its character.