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An equalizer can be incredibly useful when used appropriately, and incredibly dangerous when used inappropriately. Knowing when to use an EQ is just as important and knowing how to use it to accomplish the effect you are looking for. Every time you think you want to use an EQ you should evaluate the situation against this rule of thumb: EQ should be used to create an effect, not to solve a problem. Using an EQ as a problem solver can cause new problems when you should really just figure out what’s causing the original problem and fix that instead. Only if the problem can’t be solved in any other way should you pull up the EQ – for example, if you’re working post-production on a recording captured earlier during a film shoot, or you’ve run into an acoustical issue in a space that can’t be treated or physically modified. Rather than solving problems, you should try to use an EQ as a tool to achieve a certain kind of sound. Do you like your music to be heavy on the bass? An EQ can help you achieve this. Do you really like to hear the shimmer of the cymbals in a drum set? An EQ can help.
Let’s examine some common problems you may encounter where you might be tempted to use an EQ inappropriately. As you listen to the recording you’re making of a singer you notice that the recorded audio has a lot more low frequency than high frequency content, leading to a decreased intelligibility. You go over and stand next to the performer to hear what he actually sounds like and notice that he sounds quite different than what you’re hearing from the microphone. Standing next to him, you can hear all those high frequencies quite well. In this situation you may be tempted to pull out your EQ and insert a high shelf filter to boost all those high frequencies. This should be your last resort. Instead, you might notice that the singer is singing into the side of the microphone instead of the front. Because microphones are more directional at high frequencies than low frequencies, singing into the side of the microphone would mean that the microphone picks up the low frequency content very easily but the high frequencies are not being captured very well. In this case you would be using an EQ to boost something that isn’t being picked up very well in the first place. You will get much better results by simply rotating the microphone so it is pointed directly at the singer so the singer is singing into the part of the microphone that is more sensitive to high frequencies.
Another situation you may encounter would be when mixing the sound from multiple microphones either for a live performance or a recording. You notice as you start mixing everything together that a certain instrument has a huge dip around 250 Hz. You might be tempted to use an EQ to increase 250 Hz. The important thing to keep in mind here is that most microphones are able to pick up 250 Hz quite well from every direction, and it is unlikely that the instrument itself is somehow not generating the frequencies in the 250 Hz range while still generating all the other frequencies reasonably well. So before you turn on that EQ, you should mute all the other channels on the mixer and listen to the instrument alone. If the problem goes away, you know that whatever is causing the problem has nothing to do with EQ. In this situation, comb filtering is the likely culprit. There’s another microphone in your mix that was nearby and happened to be picking up this same instrument at a slightly longer distance of about two feet. When you mix these two microphones together, 250 Hz is one of the frequencies that cancels out. If comb filtering is the issue, you should try to better isolate the signals either by moving the microphones farther apart or preventing them from being summed together in the mix. A gate might come in handy here, too. If you gate both signals you can minimize the times when both microphones are mixed together, since the signals won’t be let through when the instruments they are being used for aren’t actually playing.
If comb filtering isn’t the issue, try moving a foot or two closer to or farther away from the loudspeakers. If the 250 Hz dip goes away in this case, there’s likely a standing wave resonance in your studio at the mix position that is cancelling out this frequency. Using an EQ in this case will not solve the problem since you’re trying to boost something that is actively being cancelled out. A better solution for the standing wave would be to consider rearranging your room or applying acoustical treatment to the offending surfaces that are causing this reflective build up.
Suppose you are operating a sound reinforcement system for a live performance and you start getting feedback through the sound system. When you hear that single frequency start its endless loop through the system you might be tempted to use an EQ to pull that frequency out of the mix. This will certainly stop the feedback, but all you really get is the ability to turn the system up another decibel or so before another frequency will inevitably start to feed back. Repeat the process a few times and in no time at all you will have completely obliterated the frequency response of your sound system. You won’t have feedback, but the entire system will sound horrible. A better strategy for solving this problem would be to get the microphone closer to the performer, and move the performer and the microphone farther away from the loudspeakers. You’ll get more gain this way and you can maintain the frequency response of your system. (Chapter 4 has moree on potential acoustic gain. Chapter 8 has an exercise on gain setting.)
We could examine many more examples of an inappropriate use of an EQ but they all go back to the rule of thumb regarding the use of an EQ as a problem solver. In most cases, an EQ is a very ineffective problem solver. It is, however, a very effective tool for shaping the tonal quality of a sound. This is an artistic effect that has little to do with problems of a given sound recording or reinforcement system. Instead you are using the EQ to satisfy a certain tonal preference for the listener. These effects could be as subtle as reducing an octave band of frequencies around 500 Hz by -3 dB to achieve more intelligibility for the human voice by allowing the higher frequencies to be more prominent. The effect could be as dramatic as using a bandpass filter to mimic the effect of a small cheap loudspeaker in a speakerphone. When using an EQ as an effect, keep in mind another rule of thumb. When using an EQ, you should reduce the frequencies that are too loud instead of increasing the frequencies that are too quiet. Every sound system, whether in a recording studio or a live performance, has an amplitude ceiling – the point at which the system clips and distorts. If you’ve done your job right, you will be running the sound system at an optimal gain, and a 3 dB boost of a given frequency on an EQ could be enough to cause a clipped signal. Reducing frequencies is always safer than boosting them since reducing them will not blow the gain structure in your signal path.