What is a compressor, why do I need one and how does it work?
-Tony P.
Santa Ana, CA

Good questions.

First, what it is: A compressor is also known as a dynamic processor. It is designed to automatically control the volume or dynamics of the sound traveling through it. Why do you need one? Compressors can improve the intelligibility of vocals and control where instruments are placed in the overall mix. Many compressors incorporate a limiter. The limiter section of the compressor catches the peaks of the audio signal. It keeps the loudest of sounds going through your system from overdriving the amps and ultimately distorting the FOH speakers.

How does it work? That’s a bit more complicated.

Threshold, Ratio, Attack, Release
These are the four basic parameters of a compressor and what you need to understand to run one correctly. “Threshold” is defined as the point where compression begins. Turning your threshold knob to the right will raise the dB level as to where the compressor kicks in. Turning this knob to the left has the opposite effect. In a high threshold setting, you will only be compressing the upper decibel levels of your audio signal. As you lower the threshold, more of the audio signal will be compressed. Setting your threshold knob to the center position is a good place to begin. Then use your ear as you raise or lower the threshold.

“Ratio” is the difference in decibel reduction from the input signal to the output signal. Your compressor’s ratio knob typically ranges from 1 to 10. For example; setting the ratio at 2 creates a 2 to 1 ratio (2:1). This means that if 2dB of signal enters the compressor, 1dB exits the compressor. A 4:1 ratio lets 4dB in and 1dB out. With these two parameters we can now adjust when we capture the signal and how much we compress it.

“Attack” controls how fast the signal is captured. This control operates in milliseconds (ms). 10-25ms would be a very fast attack. This is good for removing the sibilance or sizzle from a vocal to make it more intelligible. Slower attack times have a multitude of uses. Once again, this is a good time to use your ears.

Once you have captured the audio signal, you have to decide when to release it. The “Release” knob does just that. Also calibrated in milliseconds, a faster release time will cause the compressor to follow the signal closely so that rapid input changes (ratio) will not be lost during compression. Slower release times smooth out the overall compression effect.

The dynamic process of compression will reduce signal level. As a result of this fact, many manufacturers add a gain control to compressors in order to add back level.

Sample Settings
If I am compressing an exuberant speaker, I will set my attack time to fast, my release at medium and ratio from 4:1 to 5:1. For singers, I set my attack time to medium, release to medium and ratio from 2:1 to 4:1. Should I have an acoustic guitar, I will set my attack to medium, release to medium and ratio from 3:1 to 4:1. If I’ve got a bass guitar, I will attack the signal fast and release it fast at a ratio of 4:1 to 6:1. Kick and snare drums will be attacked and released fast at a 4:1 ratio. And should I have a lively brass section, attack and release times are set to fast at a ratio of 5:1 to 7:1. This should give you some real settings that you can use as a starting point.

Lastly be sure to connect your compressor/limiter through your channel inserts if you are working on one particular audio signal. Or, take your main mixer left and right outputs and connect them directly into the inputs of your compressor to compress the entire mix.

Originally posted 2009-08-28 03:25:34.