Most signal processors such as reverbs, equalizers and delays are designed to make an obvious change in the sound, but a compressor's action is much more subtle. When used properly most listeners won't be aware that signal processing is being used. Only if you hear the original dynamic range of a signal and compare it to the compressed version will the effect be noticable.

Yet, compressors are essential in modern audio work. Often the entire stereo mix may be compressed or limited during the mastering process. Finally, when you hear the piece of music on the radio, it passes through yet another compressor before it's transmitted.

A compressor/limiter is essentially an automatic volume control.

Imagine an engineer with a hand on a fader and eyes on an input level meter. As long as the meter stays below a certain point (the threshold), the fader will remain all the way up and the gain is unchanged. But the instant the sound gets louder, the engineer pulls the fader down by a certain amount. After the sound gets quiet again the engineer will push the fader back up. That's what the compressor is doing, except much faster and more accurately than humans can do it.

Paradoxically, by cutting the peak levels, a compressor allows you to raise the average level of a sound using the output control and make it sound louder. By using the threshold and ratio controls, you can set a stable sound that will hold its position in the mix whether the track is very loud or very quiet.

Let's imagine there's a little robot inside your compressor, controlling it with its tiny metal hand on a fader and its single glowing eye looking at the input level meter. Also imagine that the front panel controls, or settings in the LCD readout, simply tell the robot what rules it should follow.

Threshold tells it how high the input meter can rise before it has to start pulling down the fader:
If the Threshold control is turned fully clockwise (or the readout is showing it at 100%) it won't pull down its fader until the signal is very strong (up around +6dB); if it's turned counter-clockwise (or close to 0% on the LCD display), it'll have pulled the fader down even before the lowest green -30dB LED lights.

Ratio tells it how far it should pull the fader down when the signal is above the threshold level:
Should it pull it down just a little bit (compression) or pull the fader as far down as necessary to make sure the output level is never higher than the threshold (limiting)?

Hard/Soft affects how it reacts as the signal approaches the threshold:
Does it reduce it exactly by the ratio only after it crosses the threshold, or does it gradually ease into the full ratio as it gets close? The red LEDs of the reduction meter will tell you how much the robot is pulling down the fader at any time (some compressors with LCD readouts may not display this value).

Attack involves the speed of the robots's response, as does the switch. Short attack times order the robot to get its hand on the fader 1/10,000th of a second after it sees a too-loud signal; long attack times tell it to let transients less than 1/5th of a second pass.

Release tells the robot how quickly it should push the fader back up again after a loud signal has stopped; when it's turned counter-clockwise (towards 0%), it pushes the fader back up instantly, and when it's full clockwise (up at 100%), it'll take three seconds to push the fader back up to unity gain.

If the compressor is in Peak mode, the robot responds to the highest voltage peaks, and in RMS mode it will respond to the longer-term average signal level (and the Attack and Release controls have no effect). It's as if the robot is looking at a fast-acting LED meter in peak mode, and a slow old-style mechanical VU meter in RMS mode.

The Output control is simply a gain control located after our robot has done its thing.

Just to recap, the most important controls are the Threshold and Ratio settings. They both interact to get the effect you want, and that requires some experimenting. For example, if your average input signal is 0 dB, a ratio of 2:1 with a threshold of -12 dB will give you 6 dB of gain reduction, as will a ratio of infinity with a threshold of -6 dB. But the latter setting will sound more "squeezed" than the former.

Avoid common compressor mistakes:

Extreme settings will lead to extreme results. If you set an infinite ratio and turn the threshold down to -40 dB, the compressor will do what it's being told to do: turn the level way down. If you then try to compensate by cranking the Output control to its maximum, you'll amplify the noise of your mixer, EQ, mic preamp, and the compressor itself. The noise will fade itself in whenever the input signal stops, resulting in the classic pumping and breathing problems. Noise is present in every system, and improper use of any compressor will amplify it to an obnoxious level.

For low noise operation, make sure your mixer, compressor, and amplifier settings are set properly. As a general rule, you want as much gain as possible in the front of the system (at the sound source), so that a good line-level signal is travelling through the whole signal path. If you have a weak signal to start with, and then amplify it at the end of the signal path (by turning the main outputs of the mixer all the way up, for example) it will be excessively noisy.

When using a compressor on a live P.A. system, improper settings can cause feedback. Make sure that a channel is well below the feedback point when there is no gain reduction active. If you hear feedback every time the music stops, you must lower the overall level of the system.

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