Study Hall

Church Sound Files: The Basics Of Using Compressors In Your System

What compression can do to benefit a system, and the basic principles of operation

What does an audio compressor actually do?

Most types of 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 noticeable.

A compressor/limiter is essentially an automatic volume control. Imagine a sound operator/engineer with a hand on a mixing console fader and eyes on an input level meter. As long as the meter stays below a certain point (the threshold), the engineer leaves the fader all the way up and the gain is unchanged.

But the instant the sound gets louder, the engineer pulls down the fader by a certain amount. After the sound gets soft again, the engineer will push the fader back up.

This is what the compressor is doing, except much faster and more accurately than humanly possible.

Paradoxically, by cutting the peak levels, a compressor allows us to raise the average level of a sound using the output control to make it sound louder.

By using the threshold and ratio controls, we can set a stable sound that will hold its position in the mix whether the singer is whispering or screaming.

What The Controls Do
Let’s go back to the “engineer with hand on a fader and eyes on the meter” analogy, using it to explain how a compressor works. Think of it as an “automatic engineer in the box.”

The front panel controls simply tell the engineer (compressor) what rules to follow.

THRESHOLD tells him how high the input meter can rise before he has to start pulling down the fader: if it’s turned full clockwise, he won’t pull down his fader until the highest red LED comes on; if it’s turned counter-clockwise, he’ll have his hand on the fader even before the lowest green LED lights.

RATIO tells him how far he should “pull the fader down”when the signal is above the threshold level: should he 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)?

The OverEasy switch found on dbx compressors such as the model 160SL shown here, affects how he reacts as signal approaches and travels through the threshold: does he reduce it exactly by the ratio only after it crosses the threshold, or does he gradually ease into the full ratio as it passes through?

The LEDs of the gain reduction meter tell you how much the “engineer” is pulling down the “fader” at any time. If these LEDs aren’t on, his hands are in his pockets.

The ATTACK and RELEASE controls involve the speed of the engineer’s response.

Short attack times order the engineer to get his hands on the fader 1/10,000th of a second after he sees a too-loud signal; long attack times tell him to let transients less than 1/5th of a second pass.

Release tells the engineer how quickly he should push the fader back up again after a loud signal has stopped; when it’s turned counter-clockwise, he pushes the fader back up instantly, and when it’s full clockwise, he’ll take longer to push his fader back up to unity gain.

The OUTPUT control is simply a gain control located after our “automatic engineer in the box.” The INPUT/OUTPUT switch allows us to see the levels before the engineer does his job, or after.

The most important controls are the threshold and ratio knobs. They both interact to get the desired effect, 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.

Study Hall Top Stories