Audio Compressors
Audio compressors are a common component of a sound system which can improve the
quality but if set incorrectly can do more damage than good. This document
details a suggested procedure for biasing an Audio Compressor such that it will
improve your sound quality, not harm it.
What is an Audio Compressor
An Audio Compressor, or Compressor for short is an electrical subsystem that applies
Dynamic Range Compression to an audio signal. This means that when the audio level
goes above a configurable threshold, the amplitude is reduced. More simply put this means
that loud signals are made quieter, such that the difference in volume between a quiet
sound and a loud sound is reduced.
Why are they used
Audio compressors are used in a church context for a few reasons:
- Improve quality of recordings; to make sure loud sounds don't distort, and quiet sounds don't
drop below the sensitivity threshold of the recording medium
- Improve quality of music by permitting quieter sounds to be louder and be heard above background
noises or other more dominant instruments without distorting louder sounds
- Improve quality of speech; by removing the sibilance ('ess' sounds)
- Remove uncharacteristically loud noises from instruments; perhaps in the process of playing a note
like on a drum or a guitar, there would be a really short loud blast before the proper note stabilized.
- Improve quality of vocalists; reducing any breathy "p" sounds or reducing the variations in volume
when the singer varies the distance or direction of their voice with respect to the microphone.
Why can they be a problem
Being an audio component; there are limits to how much audio can be sent to a compressor before
it will electrically truncate the signal sounding like distortion, typically only on loud signals.
If more volume is required from a source, an easy mistake is to increase the input gain on a channel
too far such that too much signal is sent to the compressor; and the signal becomes distorted. More
appropriate is to increase the volume after the compressor either using the channel fader, the master
fader or perhaps even the amplifiers themselves.
Setup procedure
Compressors may have the following controls, which I recommend setting in this order.
- Output gain: set to zero. This will mean if it is suspected that the compressor is causing a problem, it can
be bypassed without changing the volume of most of the signal.
- Limiting threshold: Set to max such that no limiting is done
- Compression threshold: Set to max such that no compression is done
- Compression ratio: Set to 3
- Input signal: adjust while listening such that the lights on the unit indicate that plenty of signal is going
through the unit and only exceptionally loud sections if any peak the unit
- Compression threshold: Reduce while listening until the lights on the unit indicate that the desired portion of the signal is being compressed
Other documentation
