Compression
A compressor is a level-dependent processor. Dynamic range is the difference between quieter and louder parts of a signal. A gain-reduction display shows reduction being applied, when available.
A simplified hard-knee compression model
Static input/output level relationship for one simplified hard-knee example. Below threshold the line follows the one-to-one reference; above it the output rises more slowly.
Text equivalent
Input level increases along the horizontal axis and output level increases vertically. A dashed one-to-one reference shows unchanged level. The illustrative solid line follows it below the threshold, then has a shallower slope above threshold: larger input changes produce smaller output changes in this simplified model.
Above the threshold, the ratio describes how much less the output changes than the input in this simplified model.
Attack and release describe response timing: how quickly compression builds and how it recovers as the signal falls. Output or makeup gain adjusts the processed level afterward. Detector, knee, sidechain and program-dependent behavior vary among compressors.
LISTEN
Use a short repeating part with some level variation. Listen before adding a compressor. Then add one with a low threshold or stronger ratio so the behavior is obvious, at a comfortable monitoring level. Notice what becomes less variable and what happens to attacks or tails.
TRY
Change one control at a time: threshold, ratio, attack, release, then output. Watch gain reduction if the compressor provides a display, but listen too. Restore a less extreme state before moving on. Do not copy a preset or numeric setting as a universal answer.
APPLY
If a part has a specific level-control or envelope purpose, test compression and compare it with bypass. Adjust output so the processed signal does not win only by being louder, where practical. Record the purpose and result. The right decision may be no compressor.
Technical reference: Ableton Live Manual: Compressor describes threshold, ratio, gain reduction, attack, release and output gain for one compressor implementation. Its controls and response are an example, not a universal preset.
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