A compressor is an automatic volume control that rides gain down when a signal exceeds a threshold
The fundamental purpose of a compressor is automatic gain riding: it monitors a signal’s level and turns the volume down whenever it gets too loud, then rides it back up when the level drops. This is more accurate and consistent than a human riding a fader. The gain reduction is visible as a meter dipping in the compressor plugin — louder moments cause more reduction, quieter moments cause less. This automatic riding makes a signal’s dynamic range narrower, which makes it easier to place consistently in a mix. The compressor itself does not add loudness; it controls variation in loudness.
Examples
A vocal with uneven levels has loud consonants and quiet passages. A compressor monitors the signal, catches the loud consonants, and turns them down — the gain reduction meter dips. Quiet passages pass through untouched.
Assessment
Explain what a compressor does to a signal’s dynamic range. What does the gain reduction meter display, and which moments cause it to dip most?