A compressor's ratio sets how much of the signal above the threshold is turned down

A compressor is a level-dependent amplifier whose gain decreases once the input crosses a threshold; the ratio decides how hard the excess above that threshold is clamped. An N:1 ratio means that for every N dB of input change above threshold, only 1 dB of output change results. At 1:1 input and output are identical — a straight diagonal on the transfer graph, no compression. At 4:1, a signal 8 dB over threshold comes out only 2 dB over (three-quarters of the excess removed). At infinite:1 the output stops dead at the threshold no matter how loud the input gets — brick-wall limiting. Ratios of roughly 2:1–6:1 are typical for music; above ~8:1–10:1 the effect is usually called limiting and begins to audibly squash transients, flattening dynamic character. Ratio is independent of threshold: threshold decides where compression starts, ratio decides how steeply it clamps beyond that point, and both interact with attack and release. Over-compression side effects include ‘pumping’ (level fluctuation tied to the music) and ‘breathing’ (audible noise-floor modulation).

Examples

Threshold fixed: at 1:1 the transfer line is a straight diagonal and nothing compresses; at maximum the output flatlines at the threshold. At 4:1 an input 12 dB above threshold produces 3 dB of output above it. A snare with 30 dB of range into a 4:1 compressor (threshold at average level) has peaks pulled to ~7.5 dB above threshold.

Assessment

At 4:1, if the input sits 8 dB above threshold, how many dB above threshold is the output? What do a 1:1 ratio and the maximum (limiting) ratio each do? A 4:1 compressor with a 0 dBu threshold receives a +12 dBu peak — give the output peak, then repeat at 2:1, and describe the audible effect on a snare transient at a high ratio versus 1.5:1.