Threshold, knee, and the ratio trade-off
A compressor reduces the volume of a signal once it crosses a threshold, and the ratio determines how aggressively it reduces. A 4:1 ratio means that for every 4dB the input rises above the threshold, the output rises only 1dB. A 10:1 ratio is far more aggressive, nearly brick-walling the peak. The threshold itself is the level that triggers compression; signals below it pass through unaffected, signals above it get compressed proportionally.
The knee smooths the transition between uncompressed and compressed regions. A hard knee (0ms) creates an abrupt kink in the curve, where the compression suddenly engages. A soft knee (typically 2-4dB) gradually transitions into the ratio over a range of input levels, sounding more transparent and less pumpy. Hard knees are useful for catching wayward peaks; soft knees are essential for transparent parallel compression on a full mix.
Ratio and knee in context
On vocals, a 4:1 ratio with a 2dB soft knee and a well-chosen threshold preserves natural dynamics while taming peaks. A bass guitar benefits from a 6:1 ratio with a soft knee to maintain punch while preventing finger noise from causing wild level swings. Drums compressed heavily (8:1 or higher) require a soft knee or they sound crushed; compression is usually automated per-drum or achieved through parallel processing instead.
The interaction between ratio and knee defines character. A gentle ratio (2:1) with a soft knee is nearly invisible compression, perfect for subtle glue. A high ratio (10:1+) with a hard knee approaches limiting, protecting against clipping but sounding mechanical. Most mixing engineers live between these extremes, choosing the ratio for genre and material, then dialing the knee for the right feel.