A soft knee makes compression progressively engage below the threshold, producing more transparent results

The knee controls the shape of the gain reduction curve at the threshold. With a hard knee (knee = 0), compression switches on abruptly when the signal crosses the threshold. A soft knee starts compression progressively below the threshold: at higher knee settings, some compression occurs further below the threshold and increases gradually to the full ratio above it. This gradual engagement is perceptually smoother and more transparent — it avoids the abrupt transition that the ear detects as a ‘pumping’ artifact at high ratios. The tradeoff is that a soft knee makes the effective threshold lower than the setting suggests, so the compressor engages earlier.

Examples

Set a compressor to ratio 8:1 with a hard knee on a vocal. Hear the abrupt compression at the threshold. Gradually increase the knee — the engagement smooths out and the compressor sounds more natural at the same ratio.

Assessment

Describe the difference between hard and soft knee behavior at the threshold. When would you prefer a hard knee over a soft knee?