Compression

A singer leans into the loud notes and drops away on the quiet ones; a bass player hits some notes harder than others. In a mix those swings can make a part jump out one moment and vanish the next. A compressor acts like a very fast hand on the fader: when the sound goes above a level you choose, it turns it down by a set proportion.

The controls

• Threshold: the level at which compression begins. Sound below it passes unchanged.
• Ratio: how strongly sound above the threshold is reduced. At 4:1, a signal that goes 8 dB over the threshold comes out only 2 dB over.
• Attack: how quickly the compressor reacts once the threshold is crossed. A slower attack lets the sharp start of each note through, which keeps drums punchy; a fast attack catches it.
• Release: how quickly it stops compressing once the sound falls back below the threshold.
• Knee: whether compression starts abruptly (hard knee) or gradually (soft knee).
• Makeup gain: turns the whole result back up, since compression lowers the peaks.

A gain-reduction meter shows how many decibels the compressor is taking off at each moment.

Common uses

• Evening out a vocal or bass, so every word and note is heard.
• Adding punch to drums with a slower attack.
• Gluing a group of tracks together with gentle compression on a bus.
• Limiting: a compressor with a very high ratio and a fast attack stops peaks from going above a ceiling; limiters are a standard tool in mastering.
• Parallel compression: blending a heavily compressed copy with the untouched original, for density without losing the natural attack.
• Sidechain compression: one sound turns another down, for example the kick drum briefly ducking a bass or a pad. Exaggerated, it creates the rhythmic "pumping" heard in much electronic dance music.

Getting started

1. Start with a moderate ratio, such as 2:1 to 4:1, and a medium attack and release.
2. Lower the threshold until the meter shows a few decibels of gain reduction on the loudest notes.
3. Use makeup gain to match the loudness of the bypassed sound, then compare on and off.
4. Listen for side effects: lifeless dynamics, pumping you didn't intend, or background noise rising between phrases.

Compression changes dynamics, the musical difference between loud and soft, so use it to support a performance, not to flatten it. The word has nothing to do with file compression, such as MP3, which makes audio files smaller.