Compressor

The compressor evens out a clip’s loud and quiet passages by turning down whatever goes above a threshold. It reacts moment by moment and acts only on what’s over the threshold. For a voice that drifts on and off the microphone, the Leveller is the better fit. This page covers a clip’s compressor. A track has one of its own, on Track compressor and limiter.

Where: Clip tab ▸ Sound ▸ Compressor
Agent tools: set_audio

Smooth volume levels

Select the clip and open the Sound sub-tab of the Clip tab. The Compressor section starts closed, so open it.
Tick Smooth volume levels. It turns on at a gentle setting for speech: −18 dB, 3:1, a 10 ms attack and a 200 ms release, with no makeup.
Play the clip. Lower Threshold until the loud passages come down, and raise Ratio if you want it to act harder.
Add Makeup to win back the level it took.

The section’s header shows the ratio, such as “3.0:1”, or “off”. Reset appears there once the compressor has settings, and turns it off. Unticking Smooth volume levels also throws away its settings, so ticking it again starts from the defaults.

The controls

Control	What it does	Range	Default	Quick picks
Threshold	The level above which it starts turning the sound down.	−60 to 0 dB	−18 dB	−30, −24, −18, −12, −6
Ratio	How strongly it turns down sound above the threshold.	1 to 20 :1	3:1	1.5, 2, 3, 4, 8, 20
Attack	How quickly it reacts when the level rises.	0 to 200 ms	10 ms	1, 5, 10, 30, 100 ms
Release	How quickly it lets go when the level falls.	10 to 2000 ms	200 ms	50, 100, 200, 500, 1000 ms
Makeup	Gain added afterwards to make up what the compressor took away.	0 to 24 dB	0 dB	0, 3, 6, 12

A number typed outside the range goes to the nearest end of it.

Threshold

The threshold is a fixed level, not a distance from the clip’s loudness as the Gate’s is. A quiet recording may never reach the default of −18 dB, and then the compressor does nothing until you lower it.

Ratio

At 3:1, sound 3 dB over the threshold comes out 1 dB over it. 1:1 turns nothing down, and 20:1 acts like a limiter.

Attack

A short attack catches the start of a word or a sudden peak. A longer one lets those through before the compressor closes in. The time is in milliseconds and isn’t rounded to frames.

Release

A short release recovers quickly after a loud moment. A longer one lets go more gradually. Like Attack, it isn’t rounded to frames.

Makeup

Makeup adds a fixed gain after the compressor, so it applies even at 1:1.

Change the compressor over time

With an action selected, the controls write the action’s end, and each number moves smoothly across the action. Turning the compressor on or off changes at the action’s end, with no glide.

Good to know

The compressor acts before the clip’s volume, fades and ducking, so moving the Volume slider doesn’t change how hard it compresses.
It runs after Noise Reduction, the Gate, the Leveller and the De-esser, so the threshold is compared with sound they’ve already worked on.
Normalize in Loudness mode measures through it.
Enhance Voice sets a compressor for you, with its threshold at the speech’s typical peak and makeup that gives back the loudness it takes.
On a nest clip the compressor has no effect. Open the nest with ⌘↓ and set it on the clips inside.

With an agent

set_audio sets the same controls through its compressor argument. {} turns the compressor on at its defaults, and null turns it off. An agent isn’t held to the inspector’s Attack and Release ranges, and gives both times in its own units. See Tools: clip sound.