Noise gates and expanders: when to use which

A noise gate turns a signal down, often all the way, whenever it falls below a threshold; a downward expander does the same job on a slope, making quiet parts quieter instead of cutting them off.

Expansion, from the manual

Ableton's Multiband Dynamics documentation describes downward expansion as making quiet signals quieter, which increases dynamic range, and upward expansion as making loud signals louder, which it notes is much less common. (Ableton) Compressors shrink dynamic range; expanders widen it. A gate is the extreme case of a downward expander.

Live's Compressor also has an Expand mode that "lets you set the ratio of expansion between the input and output level." (Ableton)

What each gate control does

Using Live's Gate as the example (Ableton):

• Threshold "sets the gate's sensitivity."
• Return sets "the difference between the level that opens the gate and the level that closes it." Higher values stop the gate chattering open and shut when a signal hovers near the threshold.
• Attack is how long the gate takes "to switch from closed to open." Very short times can click.
• Hold "kicks in" when the signal drops below the threshold, keeping the gate open a moment longer.
• Release: after the hold expires, "the gate closes over a period of time set by the Release parameter."
• Floor "sets the amount of attenuation that will be applied when the gate is closed." At -inf dB a closed gate mutes the input.
• Flip reverses it: "the signal will only pass if its level is below the threshold."
• Lookahead offers zero, one or ten ms, delaying the input so the gate can open before the sound arrives.
• Sidechain lets another track trigger the gate without being heard. (More in what is a sidechain.)

When to use which

Our rule of thumb, not the manual's: reach for a gate with the floor at -inf when you want silence between hits, such as a tom mic between fills. Reach for expansion, or a gate with the floor set to something like -10 to -20 dB, when you only want the noise or spill to sit back, as with breath between vocal lines or a guitar amp's hiss under sustained chords.

What we think

The Floor knob is the most useful control most people never touch. Left at full mute, a gate makes tracks sound chopped, because natural room tone and decay vanish mid-note. Raising the floor turns the gate into an expander in all but name, and that gentler version is what most mixes actually need. Set Return before you fight with the threshold: chatter is usually a hysteresis problem, not a threshold one.