Noise Gates
A noise gate is the simplest dynamics processor. It has one rule: if the signal drops below a certain level, shut it off.
The gate watches the signal level. When the signal is loud enough — above the threshold — the gate is open and audio passes through unchanged. When the signal drops below the threshold, the gate closes and the output goes silent (or nearly silent).
Why is this useful? Because microphones pick up everything, and “everything” includes stuff you don’t want: amp hiss between guitar phrases, bleed from the hi-hat into the snare mic, room noise during vocal pauses, the hum of a noisy preamp when nobody’s playing. A gate removes that noise by cutting the signal during the silences between the parts you actually want.
Gate Parameters
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Threshold: The level at which the gate opens. Set it just above the noise you want to eliminate, and just below the quietest signal you want to keep. This is the critical setting — too high and the gate clips the beginnings of notes; too low and it lets the noise through.
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Attack: How fast the gate opens when the signal exceeds the threshold. Fast attack lets transients through cleanly. Slow attack can clip the beginning of a note — sometimes intentionally, as a creative effect.
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Hold: How long the gate stays open after the signal drops below the threshold. This prevents the gate from chattering — rapidly opening and closing — on a signal that hovers near the threshold.
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Release: How fast the gate closes after the hold time expires. Fast release = abrupt cutoff. Slow release = gradual fade to silence. A release that’s too fast sounds unnatural — the tail of a sound gets chopped. Too slow and the noise creeps back in before the gate fully closes.
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Range: How much attenuation the gate applies when closed. A range of -∞ means complete silence (the traditional gate behavior). A range of -20 dB means the signal gets quieter but doesn’t disappear entirely. Reducing the range makes the gating less obvious — the noise is pushed down, not eliminated.
Waveform with gate applied showing threshold line, signal passing above threshold, attenuation below. Attack, hold, release, and range annotated.
Remember when we talked about cutting a loud peak from a reference track in Chapter 10 so it would match your mix level? That was selective leveling — you chose when to make it quieter. A gate does the same thing, just automatically, thousands of times per second.
Expansion: The Gentler Gate
An expander does what a gate does, but gradually. Instead of slamming shut when the signal drops below the threshold, an expander increases the volume difference between loud and quiet parts. Signals above the threshold stay at their original level. Signals below the threshold get pushed quieter — but proportionally, not all-or-nothing.
Think of it this way: a gate is a door that’s either open or closed. An expander is a door that opens wider as you push harder and closes gradually as you let go.
Expanders are more musical than gates in many situations. On a vocal track, a hard gate can make the silences feel unnatural — the room tone disappears and reappears abruptly. An expander just pushes the room tone down, maintaining a more natural feel while still cleaning up the noise.
Downward expansion (the most common type): quiet signals get quieter. This is the “noise reduction” application — it increases the dynamic range by making the quiet parts even quieter.
Upward expansion: loud signals get louder. This is less common but useful for adding life to over-compressed material — it restores some dynamic range by pushing the peaks higher.
Transfer function graph comparing gate (hard cutoff below threshold) vs expander (gradual slope below threshold).
Gate vs. EQ: Throwing Out the Baby
A common beginner mistake: using EQ to solve a problem that a gate handles better, or vice versa.
Example: a snare drum mic that picks up too much hi-hat bleed. You could reach for an EQ and cut the high frequencies where the hi-hat lives — but you’d also cut the high-frequency content of the snare itself. The snare would sound dull. You’ve thrown out the baby with the bathwater.
A gate solves this more elegantly: it opens when the snare hits (loud signal), letting the full-frequency snare through, and closes between hits, silencing the hi-hat bleed. The snare keeps its brightness. The bleed disappears during the gaps.
The lesson: dynamics processors work on level. EQ works on frequency. Choosing the right tool depends on whether the problem is a level issue (noise in the gaps) or a frequency issue (too much of a particular range).
Introduction to Sidechain
Normally, a gate monitors its own input to decide when to open and close. But what if you could tell the gate to listen to a different signal?
Vocabulary
Sidechain
A detection input that tells a dynamics processor what to listen to. Instead of monitoring its own signal, the processor listens to a different source — or a filtered version of its own signal — to decide when to act.
That’s a sidechain. Instead of monitoring the signal on its own channel, the gate listens to an external source — a different track — to make its decisions. The processing still happens on the original signal, but the trigger comes from somewhere else.
The easiest way to understand sidechaining is with a gate on a single signal. Put a gate on a noisy kick drum track. Now, instead of having the gate listen to the full signal (which includes bleed from every other drum), filter the sidechain input — tell the gate to only listen to the low frequencies where the kick lives. The gate opens on every kick hit and stays closed during the snare and hi-hat bleed. Same gate, same track — but the detection is filtered. You’ve just used a sidechain.
From here, the leap to using an external sidechain signal — a completely different track triggering the gate — is small. We’ll explore that extensively in Chapter 20. Think of it like siblings: the internal filtered sidechain and the external sidechain are the same concept, just at different scales.
Signal flow showing a gate on a kick drum track with sidechain input filtered to low frequencies only. Audio path and detection path labeled.
What to Practice
- Gate a drum track. If you have a multitrack drum recording (many are available free online), put a gate on the snare mic. Set the threshold so the gate opens on snare hits and closes between them. Adjust the hold and release until the snare tail sounds natural. Notice how much cleaner the drum mix sounds with the bleed removed.
- Compare a gate to an expander. On the same track, switch between a hard gate (range at -∞) and a gentle expander (range at -15 to -20 dB). Listen to how the expander preserves a more natural feel between hits while still reducing noise.
- Try gating a vocal. Put a gate on a vocal track during a quiet section. Adjust the threshold so it opens for the vocal and closes during breaths and pauses. Notice how the room tone disappears in the gaps — and decide whether that sounds better or worse. Sometimes the room tone is part of the sound.
