Creative Gating

It’s a simple concept: set a gate at a certain threshold of volume, and only sounds above that volume will pass through. It’s a straightforward way to get rid of persistent, low-volume irritations like hums, ambient sound, and background noise. So straightforward, it seems, that many people seem to think of gates more as “utility” effects than as musical ones. But just as compression (as in dynamic range compression) is now viewed as at least as much an art as a science, there is much to recommend the creative possibilities of the humble gate.

(For a quick primer, check out this two-part series from Sound on Sound.)

Even though gates are generally used to reduce the volume of undesirable parts of an audio signal such as background noise (also known as “noise gates”), they are actually part of a class of effects known as dynamic range expanders. (The term “dynamic” is often used to refer to volume in music and audio.) How does expanding the dynamic range reduce volume? Well, by making quiet sounds even quieter, expanders actually increase the distance between the loudest sounds (i.e. those above the threshold and therefore unaffected by the gate) and the softest.

Imagine standing next to a drummer performing in a large, empty acoustic space. They play a drum beat that contains both loud sounds (for example, played on the kick and snare), and soft sounds (played on a closed hi-hat). The dynamic range is defined as the range or distance between the loudest sound (let’s say the thwack of the snare) and the softest sound (the tap of the hi-hat).

To expand the dynamic range, we physically move the hi-hat farther away from where we stand, so it sounds even quieter than before (let’s assume our drummer has extraordinarily long arms). If the softest sound before was the sound of a hi-hat a few feet away, now the softest sound is that of a hi-hat, say, fifty feet away. The loudest sound is unchanged (the snare is still right next to us), but there is more of a difference–a wider dynamic range–between the snare and the distant hi-hat, than between the snare and the close hi-hat.

To extend the analogy, compressors work in just the opposite fashion: we would take the loudest sound–the snare drum–and move it away from where we stand until it was closer in volume to the hi-hat right next to us. This rearrangement reduces or compresses the dynamic range, meaning that there is less of a difference between the volume of the snare and the hi-hat than before.

The most extreme version of the compressor, called a limiter, prevents any sounds above a given threshold from passing through (normal compressors decrease the volume of sounds above the threshold but do not impose a hard limit). Similarly, the most extreme version of the expander is the gate: instead of just making soft sounds softer, it makes them completely inaudible. In our analogy, it would be like taking the hi-hat out through the doors at the opposite end of the space and down the block.

That said, gates–and by extension, all expanders–are about much more than just changes in volume and dynamic range. One of the most famous creative gating effects is gated reverb, an important part of the huge sound of 1980s-era snare drums on many records. A conventional reverberation effect simulates the gradual decay in volume of a sound in a resonant space. Gated reverb involves aggressively cutting off the reverb before it has naturally decayed, resulting in a larger-than-life burst of energy that fades out quickly. Check out the snare on almost any record from the 1980s by Prince, Bruce Springsteen, or Phil Collins:

The “gated” in gated reverb refers to the use of a gate to dramatically cut off the reverb after it passes below a certain threshold (but well before it would naturally fade out). However, even though the gate is affecting the reverberated version of the audio, it’s actually responding to (or detecting) the volume of the drum before reverb is applied. The much shorter duration of the dry signal causes the gate to close on the reverberated signal before it has itself decayed, giving us the characteristically abrupt cutoff.

The principle of using one signal to control another is at the heart of many more familiar techniques, such as a modulation using an LFO or side-chaining. When gates are involved, the process is often described as “envelope following.” Envelope following means that changes in the volume of a signal (such as the attack and decay of individual notes or drum hits) are linked to the opening and closing of a gate. The gate is then applied to other tracks, essentially allowing one to apply the rhythmic pattern of one track to another.

For example, in “Upside Down” by Diana Ross, the rhythm of the strings is triggered (or “keyed”) by Nile Rodgers’s guitar:

Another great example is “Everybody Dance” by Chic. Listen to the solo section, beginning around 4:00:

In electronic music, this is often achieved by applying a gate to a sustained sound (such as a synth pad) and triggering it with a more rhythmic track, such as a drum track or step sequencer. This control layer may or may not be audible, just as kick sounds used for aggressive side-chain compression in a mix may be completely different from the actual kick track. Sometimes the gate is even inverted to produce complementary rhythms: whenever the first layer is playing, the second layer is not, and vice versa.

And what about using a reversed gate on its own: how might one use a gate to pass only sounds below a certain volume threshold? This is not an application that comes up often, but it’s an interesting question. Let’s say one wanted to create a sustained pad-like background layer from a dynamic source source (for instance, turning a drum beat into a wash of metallic cymbal sounds). One approach might be to use such a “reverse gate” to cut out the loudest attacks from the source, and then fill in the gaps with heavy reverb effects (and probably some compression as well). Although there exist specialized plug-ins that can do this, a bit of cleverness can easily flip the functionality of a standard gate.

Recall that combining two versions of the same signal with opposite phase cancel each other out. We can use this principle to cancel out sounds above the threshold of the gate, leaving only those below the threshold. Simply copy the same audio to two tracks and invert the phase of one version. Then apply a gate to the inverted version. When the sound is above the threshold the gate will open, and the two out-of-phase versions of the track will play simultaneously, resulting in silence. When the audio is below the threshold the gate will remain closed, meaning that the only audio we hear is the original, in-phase audio without the gate applied. Unlike compression, this technique will introduce silences where the loud sounds once were, but the attacks will be removed completely.

Gates can even be used to toggle between different inputs. The most famous example is the lead vocal on David Bowie’s “Heroes,” using a technique known as multi-latch gating devised by producer Tony Visconti. On this track, Bowie’s vocals are captured simultaneously by three microphones at varying distances, but the volume at which Bowie sings determines which microphone captures the sound. Each microphone input passes through a gate whose threshold is proportional to its distance from the singer. When Bowie sings quietly, the closest microphone captures his voice, but as his voice rises in intensity, the gate on a more distant microphone is opened, and the gate on the closer microphone is closed.

What ends up happening is that Bowie has to practically yell to be picked up by the farthest microphone, but the physical distance prevents the microphone from being overloaded while simultaneously adding more room ambience. In the context of the song, it allows for a powerful, emotional performance that is also somehow remote and alienated. It’s hard to describe, but it works–and at the center of it all is an elegant application of one of the simplest and perhaps most underrated tools.