Noise gates, usually called just “gates,” are dynamics processors that you use when you want to automatically turn off a channel if the signal is not present.
They perform this magic by “looking” at the input signal, and if it is below a certain level, the gate is closed.
When we say that the gate is closed, what really happens is that the gain of a device in the audio path called a Voltage-Controlled Amplifier (VCA) has been set to minimum.
Because the various signals you might want to gate are all different, most gates provide a handful of control knobs that let you tailor the gate’s action to your liking.
This article will discuss what those knobs are, and how they affect the gate’s response.
I’ll assume that the gate is connected to a console’s insert loop, and that the channel’s input level has been set properly, and I’ll also use a drum as the example input, since gates are most commonly used on drums.
A typical full-featured gate will have the following controls: threshold, attack, hold, decay and range. Additionally, there may be high- and low-frequency controls.
Less expensive gates may not include one or more of these controls. There are usually LED indicators which tell whether the gate is open or closed. Some gates use an LED bar-graph display to tell you how much attenuation the gate provides.
I find it easiest to start when all of the gate’s controls are set for “no gating.”
Start with the Threshold set to Minimum (largest negative number!), and attack time at minimum, (shortest time), hold time at maximum (longest), decay time at maximum (longest) and the range to maximum (largest negative number). Set the low-frequency control to minimum, and the high-frequency control to maximum.
As the drummer slowly bangs on something, raise the threshold. At some point, the gate will close and no sound will pass through the gate.
The threshold control “looks” at (the envelope of) the input signal, and if the input signal is below the threshold, the gate is closed. The gate opens when the signal is above the threshold.
So, the trick is to find the point where the drum opens the gate, but the ambient noise on the stage, or the next drum, doesn’t. With a little practice, you’ll find it’s easy to narrow that down.
The threshold control is calibrated in decibels. 0 dB refers to the gate’s nominal input level. This means if your gate has +4 dBm I/O, then if you set the threshold to 0 dB, the gate will open when the box input exceeds +4 dBm.
If the threshold is set to -15 dB, then the gate opens when the input is above -11 dBm. Likewise, if your box has -10 dB I/O, then the 0 dB mark means -10 dB.
Next is the attack time. The attack time control sets the time (usually in microseconds, or milliseconds) the gate takes to go from closed (maximum attenuation, as set by the range control) to open (zero attenuation).
The gate doesn’t “wait” the attack time before snapping open; rather, it smoothly ramps the attenuation from max to zero in the attack time. This is analogous to starting with the channel fader on minimum, and fading up to unity, in microseconds.
If you’re playing along at home, you may notice that if the threshold is set so that it’s barely below the drum input level, the gate will “click” as it opens. You can mitigate that click with the attack time control.
Here’s what happens: when the signal goes above threshold, the gate is told to open. If the attack time is too fast, the gate output wants to switch instantly between 0V and some non-zero value—maybe a couple of volts. Now, if you use the attack time to slow down the attack, that voltage doesn’t change “instantly,” but rather smoothly ramps up.
So, the trick is to set the attack time fast enough to capture the drum’s transient, but not so fast that the gate clicks.
The hold time control is obvious—it’s simply how long (in milliseconds) the gate remains open once it’s fully open. Too short a hold time, and you clip the end of the drum’s ring. Too long, and the gate may not close, or you’ll get excessive ring, or what have you.
You can also get weird clacking noises from the gate as it chatters. Too short a hold time and you’ll find the gate might try to open and close and open and close quickly.