Expansion and gating are forms of dynamics control. Dynamics control is the process of automatically adjusting the volume control on an audio channel. The gain level of the channel is automatically adjusted by the process based upon the level of the audio signal passing through the processes detector. An expander is a dynamics control processor that lowers the volume of the signal when the overall level of the signal decreases. As a result, it tends to expand the dynamic range of the signal. A gate is a closely related process that essentially “cuts off” the incoming signal when the level drops below a predetermined level.

The question is, why would we want to make soft sounds softer? Well, it is often the case that the sound we want to record is contaminated by (sometimes much) lower level sounds from interfering sources. This can be bleed from other instruments when you are recording multiple instruments at once with multiple microphones. It could be background noise, like room noise or air handler noise. Or, it could be electrical interference like buzz or hum. If the recording environment is well constucted, none of these problems will arise, but often the recording environment is not ideal.

When the primary instrument (or voice) is at normal volumes, it masks the noise and makes the low level noise inaudible. So, when the sound level in the channel is high, you don't notice the noise, and it is not a problem. When the primary instrument (or voice) is not generating sound, the noise is no longer masked and can become both audible and irritating. If the channel also has a compressor or some other dynamics processor (like a guitar amplifier) in line (see All about compression to learn more about compressors), the very low-level noise signal may be amplified significantly, and as a result actually contribute a large amount of noise to the mix when the primary instrument in the channel is not generating sound.

The expander/gate is a processor that allows you to automatically mute or attenuate the noisy low level signal in between the (relatively) loud parts that you want. If the expander/gate is placed in the signal chain before any compressor or limiter, it can be used to remove the low-level noise before it gets amplified by the compressor (or other dynamics processor like a guitar amp). Noise gates are quite effective when used as the first dynamics processor in a chain, and are relatively ineffective when used after a compresssor because the compressor removes the dynamic range that the gate uses to distinguish between the signal and the noise.

There are many different forms of exapanders and gates, all with different parameters. In this tech page, we will only address the most important parameters that you are likely to find on an expander/gate, and in fact, these are the parameters that you will find on the expander/gate that is part of ChannelStrip and MIOStrip. The primary parameters on a expander/gate are:

Channel Strip Gate Controls & Graph
• Threshold
• Ratio
• Attack
• Release

The Threshold parameter allows you to select the level at which the gate will begin to function. When the detected level is below the threshold you select the gate will begin to attenuate the signal. If the detected level is above the selected threshold, the signal will pass through with no attenuation.

The Ratio parameter allows you to control the amount of attenuation applied to the input signal when the detector level is below the theshold. Higher ratios mean that the signal will be cut off more quickly as the detector level drops below the threshold. The gate in ChannelStrip has a fixed ratio — there is no control available for this parameter.

The Attack parameter controls the rate at which the detector tracks increases in signal level. For most applications you want the attack parameter to be instantaneous; this corresponds to a value of 0, or with ChannelStrip “auto”. By keeping the attack parameter set to allow for an instantaneous attack, the gate will release at very beginning of the sound that you want to pass through the gate. If the attack is set to have a longer time constant, the gate will have the effect of muting the inital transients of the sound that it lets through. This can be useful in a creative application of the gate; it can be used to make the attacks of a sound less sharp, and it can also be used to change the feel of the material that passes through it -- it has the effect of delaying the onset of impulsive sounds.

The Release parameter controls the rate at which the detector falls back from the measured peak level. It effectively controls the rate at which the gate closes as the sound level drops below the set threshold. For very impulsive sounds (like drums) it may be useful to set the release value quite low -- this has the effect of muting the channel in between the impulses of the drums, and it leads to a very dry sound, with no reverb and very short decays. If the release value is too low, it can lead to substantial distortion — in the extreme case, it can even sound broken. Large values of the release parameter will serve to cause the gate to “skip” over short segments of sound that are below the threshold. Longer release times may be appropriate for use on vocal mics; this will have the effect of muting the mike on long pauses, but not chopping up the silences between words. For guitars and other similar instruments, intermediate values are most appropriate; tune the release to taste — you will generally be shooting to have the release time short enough to mute the instrument while it is not playing but long enough that the muting action sounds like a very smooth fade as opposed to a rough cut-off of the sound.

As we alluded to in the preceding paragraphs about the parameters of the expander/gate, there are other applications besides noise-reduction.

By carefully adjusting the attack and release parameters of the expander/gate, you can achieve many creative effects including:

• reverb reduction
• shortening the decay characterisics of impulsive instruments (such as drums)
• changing the rythmic feel of existing tracks
• synthesising elements from a triggering source

The last application listed relies on having a sidechain key input. This application allows you to open the gate via different souce than the sound passing through the gate. One way that this is used is adding an EQ'ed noise source to a snare drum sound every time the snare hits. This is extremely useful if the recorded snare sound is weak.

As an example, here are some drum mixes that show the before and after results that can be achieved with the ChannelStrip plugin:

Drum mix with no processing

Drum mix with processing (gate, comp, EQ)

If the gate has a sidechain, the detector can be driven with a different signal than the signal that is being gated. This allows you to accomplish all sorts of interesting things that would be virtually impossible otherwise. Basically, a gate with a sidechain decouples the level detector from the input signal. When people say “sidechain”, they usually mean one of two things:

• One meaning of sidechain is an independent input feeds the detector, and can be connected to any arbitrary signal source. When the compressor has this kind of sidechain it can be used to implement things like the trigger described above.
• The second meaning of sidechain is that the gate has an additional integrated signal processor between the input signal and the detector. This signal processor is usually an EQ, and it allows you to make the gate react in a frequency-sensitive way.

The expander/gate in ChannelStrip and MIOStrip actually implement both types of sidechains; the sidechain can be fed from an external key input or the input signal and the processors integrate a sidechain EQ before the detector.

Having an EQ integrated into the detector is particularly useful, because it allows you to make the gate more or less sensitive to certain frequencies. This can be used, for example, to make the gate only open for the signal of interest, while ignoring other (possible quite loud) sound sources that appear in the channel.

Careful application of the gate as a signal processor can make your mixes crisper and tighter, with (substanitally) less noise and grunge while still revealing the low-level details. Overapplication of the gate can make your mixes dry and lifeless or even distorted. As with most signal processors, it requires experience and taste, but once you get it right, you will be shocked at the difference.

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