Rack mount Expander/Gates like the Presonus ACP88 can be an invaluable
tool with the right knowledge.
In order to get the most out of your recording software or effects units, you’re going to need to know the ins and outs of what they can bring to the table. Two of these such tools are Gates and Expanders which are primarily, though not only, noise management devices. Unwanted noise is the enemy of all parts of the recording process. Unwanted noise can be caused by the recorded environment or can be generated by, or in, the recording/processing circuitry or storage medium. This problem is magnified when you are dealing with multi-track recording as the noise on each track can be added to the noise on all the tracks. This means that the volume of the noise can go well passed audible and into the really annoying region. Not really the way you want to present your recordings.
Once noise is part of an electrical signal it is almost impossible to remove it, and if you do you will probably change the tone of the sound. It makes sense then to manage the noise that can be present on the signal before it gets there and to reduce the amount of noise contributed by each track to the mixdown, as much as possible. In particular, if there is no signal content during a quiet section, for example the quiet sections between vocal phrases, then the track is only contributing unnecessary noise to the final mix.
A Noise Gate can easily be used to help manage the noise on a track. The controls come if the form of Release, Attack, Hold and Ratio.
A Release Time control is used to get around the issue of cutting off low level sounds. This allows the user to set the time over which the gate will close. This stops the sound or noise from suddenly stopping, instead the sound or noise gradually fades.
The Attack Time control is a less vital control, but it is still very useful. The Attack Time control is used by the user to set the time it takes for the gate to fully open after the signal exceeds the threshold level. Low frequency sounds are likely to be distorted if the gate opens too quickly. This is because the sudden change in gain could be applied to signal part of the way through the frequency cycle, resulting in an audible click. Conversely, if the Attack Time is too long, fast attack sounds, like a snare, will have less impact.
Floor or Ratio Control
This control allows a certain amount of bleed from the original signal, even when the gate is fully closed. This can be useful if you want to retain some of the ambience on the track, but you want to reduce the amount of it.
This control allows the user to set a minimum time between the release threshold being crossed and the gate begining to shut. This control can be useful to stop the gate "chattering" open and closed when the source signal is constantly changing amplitude.
This is effectively having two thresholds in the unit, although you will only have one threshold control. In a noise gate the release threshold is several dB less than the attack threshold. This is a very useful way of addressing "chattering" gates.
This allows the gate threshold to be controlled from a signal other than the source signal being treated. This has been used to create some interesting effects. For example use a bass drum signal to control a bass guitar track to help tighten up the rhythm section.
This is a facility some gates have that allows them to behave similarly to a compressor. Without a side-chain input the dynamics of the part can be reversed, making the louder songs quieter than the quiet sounds. This is done by closing the gate when the signal exceeds the threshold and using the floor or ratio control to set the minimum signal level.
Equalizers or Filters in the Side Chain
This allows the gate to respond to some frequencies more than others. This can be useful when trying to trigger the gate from a source that might have spill from several sources as well as the desired trigger sound, for example drum kit mikes. For example, a snare mike is likely to pick up spill from drums, or more than likely the hi-hat. Using an unfiltered snare mike signal for a side-chain trigger may result in false triggering ot the gate due to spill from the hi-hat.
This is basically the opposite principle to compression, Above the threshold the signal is untreated, below the threshold the signal is attenuated by a ratio setting. For example, if an expander ratio of 1:4 is chosen, for every dB the input signal falls by, the output signal will drop by 4 dB. This can be particularly useful for bringing back more natural dynamics to an over-compressed signal.