A Guide to Expanders in Live Sound Reinforcement
An expander is rather like a compressor in reverse (it increases, rather than reduces, the dynamic range of a signal). Another way of looking at it is as a noise gate that uses volume reduction rather than total muting of noise.
Usually an expander is a 19″ 1U box with knobs or buttons (and more often than not a couple of LED meters) on the front. Some compressors have basic expanders or noise gates included in the channel facilities.
An expander increases the dynamic range (the difference between quietest and loudest) of a signal.
It uses a voltage controlled amplifier (VCA) either to increase the gain when the signal exceeds a threshold, or to reduce the gain when the signal falls below a threshold (usually the threshold is user-determined). The principle of operation is the same in either case: the gain applied below the threshold is proportionately less than that applied above it. An expander that reduces the signal below a threshold is sometimes called a downward expander.
It can have controls for:
The level below which gain is reduced.
How quickly full gain is achieved once the signal reaches the threshold. On most sounds (particularly percussive sounds), the attack should be very fast to avoid cutting off the beginning of the sound. Very low frequency sounds may need a slightly slower attack to avoid audible switching. However, longer attack times are seldom needed except for special effects.
Certain important parts of the sound (decaying resonance & natural reverb tails) may fall below the threshold. If the gain is reduced too abruptly, the cut off may be audible (and unnatural).
The amount by which gain is reduced below the threshold, typically ranging from 1:1 (unity) to over 10:1.
If an expander is used on a small rack-mounted tom, low-frequency sound from the kick drum may exceed any usable threshold. A filter enables the user to tune out (or sometimes tune in) a frequency range, so that full gain is only applied when sounds in a particular frequency range exceed the threshold.
Most multi-channel expanders allow coupling of each pair of channels for stereo use. If expansion is applied to left and right channels independently, apparent drop-out of one or other side can result whenever the signal on one side is above and on the other side below the threshold.
If all else fails, read the manual!
Expanders and noise gates can have a disastrous effect if set up badly, and are at best problematic in live performances: spill from other instruments or from monitors can make it impossible to set effective thresholds. With care they can deal with very situation-specific problems (for example, buzzing from a bass guitar can be silenced between songs by careful use of an expander with a filter). However, they are most commonly used on drums, either to reduce spill between drums, or to reduce the ringing of undamped toms. In this application, they are usually used on channel inserts.
To use an expander on a drum:
Although the cut-off will still be audible, this will be much less noticeable when the whole kit is being used and the whole band is playing. A little reverb on the drum will help make the cut-off less noticeable during solos.
Other than on drums, leave well alone.
Most expanders can be set up as noise gates (which is essentially how one is being used in the descriptions above), and they don't have a more practical application in live music. If the drums are well tuned and damped (or if ringing toms are an intentional part of the drummer's sound), you probably don't need one. Otherwise, one channel for the kick drum and one for each tom can be useful.
One that has at least all the controls described above. Four channels is usually enough (which might mean you need two expanders, not one).