The question sent in to us is simple – “How does the expander helps my live sound recording?” Once again we bear in mind that not all readers have English as their first language, but the meaning is clear.
First of all, an expander will not help at all in live sound recording. It might when you are mixing a live sound recording, but the original recording should get as close to the original signals from the mics and DI's as possible.
But what about actual live sound, in front of an audience?
Well we can simplify this a little and consider the noise gate, which is closely related to the expander and is used widely in live sound.
Basically the noise gate cuts the level when the signal drops below a certain threshold. It will be set by the operator so that when an instrument is playing, the gate on that channel will be open. When the instrument is not playing, the gate will be closed.
In recording, the gate is often set so that it is either fully open or fully closed. In live sound it is safer to set it so that, when closed, it reduces the level of the signal but doesn't cut it completely. A 10 dB reduction is well worth having and means that a mic is never dead completely.
It is worth noting that noise gates are used individually on channels, not on the whole mix. If you want to gate ten channels, then you need ten channel's worth of gates (i.e. five 2-channel units).
The benefit of gating in this way is that all of the spill from the other instruments that gets into, say, the saxophonist's mic, is lowered in level when the sax is not playing. Spread over the entire band, or as many mics as you have gates for, the result is a much cleaner sound.
The benefit of an expander rather than a gate is that the expander offers a smoother transition into level reduction but, as you would expect, it costs more in the form of a hardware unit. It would be better to have enough gates than not enough expanders.