Stephen - over the years you've answered several of my questions and have taught me a lot. I have another question if you've got the time. I hear everyone talking about compression for dynamics, but I never hear anyone talking about using expanders. Why do people use them? Are they used frequently? Do you use them? When is it good to use one? Thanks for any information you can provide.
Expanders are most often used to help suppress noise or unwanted bleed in a signal, but can also be used to increase the dynamic range of a signal. Compressors, on the other hand, decrease the dynamic range of a signal, and are most often used to help smooth out the levels of a signal.
The most common type of expander is a downward expander. It behaves very similarly to a gate, except that it's not a simple on/off type of process. Like a compressor, it has a ratio. When a signal falls below the threshold on an expander, the signal level is reduce proportionally according to the ratio. So, if you use a 1:2 ratio, then for every 1 dB that the input falls below the threshold, the output will be pushed down 2 dB lower. This is useful for lowering the noise from a guitar amp, for example, in between guitar notes, or reduce bleed from cymbals on a tom or snare microphone. Since it's not an on/off type function like a gate, a downward expander is useful for lowering noise is a more subtle way. You also usually have control over attack and release, so you can determine how fast it reacts to signal level changes. Some downward expanders also have a range setting, which allows you to set the maximum amount of attenuation (so it never turns completely off).
My favorite use of downward expanders is when I'm trying to reduce heavy handed ride cymbal bleed getting into the toms or snare mics, but a standard gate is too drastic and noticeable. In those cases, when the gate turning on/off is too obvious in the mix, then I'll switch to an expander and set it so that it reduces the level of bleed between hits, but only by a certain amount (using the range setting).
An upward expander is a bit different in that it works on signals that go above the threshold, like a compressor does, but instead of attenuating the output, it multiplies the output by the ratio you set. It is not used very often, but can be used to try to restore some dynamic range to dull or lifeless recordings, and sometimes can help recover some dynamics for signals where too much compression was used.