An expander is a more sophisticated form of gate. Whereas a gate is either
on or off, an expander increases the dynamic range when the signal is below
- When the signal is above the threshold level, there is no change.
- When the signal is below the threshold, the quieter the signal gets, the
expander will make it even more so.
The expander has a ratio control like a compressor. 1:1 means no expansion;
20:1 means that the expander is working almost like a gate.
The expander isn't as useful a device as one might think. The problem is noise
modulation – as the signal level goes up and down, below the threshold, the
noise level will also go up and down.
This is an unpleasant effect known as 'breathing' or 'pumping'. Breathing and
pumping can be evident in the compressor, but since it is high level signals
that are being processed, they are better able to mask the noise.