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 the threshold.
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.
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