Expander

An expander works opposite to a compressor in that it operates when the signal falls below the threshold. The result is that the output level drops off more steeply than the signal above the threshold - the dynamic range is proportionately increased.

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Expanders can be used in place of noise gates, as the drop off of the signal level below the threshold produces a more subtle and transparent effect rather than a sharp cut off of a noize gate. An expansion ratio is the inverse of a compression ratio for example, instead of 4:1 as seen with a compressor; an expander will have a ratio of 1:4.