Expander  --  An audio amplifier designed to provide an output level that increases out of proportion to the input level. An expander does the opposite of a compressor; it expands signals above a set threshold. If sounds above a threshold are expanded, it is equivalent to attenuating those below it. It can thereby reduce noise or low-level background sounds and when so used is called a noise gate. By attenuating decaying sound, reverberation time can be reduced.