Noise gate   

The noise gate is a special case of a type of processor known as an expander. 

When the signal input to the noise gate falls below a preset threshold the gate "closes" by reducing the output gain to zero at a rate defined by an attack control. When the signal level exceeds the threshold the gate "opens" by restoring full gain at a rate defined by a release control. 

The basic usefulness of noise gates is to reduce noise in a signal by only passing the signal when it is at a significantly high level. This level effectively masks the underlying, unwanted noise which may be present in the signal. When the signal falls below this level the noise floor may become audible but the closing of the gate blocks it.