Dynamic Processing  

Signal processing devices that change their operating parameters in reaction to changes in the signal input are known as dynamic signal processors. 
Several reasons for control of dynamic range commonly occur:
make a signal fit within optimum operating ranges of equipment, or recording tape  smoothing out the dynamic range of a signal can increase its average intensity resulting in more subjective volume and presence increasing dynamic range can "tighten up" percussive sounds and remove unwanted noise 
reduces gain as input level is increased above some threshold see
limits output to some maximum level regardless of input level above some threshold 
reduces gain as input level is decreased below some threshold see
noise gates 
noise reduction 
in analogue systems is often a type of companding 
and dynamic filtering