Dynamics processors are used to control or "smooth out" the dynamics of an audio signal and can be used on individual channel or a whole mix. With careful use of compression, an engineer can have greater control over the variation in loudness of an instrument or mix and therefore safely make it louder overall, without loosing the expression and musicality of the performance. Compressors are regularly used both live and in the studio.
Compressor controls include Compressor Threshold, which determines at what level the compressor begins working, as well as Ratio, Attack and Release controls that control the how quickly and for how long the compressor affects input signal.
Gates are like an extreme compressor used as a "gate" that only opens to sounds above a certain volume level.The Gate Threshold control determines the dB level at which the signal will pass through the compressor and to the rest of the mixing desk. This allows the sound engineer to cut all signal from microphones that are not being used, effectively 'muting' them without having to mute the channel on the desk.