The gain knob controls the sensitivity of the mic. Because various mics with different sensitivities can be used with the wireless beltpack, the gain knob allows all of them to be calibrated for the right situation. In addition, people speak at different volume levels and the gain knob can be used to adjust for that as well. If the system volume is all the way up and you are still not loud enough, simply use the key to turn the gain knob clockwise. If you are too loud or you are hearing distortion, turn the gain knob counter-clockwise. Wireless beltpacks are shipped with the gain set at halfway.