The Sound Engineer as a Feedback System

At a concert the sound engineer stands in the middle of the room, behind the desk, and does the same thing all evening. He listens to how the room sounds, compares what he hears with how it ought to sound, turns a knob and listens again. And the sound keeps changing all the time. The room fills with people, and their bodies eat up some of the high frequencies and the echo, so that the sound set up at the soundcheck in an empty room sounds different by the middle of the evening. The singer moves closer to the microphone, the guitarist quietly turns himself up, the drummer in a frenzy hits harder, and the moment some microphone catches its own sound from the speakers, a high feedback squeal cuts through the room, and it has to be removed within a second, before the audience clutches its ears.

Engineers call this kind of work a feedback loop. Measure, compare with the target, correct, measure again, and so on without end. A thermostat that turns on the heating when the room gets colder than the set temperature works on this principle, and so does an autopilot that brings a plane back on course. A sound engineer in a room is the same loop, only his measuring instrument is his ears and his actuator is his hands.