Reverberation Time Control by Direct Feedback

A system for increasing the reverberation time in halls is described which uses the feedback of a delayed signal. The delay is obtained by the location of a microphone or microphones at a distance from a loudspeaker or loudspeakers, thus using the time taken by the sound to travel the distance between them. Feedback is then used to produce a long decay. Satisfactory results were obtained when this system was used in a small hall. In order to increase the reverberation still further, without instability, a method is used which phase modulates any signal at a very low frequency and thus prevents any build up due to feedback because the phase of that signal is continually changing.