Improvement of Acoustic Feedback Stability in Public Address Systems
Manfred R. Schroeder · The Journal of the Acoustical Society of America · 1959
It has been shown previously that the frequency response of a large room has fluctuations of an average magnitude of 10 db. A related result which has been obtained recently is that the highest peak of the response in the audio band is typically about 12 db above the (constant) mean gain. Since a public address system can become instable when the maximum loop gain exceeds 0 db, the gain could be increased accordingly if the fluctuations of the room response could be eliminated. Unfortunately, this cannot be done. It has been demonstrated both theoretically and experimentally that the response fluctuations in large rooms are an inherent property of the multiple path transmission and result from their statistical interference. However, by inserting suitable electronic circuits into the feedback loop (between microphone and loudspeaker) the effect of the response fluctuations can be eliminated. With the help of such a circuit, we have been able to increase the gain by up to 14 db above the “singing” point before instability occurred. The technique will be described in detail. A demonstration of this method is planned.