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Providing Foldback with Out-of-Phase Loudspeakers

Providing adequate foldback sound level to performers on the stage or to dignitaries seated behind the podium has been a long-standing problem for sound reinforcement engineers. The level required is generally well above that which can be provided and still avoid sustained feedback or singing. This problem can be overcome in many cases by connecting loudspeakers in an out-of-phase circuit configuration and placing the microphone in the resulting area of cancellation. Actual field tests providing foldback to dignitaries behind the podium resulted in as much as 15 dB additional gain before feedback. Anechoic chamber and field test results on variations of this method are described.

JAES Volume 19 Issue 4 pp. 306-309; April 1971
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AES - Audio Engineering Society