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Providing Foldback with Out-of-Phase Loudspeakers

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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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Scott Dorsey
Scott Dorsey

Comment posted August 16, 2021 @ 15:38:29 UTC (Comment permalink)

The author places two conventional sealed-box loudspeakers back-to-back and drives them with opposite polarity wiring to create a figure-8 pattern with a deep null in the plane on which they sit.  This allows two speakers to be placed directly behind a lectern so that PA monitor audio can be heard by listeners on stage without much leaking into the lectern microphone.

This technique is extremely effective and seldom used today.  Other later variants have come out of this such as putting two floor wedges on opposite polarities so that a lectern microphone can be very carefully placed at the null between them.

These techniques can give more than 20dB improvement in gain before feedback but are no longer taught today.  If you use them in convention jobs they can delight and amaze the house crew, just make sure that the microphone is fixed in place and cannot be moved out of the null.

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