The problem of acoustic feedback in public address systems has been with us since the advent of the electronic amplifier (and, of course, the loudspeaker and microphone). Various schemes to ameliorate the problem have been proposed: filters, frequency shifting, directional transducers, etc. One possible technique that has not been explored in the past is to allow the loudspeaker directivity to be modulated in time. The modal excitation of the room can be time-varied, thereby averaging the response at a point. Such directivity dithering reduces the peak response in the system transfer function (between loudspeaker and microphone). We have constructed a three-octave digital beamforming loudspeaker array whose directivity can easily be modified as a function of time. The presentation will discuss the theory of the use of the modulated directivity and report on the design and construction of the beamforming loudspeaker array.
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