Beam dithering: Acoustic feedback control using a modulated-directivity loudspeaker array

Acoustic feedback instability is a fundamental limitation in public address systems and hands-free full duplex teleconferencing systems; it occurs at the resonant peaks of the acoustic signal path. These resonances can be reduced by modulating the directivity of the loudspeaker or microphone, as in a digital beamforming loudspeaker array with time-varying shading coefficients. Beam dithering can be accomplished by time-stepping through a sequence of discrete Chebyshev taps. For each time interval, the array is weighted with a distinct set of Chebyshev coefficients. As the taps change, the beam pattern changes, thus continually altering the modal excitation of the room and thereby smoothing the room response. The authors include a summary of linear beamformer theory, a mathematical development of the array weighting scheme, an analysis of the dithering method, a diagrammatic explanation of the beamformer implementation and some experimental data.<>