A study on howling suppression by automatic directivity control of a microphone array

Howling is one of the most annoying problems for public-address systems in a reverberant sound field. A new technique to suppress howling has been developed, in which the direction of the strongest coupling between the microphone and the loudspeaker is constantly detected to lower the sensitivity of the microphone in that particular direction. For detecting the direction of coupling, a reference signal, whose spectrum is modulated to spread over the frequency range of the public-address system, is combined with the acoustic input to be reproduced from the loudspeaker. This combined reference signal is eventually picked up by an array of microphone units. The direction of the coupling is detected from the difference in the arrival time of the reference signal between these microphone units. To lower the sensitivity of the microphone array for the particular direction, the directivity of the microphone array is controlled by individually delaying the outputs of each of the microphone units and then combining them as the output of the microphone array.