Abstract
Under some conditions of background noise and/or reverberation, increasing the directionality of hearing‐aid microphone systems can lead to gains in speech intelligibility. Conceptually, directional microphone systems are realized by combining the outputs of multiple, spatially distributed omnidirectional sensors. Two approaches to obtaining increased directivity D derive from the classical antenna theory: (1) With uniform or tapered sensor weightings, increasing the spatial extent L of the sensor array increases D. The maximum D is limited to 2Lf/c, where f is frequency and c is the speed of sound. (2) With arbitrarily small arrays, broadband “superdirectivity” can be achieved whereby the maximum D is independent of frequency and equals the square of the number of sensors in the array. A conventional hearing‐aid directional microphone exploits suboptimum superdirectivity; two closely spaced sensors attain D = 4.5 dB up to about 4 kHz. We shall review the basic principles and limitations of such microphone systems and report on our effort to develop a four‐sensor array (using two conventional directional microphones) that achieves D = 8.5 dB and a 3‐dB beamwidth of ± 37° for f < 4 kHz with an array length L = 4.1 cm that is practical for hearing‐aid applications. [Work supported by NIH.]
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