Multimicrophone adaptive beamforming fo reduction in hearing aids *

To reduce interference in monaural hearing aids from sound sources that are spatially separated from a target source, we are investigating methods for combining information from multiple microphones . In this paper, we describe an adaptive beamforming method that functions to preserve target signals arriving from straightahead of a microphone array while minimizing output power from off-axis interference sources . In a preliminary evaluation of a two-microphone system, sentence intelligibility tests were administered to normal-hearing subjects using processed and unprocessed materials from simulated environments in which the target was on-axis, the interference (speech babble) was 45 degrees off-axis, and the reverberation mimicked that of a living room, a conference room, and anechoic space . Compared to listening through a single microphone, the two-microphone beamformer reduced the target-to-interference ratio required to achieve 50 percent keyword intelligibility by 30, 14, and 0 dB in the anechoic, living-room, and conference-room conditions, respectively . The corresponding improvements over binaural listening (one microphone to each ear) were 24, 9, and 0 dB . Further tests in the living-room environment using the same beamforming system but with filter impulse responses shortened by a factor of four (which would decrease the adaptation time by a factor of four) decreased the improvement by 5 dB . These results are sufficiently encouraging to warrant further tests involving more realistic reverberant conditions, multiple sources of interference, and time-varying acoustic environments. This research was supported by a grant from the National Institute of Neurological and Communicative Disorders and Stroke, Grant No. RO1 NS21322 . INTRODUCTION It is well known that impaired listeners have difficulty hearing in acoustic environments that contain background noise and reverberation . For many of these listeners, the underlying causes of this difficulty are not yet fully understood . However, for listeners who perceive the acoustic world monaurally, at least one major cause is obvious . Because such listeners are sampling the acoustic environment at only one point in space, their ability to localize sound sources, and to selectively attend to one of them, is seriously degraded. The importance of sampling the acoustic environment at more than one point in space is indicated not only by straightforward physical considerations, but also by the wealth of data on the advantages of binaural hearing in humans (2, 9, 16, 18, 20, 21, 24) and by the ubiquity of spatially diversified sensing systems in all sense modalities and all animals. Most previous attempts to reduce the effects of environmental noise using single-microphone systems have failed to improve speech intelligibility (19), even when reverberation is excluded . Given that interfering sound sources are almost always spatially separate from a target source, processing that uses a spatially-extended sensing system to exploit directional differences can help separate target and interference signals for presentation through a hearing aid. In this paper, we report briefly the results of