Development and assessment of two fixed-array microphones for use with hearing aids

Hearing-impaired listeners often have great difficulty understanding speech in situations with background noise (e.g., meetings, parties). Conventional hearing aids offer insufficient directivity to significantly reduce background noise relative to the desired speech signal. Based on array techniques, microphone prototypes have been developed with strongly directional characteristics to be incorporated into the frame and the "temples" of a pair of eyeglasses. Particular emphasis was on optimization and electronic stability. Computer simulations show that a directivity index of more than 10 dB can be obtained at the higher frequencies. Simulations were verified with free-field measurements. To investigate the influence of the human head on directivity, two portable models were also tested with a KEMAR manikin. The measurements show that the two models give an improvement of the signal-to-noise ratio of approximately 7 dB in a diffuse background noise field compared with an omnidirectional microphone. For the clinical assessment of these microphone arrays in the diffuse noise field (simulating a cocktail party situation), the speech-reception threshold in noise for simple Dutch sentences was determined with a normal single omnidirectional microphone and with one of the microphone arrays. The results of monaural listening tests of 30 subjects with normal hearing and 45 subjects with hearing impairment show that the microphone arrays give a mean improvement of the speech reception threshold in noise of about 7 dB compared with an omnidirectional microphone.