DEVELOPMENT AND ASSESSMENT OF 2 FIXED-ARRAY MICROPHONES FOR USE WITH HEARING-AIDS

Hearing-impaired listeners often have great difficulty understanding s peech in situations with background noise (e.g., meetings, parties). C onventional hearing aids offer insufficient directivity to significant ly reduce background noise relative to the desired speech signal. Base d 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 th at a directivity index of more than 10 dB can be obtained at the highe r frequencies. Simulations were verified with free-field measurements. To investigate the influence of the human head on directivity, two po rtable models were also tested with a KEMAR manikin. The measurements show that the two models give an improvement of the signal-to-noise ra tio of approximately 7 dB in a diffuse background noise field compared with an omnidirectional microphone. For the clinical assessment of th ese microphone arrays in the diffuse noise field (simulating a cocktai l party situation), the speech-reception threshold in noise for simple Dutch sentences was determined with a normal single omnidirectional m icrophone and with one of the microphone arrays. The results of monaur al listening tests of 30 subjects with normal hearing and 45 subjects with hearing impairment show that the microphone arrays give a mean im provement of the speech reception threshold in noise of about 7 dB com pared with an omnidirectional microphone.