ON THE POTENTIAL OF FIXED ARRAYS FOR HEARING-AIDS

Microphone arrays with fixed (time-invariant) weights are directed at enhancing a desired signal from one direction (straight ahead) while a ttenuating spatially distributed interference and reverberation. Using the theory of sensitivity-constrained optimal beamforming [Cox et al. , IEEE Trans. Acoust. Speech Sig. Process. ASSP-34, 393-398 (1986)], f ree-field arrays of head-sized extents were studied. The key parameter s affecting array design and performance are the set of transfer funct ions from the target direction to each array microphone [H(f)] and the intermicrophone cross-spectral densities for isotropic noise [S(zz)(f )]. Design variables included the orientation of the array, the number , and [as motivated by Soede, Ph.D. thesis, Delft University of Techno logy (1990)] the directionality of the microphones within the array, a nd the complexity and robustness of the required processing. Performan ce was characterized by the broadband intelligibility-weighted directi vity (gain against isotropic noise) and noise sensitivity (reflecting the array's sensitivity to uncorrelated noise, as well as device toler ances). For broadside orientation, a variety of arrays based on cardio id and hypercardioid microphones gave very similar performance. They c an provide directivities of 7-8 dB with easily implemented weights (si mple scalars). For endfire orientation, as Soede (1990) recognized, si milar directivities result with weights based on analog gains and pure time delays. However, with weightings chosen independently for each f requency, directivities up to approximately 11 dB may be obtained, alt hough the increased noise sensitivities of these arrays require practi cal evaluation. Because of sound diffraction, placement of arrays onto the head potentially impacts both their design and performance. In-si tu measurements of H(f) and S(zz)(f) as well as simplified theoretical models are suggested to explore the optimization of head-mounted arra ys.