R. W. Stadler
W. M. Rabinowitz
Res. Lab. of Electron., MIT, Rm. 36-789, Cambridge, MA 02139
Microphone arrays with fixed (time-invariant) weights are directed at enhancing a desired signal from one direction (straight ahead) while attenuating spatially distributed interference and reverberation. Using the theory of sensitivity-constrained optimal beamforming [Cox et al., IEEE Trans. Acoust. Speech Signal Process. ASSP-34, 393--398 (1986)], free-field arrays of head-sized extents were studied. The key parameters affecting array design and performance are the set of transfer functions from the target direction to each array microphone and the intermicrophone cross-spectral densities for isotropic noise. Design variables included the orientation of the array, the number and [as motivated by Soede, Ph.D. thesis, Delft Univ. of Technol. (1990)] the directionality of the microphones within the array, and the complexity and robustness of the required processing. For broadside orientation, a variety of arrays based on cardioid and hypercardioid microphones gave very similar performance. They can provide broadband intelligibility-weighted directivities (direct-target gain against isotropic noise) of 7--8 dB with easily implemented weights (simple scalars). For endfire orientation, as Soede recognized, similar directivities result with weights based on analog gains and pure time delays. However, with weightings that are chosen independently for each frequency, directivities up to ~11 dB may be practical. [Work supported by NIH.]