RESTRICTED GEOMETRY ACOUSTIC ARRAYS FOR HIGHLY DIRECTIONAL PATTERNS

We consider the design and performance of a restricted geometry, 2- or 4-element (termed 'small') acoustic array as a beamformer. These arrays are designed to be mobile, for individuals who need to communicate in noisy environments. An acoustic array is made as directional as possible (thus 'forming the beam'), so as to reduce (or eliminate) any unwanted narrowband acoustic signals which impinge on the array from undesirable directions. The directionality of multi-element arrays, and thus the capability for interference rejection, is greatly superior to that possible with a single-element array. Enhanced directionality comes from the extra knowledge gained when acoustic signals are spatially sampled. 1.2 From intuition, we expect such a results since humans have 2 ears to hear with, and we do not have the 'extra' one merely for redundancy. That we know more about the incoming signal comes at the cost of a higher complexity in handling the signal itself. This added expense includes not only the cost of the added sensors, but also the cost of the interconnections required by the multi-element arrays. We introduce some measures of directionality, which we use to show that 4-element acoustic arrays are much more directional than 2-element arrays. A sensitivity analysis shows that for human speech, the narrowband beamformer can be implemented for speech-band use with only a slight decrease in the array directionality.