Prospects for digital microphone arrays

The practical application of microphone arrays benefits from the use of Micro-Electro-Mechanical (MEMs) devices. Many of these have digital outputs, which can be multiplexed and fed via serial lines into digital processors, which can implement beamforming and provide multiple outputs in a variety of digital or analog formats. MEMs microphones are low-cost, and it is now possible to economically meet the spatial sampling theorem up to appreciable audio bandwidths, at least for compact designs. Practical construction requires a solid baffle, which means that either a small enough baffle is used so that diffraction may be ignored or the diffraction must be compensated for in the beamformers. Simple geometric baffles such as a sphere allow the diffraction to be calculated analytically, but in general, the diffraction must be measured or modeled and compensated for. This paper presents two microphone array designs based on these principles. The first is a commercially available free-field “shotgun” microphone, which produces a highly directional beam, and a second complementary beam for subsequent interference cancellation. The second design is a prototype diffraction-compensated cubic array for recording 3-D sound. The array has 16 microphones per face and can produce fourth-order spherical harmonic outputs up to 7 kHz.