INVERTED CARDIOID TOPOLOGY FOR MULTI-RADIUS SPHERICAL MICROPHONE ARRAYS

The design of microphone arrays yielding constant directivity over wide bandwidths produces a tradeoff between narrow element spacing for aliasing performance at high frequencies, and wide spacing for white noise gain performance at low frequencies. In the design of spherical microphone arrays, free-space cardioid microphones allow embedding of spheres at multiple radii, providing constant directivity over wider bandwidths. The physical size of the microphone capsules and cabling limits the practical lower bound on sphere radius and consequentially how high the resulting aliasing frequency can be pushed. This paper proposes an inverted cardioid topology in which the main lobes on the inner sphere capsules face radially inward, instead of outward, allowing denser packing of elements. Practical measurements on a dual-sphere design, targeting third-order soundfield capture, investigate the self-shadowing behavior and show near constant modal beamformer directivity between 400 Hz and 8 kHz.