Effects of geometry dependent diffraction on the performance of object-mounted Linear Differential Microphone Arrays

Effects of geometry-dependent diffraction on the performance of object-mounted Linear Differential Microphone Arrays (LDMAs) for far-field sound waves are investigated through the Finite Element Method. This is done by studying the acoustic pressure gradients corresponding to different orders of LDMAs and placement (azimuthal and radial) on an object (such as human head or voice activated devices). An optimized frequency-dependent adaptive-dimension finite element technique, innovated by the authors, is used to simulate acoustic scattering for two simple head-model geometries (sphere and ellipsoid). These simulations are conducted for frequencies ranging from 20 to 20 000 Hz. Verification of results is done by comparisons to analytical solutions (for the sphere) and literature data. Results show that object-mounted second order radial LDMAs are greatly affected by object geometry. Maps of directivity patterns show that object-mounted first order azimuthal LDMAs completely attenuate sound from sources located at the sides of the listener. A modified LDMA signal processing method is proposed to lessen the effects of scattering for object-mounted first order azimuthal LDMAs by incorporating frequency-dependent weighting functions. The findings of this article provide an understanding of using object-mounted LDMAs for designing directional sound-sensing devices.