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.

Motor and mechanical bases of the courtship call of the male treehopper Umbonia crassicornis.

This study is a physiological, anatomical and biophysical analysis of how plant-borne vibrational signals are produced by the treehopper Umbonia crassicornis During courtship, males and females engage in a vibrational duet, with each producing a characteristic call. For males, this consists of a frequency-modulated tonal signal which is accompanied by rhythmic broad-band clicks. Although previous studies have described these complex signals in detail, little is known about how they are produced. By combining video recordings, electromyograms, dissections and mechanical modeling, we describe the mechanism by which the male produces his courtship signal. High-speed videos show that the tonal portion of the call is produced by periodic dorso-ventral movements of the abdomen, with a relatively large amplitude oscillation alternating with a smaller oscillation. Electromyograms from the muscles we identified that produce this motion reveal that they fire at half the frequency of the abdominal oscillation, throughout the frequency modulation of the tonal signal. Adding weight to the abdomen of a calling male reduces the frequency of motion, demonstrating that the abdominal motion is strongly influenced by its mechanical resonance. A mathematical model accounting for this resonance provides excellent qualitative agreement with measurements of both the muscle firing rate recorded electrophysiologically and the oscillatory motion of the abdomen as recorded in the high-speed video. The model, electromyograms and analysis of video recordings further suggest that the frequency modulation of the abdominal response is due to a simultaneous modulation in the muscle firing rate and a fluctuation in stiffness of the abdominal attachment.