Effects of the source location on numerical biosonar beampattern predictions for bat noseleaves

Abstract

Approximately 300 bat species are known to emit their ultrasonic biosonar pulses through the nostrils. In these animals, ultrasound is produced by the larynx, propagates along the vocal tract, exits through the nostrils, and is finally diffracted by intricately shaped baffle structures known as “noseleaves.” Noseleaf geometry determines diffraction and hence the spatial distribution of the emitted ultrasound. As a consequence, numerical predictions of the noseleaves' acoustic function can be made based on the digital models of noseleaf shape. To limit model size and computational effort associated with numerical beampattern predictions, the vocal tract is often only partially included in these models or left out completely. In order to investigate the effect of source placement within a complete or partial vocal tract attached to a noseleaf shape on the numerical beampattern prediction, the noseleaf of the Great Roundleaf Bat (Hipposideros armiger) was studied as a model. Numerical beampattern estimates were obtained for a single monopole source positioned near the vocal folds or closer to the nostrils. Two monopoles sources placed in each nostril were also investigated. It was found that source positioning could impact the beampattern whenever they broke the symmetry in the near-fields of the two nostrils.