Numerical Study On Beamforming Emission Baffle Shapes In A Horseshoe Bat

Around 300 bat species are known to emit their ultrasonic biosonar pulses through the nostrils and listen to the returning echoes. This nasal emission coincides with the presence of intricately-shaped baffle structures -so-called "noseleaves" - which surround the nostrils. The two largest groups of bats with noseleaves are the New World Spear-nosed Bats (Phyllostomi-dae) and the Old World Horseshoe Bats (Rhinolophidae). While noseleaves have been frequently hypothesized to affect the shape of the animals' radiation patterns and some prior experimental evidence indicates that these noseleaves have such an effect. The experimental pilot data collected so far has been scarce and limited to observations from coarse, poorly controlled manipulations. The noseleaf studied here belongs to a horseshoe bat (Rhi-nolophus rouxi). The acoustic effects of the three parts ("lancet", "sella", "anterior leaf") of the noseleaf as well as the effect of the furrows in the lancet have never been analyzed.In this thesis a first numerical acoustical analysis of the noseleaf of Rhi-nolophus rouxi is presented. It is shown that all three distinctive parts of its noseleaf have effects on the acoustic near-field as well as on the directivity pattern. Furthermore, it is demonstrated that furrows in one of the parts (the lancet) also exert such an influence. The underlying physical mechanisms suggested by the properties of the estimated near-field are cavity resonance, as well as reflection and shadowing of the sound waves emitted by the nostrils. In their effects on the near-field, the noseleaf parts show a tendency towards spatial partitioning with the effects due to each part dominating a certain region. However, interactions between the acoustic effects of the parts are also evident, most notably, a synergism between two frequency-dependent effects (cavity resonance and shadowing) to produce an even stronger frequency selectivity. The research work and findings are as follows :1. A digital representation of the shape of a noseleaf sample from the horseshoe bat is obtained by means of high-resolution X-ray computer tomography. From these X-ray images, a stack of tomographic cross-sections is derived by a three-dimensional cone beam reconstruction method. Cross-section images coded with 256 gray values are pre-filtered using an isotropic Gaussian smoothing kernel and then thresholded to classify pixels as representing either air or noseleaf tissue. The resulting binary voxel representation of the noseleaf is transcribed into a finite-element mesh with linear cubic elements.2. Using suitable Boolean operators a complete set of all possible combinations of the presence or absence of each of the three parts is created. This set contains a total of eight shapes, the original shape plus seven shapes in which either one, two or all three parts have been removed. To answer the questions whether the furrows of the lancet have an acoustic impact, the air volumes inside the furrows are filled completely with hand-placed voxels representing noseleaf tissue. Using numerical analysis on these nine noseleaf structures we obtain the numerical estimate of the near-field magnitude and phase as well as far-field directivity.3. All studied parts of the noseleaf are found to have an effect on the acoustic near field as well as on the far-field directivity pattern. The same is true for the studied part shape feature, the furrows on the lancet :In the near field, a significant increase in the field magnitude is observed inside the lancet furrows. The results obtained here seem to indicate that both sella and anterior leaf work as shielding or reflecting baffles, which cause reflections on the side facing the source and cast shadows on the opposite side. The effect of the sella on the near field above it and hence its interaction with the lancet can be explained as the casting of a shadow on the lancet by the sella.In the far field, the acoustic effects of lancet, sella and anterior leaf stand out for their impact on the directivity pattern (lancet for low frequencies, sella and anterior leaf overall) and interaction with each other. The lancet in conjunction with the sella introduces a frequency-specific widening. The primary function of sella and anterior leaf appears to be an overall focusing of the beam.4. The finding that the lancet furrows are to a large extent responsible for the acoustic effect of this noseleaf part, demonstrates that structural details of the noseleaf parts should be considered as candidate features of potential acoustic relevance. It is shown that the horizontal furrows in the lancet act as half-open acoustic resonance cavities. Here, the spatial and spectral specificity of the increased sound field amplitudes required as experimental evidence for a cavity resonance has been demonstrated. Furthermore, the employed methods allow it to quantitatively describe the impact of the furrow resonances on the directivity function, the distribution of sound energy with direction, without any confounding additional changes to the geometry. In these results, the resonance amplitude correlates with the extent to which the directivity function is changed at different frequencies, which allows us to link the physical effect (cavity resonance) to the functionally relevant system property (the directivity function). This demonstrates that (a) animals can use resonances in external, half-open cavities to direct sound emissions, (b) structural detail in the faces of bats can have acoustic effects even if it is not adjacent to the emission sites, and (c) specializations in the biosonar system of horseshoe bats allow for differential processing of subbands of the pulse in the acoustic domain.For the lower furrow, it is noteworthy that two frequency selective effects. resonance and shadowing, reinforce each other and hence create an even stronger frequency-selective behavior.