Beamforming By Outer Ear Extensions In Bats

Skin extensions of pinna rim occur in several bat species like Nyctalus, Tadarida, Megaderma and some other genera. A particular set of flaps that broaden the pinna rim laterally are present in a group of bats belonging to the genus Megaderma. Directivity and sound diffraction of the pinna of the Megaderma spasma are studied to predict the the role of these outer ear extensions in the biosonar system.In order to characterize the acoustic function of a bat pinna fully, it is necessary to obtain a high-resolution estimate of its directivity pattern, i.e., the sensitivity of the ear as a function of azimuth, elevation, and frequency. For this purpose, geometrical data on the biological shapes has been collected and used as a basis for numerical simulations of the diffraction phenomena.Samples of ears of the bats are obtained from the 3 bat carcasses belonging to the species, Megaderma spasma. Hence, totally 6 pinna samples (3 left ears and 3 right ears ) are collected for the purpose of study. After the specimen is subjected to the fan beam scanning with X rays and then reconstructed to a stack of images using the cone-beam reconstruction algorithm. The tissue-air boundary in the cross-section images is clearly visible and the images are processed into the binary images to be meshed out for the later numerical simulations. The acoustical near-field in a immediate cuboid vicinity of pinna surface is got with the finite element method coupled with the infinite element method for representing the absorbing boundary conditions. The finite element results are forward projected into the free space and the far-field system properties such as the directivity pattern are estimated. The mesh representation of the pinna shape offers a ease that experimental shape alterations can be operated digitally such as the removal of the outer ear skin extensions in the Megaderma spasma. So both the near and far field of the manipulated pinna come out with the second simulation.The beam pattern of the Megaderama's pinna has one main lobe over the all frequency range and smaller side lobes start to turn out at higher frequencies. In general, the beam width decreases with frequencies. Lower frequencies have wider beam than higher frequencies consistent with their larger wavelengths. The beam shapes could be described in the form of the direction of the eigenvector which are calculated within the domain of the half power beam. It is found that the beam is narrower in elevation than in azimuth in a general view although they are not exactly aligned . Despite of the differences, there are similarities on the above features between beam patterns of individuals of 6 outer ear.Apart from the properties of the beam pattern in the original ear, the functional role of the skin extensions in the pinna rim is more important in this study. By removing the pinna rim extensions in a digital representation of the ear, it is found that the flap is responsible for narrowing the beam pattern of the ear in the direction in which it is added to the pinna. In fact, the skin extensions enlarge the aperture of the ear in horizontal direction. On the directivity map , the beam is wider in the azimuth direction at the same frequency than the original beam that results from the ear with the extension . This effect is seen over frequency. This is consistent with the general relationship between the diameter of an aperture and beam width: the larger the aperture, the smaller the achievable beam width. Moreover, this is a robust effect consistent seen between specimens. The 6 ear pinnae in this study conform that the outer ear extensions in the pinna rim of the Megaderma play the important role in the far field beamforming.The studied bats are predators who follow sounds generated by their vertebrate prey -so called passive sonar in the context of biosonar system in bats - it may hence be speculated that accurate sound localization at not too high frequencies is important to them. Narrow beam could be in important feature for active and passive sonar . Narrow beams would aid detection of weak signal (all sensitivity in one direction) and precise localization, but it is not good for being alerted to sounds from any direction, the bat needs to scan (with ears or head) to achieve this.