Numerical Study On Beamforming Shapes In A Lesser False Vampire Bat And A Rickett’s Big-footed Bat

Bats have a mature and sensitive biological sonar system after long time evolution, all of the Microchiroptera bats can use it for location, predation or environment detection. Ultrasonic produced by the laryngeal vocal cords and spread outward through oral or nasal, then sounds meet with obstacles will be reflected and received by bat’s ears. The present study results on experimental and numerical shows that some apparatuses (e.g. noseleaf etc.) on bats head can influence the acoustic emission in the process of ultrasonic spread outward. In this paper, we use numerical method to analysis the influence of two parts (lancet and anterior leaf) in the noseleaf of a Lesser vampire bat on the emission sound field, as well as different mouth opening angle of a Rickett’s big-footed bat.The research work and methods as follows:(1) Capture bats samples and construct three-dimensional digital structure. The bats samples are collected in the outdoor, and then obtain digital representations of the shapes of the noseleaf sample from the Lesser vampire bat and head sample from the Rickett’s big-footed bat by means of high-resolution X-ray computer tomography. From these X-ray images, stocking of tomographic cross-section are derived by a Feldkamp’s cone-beam reconstruction algorithm, and pre-filtered using an isotropic Gaussian smoothing kernel and then thresholded to obtain binary images, superimposing these binary images gains three-dimensional digital structures which consist of voxels. Two parts of noseleaf of the Lesser vampire bat are eliminated respectively by means of computer image processing technology and get three shapes of noseleaves eventually, ie. primary noseleaf, noseleaf without lancet and without anterior leaf. For the Rickett’s big-footed bat, we get five digital structures of heads which have different mouth opening angle through rotate the mandible position.(2) The calculation of the acoustic field. Add the excitation source in proper position based on all structures, and whole calculation field can detach into two fields that are near-field and far-field. Near field by using finite-difference time-domain method and meshing the whole field, the side-length of grid cells are equal to voxels of digital structures, at same time we should set up the perfectly matched layer to imitates open field. The results of far-field are using the Kirchhoff integral formula based on results of near-field. Results of near-field are represented by three mobile and mutually perpendicular plane, different colors represent different field amplitude. The results of Far-field represented by radiation pattern, and the far-field amplitude unrelated to the shape of the patterns, here we only care about the radiation pattern shape.(3) The analysis of numerical results. Quantitative analysis of the calculated results needs through the directional correlation coefficient and the directivity index of radiation pattern. Finally, discussion of the calculation results may need to use the existing physical theory and found simple corresponding relationship by the complex relationship.The findings of the research work are as follows:(1) The two parts (lancet and anterior leaf) of noseleaf in a Lesser vampire bat have significant influence on the sound radiation pattern. The two parts plays a different role in the radiation beam and influence sound field in different frequency range:lancet can influence the scope of radiation pattern and make it smaller in lower frequency range; anterior leaf can change the directivity of radiation pattern in higher frequency domain.(2) The mouth opening angle of the Rickett’s big-footed bat has influence on the acoustic field. The directivity index of pattern is increases with the increase of frequency under the same mouth opening angle; while the directivity index can increases with the increase of the mouth opening angle under the same frequency. That is to say, the Rickett’s big-footed bat can control the directivity of radiation field through changed mouth opening angle.