马来假吸血蝠耳屏声学特性的数值研究

Numerical study on the acoustic characteristics of the tragus in a lesser false vampire bat,Megaderma spasma

马来假吸血蝠通过结构复杂的耳廓结构接收回声定位脉冲信号.利用高分辨率X射线微型CT机对马来假吸血蝠耳廓结构进行扫描,得到其耳廓结构的断层扫描图像,通过高斯滤波器和二值化处理,并利用三维数字结构重构技术获得耳廓结构的三维数字模型.手工处理这种数字结构,得到原始耳廓结构、无耳屏耳廓结构的三维数字结构.运用数值计算的方法,得到两种耳廓结构不同频率下的近场声压幅度分布图和远场接收方向性波瓣图,比较不同耳廓结构对近场声压分布和远场辐射的影响.结果表明:耳廓内的耳屏对频率有选择性的影响,在频率为50kHz和70kHz处,耳屏结构对近场声压分布的影响最大;对于远场接收方向性波瓣图,低频率和高频率时,耳屏的作用是拆分主瓣,中频时则合并旁瓣.

The lesser false vampire bat,Megaderma spasma has complicated pinna structure with which the bat receives the echolocation pulse.A comprehensive acoustic characterization of the spatial sensitivity of the tragus in the pinna from a lesser false vampire bat has been obtained with the means of numerical methods and visualization techniques.Hundreds of cross-section images of the pinna shape were acquired through a high-revolution x-ray mic-rotomography.In order to clear away the glitches and noises,cross-section images were processed by an isotropic Gaussian smoothing kernel and binarization processing.The modified images were converted into another digital rep-resentation.This kind of digital representation was composed of cube voxels.The original and other modified pinna shapes of the lesser false vampire bat were obtained by manually handling these voxels.The first one was the original structure of the pinna in lesser false vampire bat.Compared with the original structure,the second one had no tragus in pinna.Furthermore,setting up a finite-difference time-domain model of diffraction from which directivities were predicted by virtue of forward wave-field projections based on a Kirchhoff integral formulation.Digital shape manipulation was used to study the role of the tragus in detailed numerical experiments.We show that the tragus in the pinna authentically had an impact on the acoustic near-field sound pressure magnitude as well as the far-field di-rectivity patterns.In the effects on the near-field sound pressure magnitude,the most significant differences between the present and absence of the pinna appeared at the frequency of 50 kHz and 70 kHz.With the absence of the tragus in the pinna,changes in the far-field directivity patterns are the number of side lobs decreased in the range of low fre-quencies and high frequencies,however it’s obvious to see some side lobs combine into a main lob in the intermediate frequency.In addition,the tragus in the pinna from the lesser false vampire bat expresses clear and definite frequency selectivity.