Numerical Analysis Of A Biomimetic Microphone Baffle Model Based On Bat Ears

With evolution and natural selection, most bats have developed an ultrasonic echolocation system with high performance, so called biosonar, which comprises three primary parts:signal sender (mouth or nostril), signal receiver (external ear) and signal processor(brain). Around 300 bat species of the over thousand species ever known are found emitting their ultrasonic biosonar pulses through the nostrils and listen to the returning echoes. The signal sender of a bat is usually surrounded by complicated wrinkles (noseleaf) whose position and structure are considered able to change the beamforming of ultrasonic pulse. Experimental studies have suggested noseleaf really has complicated acoustic function. However, the variability and diversity of biologic structure or seif-adjustability, coupled with the limitation of bionic work condition, the former studies are just qualitative physical analysis quite a few phenomena based on observation and rough survey. In fact, there are varieties of bats in the world, but many species have not been studied yet. Study on woolly horseshoe bat (Rhinolophus luctus) is restricted to its sound spectrum, and no further works have down; this thesis studies the acoustic function of noseleaf structure and outer ear of a woolly horseshoe bat.This thesis makes acoustic study of the noseleaf and external ear of a woolly horseshoe bat by numerical implementation for the first time. It mainly analyzes the sound field distribution of sender and receiver, analyzes the acoustic function of flap, a unique structure on the noseleaf, of the woolly horseshoe bat, and for the first time combines the physical acoustic with the biological behavior to interpret the foraging behavior from the perspective of the sensitive directivity of the bat's emitting soundfield. It also presents a bionic bat ear model based on the study of the acoustic function of outer ear and the practical echolocation process of flap, designs and looks into two models with and without flap, puts emphasis on the acoustic function of the bionic model with flap in receiving ultrasonic pulse, and discusses the effect of bionic model's parameters on far field. The models can be applied to be an ektexine of device such as microphone or transducer in order to adjust the direction and strength in ultrasonic echolocation at low frequency, so as to detect the area beyond the primary detection orientation.The main research works and results are:1. Acquisition of original biological data. In experiment, high-resolution X-ray micro CT machine is adopted to scan samples of noseleaf and outer ear of a woolly horseshoe bat to get original digital shadow images. Then the cross-section view of the noseleaf and outer ear are worked out by the method of cone beam reconstruction. These images are then processed through Gaussian filter and boolean process, and images with clear definition of air and biological tissue could be got. With three-dimensional digital image processing technique, three dimensional digital images attribute of the woolly horseshoe bat's noseleaf and outer ear are acquired, and then numerical calculated by the finite element method.The numerical calculation shows that the constant frequency(CF) and frequency modulated(FM) play difference roles in the emitting sound field of Rhinolophus luctus. There are two mainlobe and function of frequency driven scanning in constant frequency band, the sound field focuses on searching and judging the characteristics of target; there are multiple lobes in frequency modulated band, and the sound field distribution changes with frequencies, mainly focus on demarcating the target's orientation. The conclusion is consistent with the experimental result of the biological behavior.2.With the three-dimensional image technique, two kinds of comparison are gotten after eliminating the flap from noseleaf; refer to the noseleaf s movement in the video of the woolly horseshoe bat, rotates the flap at a certain degree by same technology, and a series of static condition are gotten in the way to simulate dynamic process. Near field distribution of sound pressure and phase are simulated using same finite element processing, then the far-field sound pressure distribution could be gotten by Kirchhoff integral under the premise of no attenuation, a basis for analyzes the acoustic role of flap.The numerical results show that flap is very important on the constant frequency band, it affects the beamforming in far field, and is one of main components to format the frequency driven scanning; flap rotating can change the distribution of sensitive directivity of the bat sound field to help make better use of energy.3. The results suggest, the receiving sound field of Rhinolophus luctus corresponds to the emission performance, represented by the significantly different sound field distribution in CF and FM band.There has two mainlobe in CF band and only one mainlobe in FM band, and the distribution of sound field in CF band has obvious phenomenon of adaptation to the Doppler effect. But there is only one mainlobe in receiving field in FM band, different from the emitting field that has multiple lobes which change greatly with frequencies.4.Based on digital image technology and the acoustic characteristics of the Rhinolophus luctus, a variable simple obliquely truncated bionic ear model is designed. The simplified model could provide prospective improvement of the performance of wireless positioning.Numerical results of Bionic bat ear model show that the characteristic of frequency scan can be achieved by a simplified artificial ear model, and can have better performance if we adjust the parameters carefully. The bionic model with flap can meet the requirement of more complex distribution in sound field, flap can significantly change the sound field distribution of the model, presenting a powerful acoustic role. Double flaps can effectively restrain the sidelobes and regulate mainlobe, so have strong application value. The beneficial sound field distribution in acoustic appears only when the parameters have value within a certain range. The sound field of changed flap offers idea for designing the adaptive wall of device such as microphone or transducer.