Numerical Research On The Acoustic Features Of The Vocal Tract In Horseshoe Bat

Humans get inspiration from biological systems, first, we need to study the biological and technical devices whether there are common characteristics. Bat is a very common species in mammal and as a general creature in bionics research. In its adaptation to the natural evolutionary process, bats evolved a highly efficient ultrasonic echolocation system. Research results accumulated by our research group at Shandong University over the past years, together with some additional clues from research conducted elsewhere, strongly indicate the ubiquity of time-variant physical processes in the biosonar system of horseshoe bats. In particular, indications of such processes have been found in key physical mechanisms at the stages of ultrasound generation, ultrasound emission, as well as reception.Bats produce their ultrasonic pulses following a standard mammalian mechanism that consists of a fluid-structure interaction between pairs of membranes and a pressured air stream produced by compression of the lungs. Nevertheless, the ultrasound generation of bats stands out by virtue of a unique and hitherto unmatched combination of features that includes miniaturization, high output amplitudes, as well as repeatability and reliability. All these features are the outcome of time-variant physical processes that are evident from the time-frequency patterns of the biosonar signals. It is widely believed that the frequency modulations that are seen in the biosonar pulses of bats are the result of a varying tension of the vocal folds. Furthermore, it has been shown already that bats use a special types of fast muscles in order to be able to vary the tension of the vocal folds within the duration of their biosonar pulses, i.e., on a time scale of milliseconds. Hence, ultrasound in bats is generated by time-variant physical processes and as the resulting time-variant wave packets travel through the remainder of the biosonar communication channel, they are likely to trigger more time-variant effects. Hence the sound generation mechanisms can be seen as the first in a cascade of time-variant mechanisms, some of which are triggered by upstream time-variant behaviors and others which have independent sources but may still interact with time-variant effects passed down to them.Bat biosonar systems include ultrasound generation, transmission and reception. Because of the importance of ultrasound production as a source of time-variant behavior, in order to investigate the physics of the bat ultrasound generation, we have created accurate digital models of the bat vocal tract and larynx geometry (through micro-CT scans) to study the procession of the ultrasonic generation. The ultrasonic waves generated in the vocal tract spread with a filtering function, the internal vocal tract having a complex structure, inside each of the cavity both have a transmission of ultrasonic action.Unlike traditional biological methods to study the bat sonar system,we used numerical analysis to study the acoustic role of the vocal tract in horseshoe bat, numerical analysis is a new method with the computer technology and the mathematical modeling combination, has been widely used in the simulation calculations. The most critical aspect of numerical calculation method is to construct the model for research object, first of all, we use X-ray micro-CT scanner to scan the sample of the bats, the bat’s tomographic images obtained through a reconstruction algorithm, a stack of cross-section images are derived by the three dimensional cone beam reconstruction method. The tomographic images contain the bat’s internal information. They are pre-filtered by an isotropic Gaussian smoothing kernel and threshold to classify pixels as representing either air or bat’s tissue. We can get3D numerical model of the vocal tract using VTK modeling. The different gray values in3D digital model of the vocal tract represent different bat’s tissue, using Boolean logic to change the voxel’s value to filled the cavity within the vocal tract, we can get the different internal structure of bats. Secondly, the grid structure can be used for3D digital in finite element method. We use the FEM method to solve the Helmholtz equation of the sound field and get the near sound field distribution in the vocal tract. By analysis the changes of the sound pressure in the internal vocal tract, we can get the acoustic in the vocal tract of horseshoe bat.The results of these experiments suggest several important conclusions regarding the acoustical properties of the vocal tract in horseshoe bat. First, the vocal tract acts as a filter which can selectively attenuate further propagation of the sound at the second harmonic along the main channel. The transfer function of the intact vocal tract indicates that most energy is at the second harmonic and its center frequency is accord with the sonar signal. Second, our data suggest that the tracheal chambers may act as a Helmholz resonator tuned to attenuate the second harmonic and they do this only for tracheal sound. In this position, a Helmholz resonator may return the acoustic energy entering it at its resonant frequency to the main channel. The tracheal chambers have the effect of the acoustical impedance. They can attenuate the sound in the trachea and prevent it from being reflected from the lungs back towards the cochlea. It may be important to prevent the sound from stimulating the cochlea and protect the bat’s auditory system. Taken together, the results of these experiments suggest that the vocal tract act as a filter in the sound propagation.