Numerical Research on the Acoustic Features of the Vocal Tract in a Horseshoe Bat, Rhinolophus Hipposideros

Ultrasonic system is the major feature of the most bats and the vocal tract of the bats play an important in the system. Most prior study in the vocal tract of the bats are mainly based on the biology approach. In this paper, we use the numerical method to investigate the acoustics role of the vocal tract in a horseshoe bat, Rhinolophus hipposideros. 3D digital model of the horseshoe bat vocal tract is constructed by using tomography scanning technique, and then the sound field distributions in nostrils are figured out by solving the Helmholtz equation based on the finite element method. The horseshoe bat vocal tract has very complex structure and the acoustic role of the nasal cavities and rigid tracheal chambers in the vocal tract was investigated by determining the effect of their selective filling on the nasal emitted sonar pulse and on sound traveling backwards down the trachea. The result demonstrated that (a) the transfer function of the vocal tract was consistent with the real sound spectrum which has a peak at the second harmonic, (b) filling the dorsal and both lateral tracheal chambers had little effect on the emitted vocalization, but caused the level of the second harmonic component and (c) filling nasal chamber had no effect on the tracheal sound but the effect of this treatment on the nasally emitted sonar pulse was changed.