Studies Of Sound Propagation In The Acoustic Trachea: An Experimental, Anatomical And Numerical Approach

Bush-crickets (Ensifera: Tettigoniidae) rely on the perception of sound to detect and localise predators and potential mates, and this has led to the development of complex ears. This is not confined to bush-crickets, and a variety of sound detection and localisation mechanisms have arisen in other ensiferans and tetrapods. This thesis aims to summarise the literature across these two groups to provide an overview of auditory anatomy and directional hearing. Bush-crickets possess ears in their forelegs to detect and localise sound predators and potential mates. Each ear consists of two tympanic membranes which are exposed to sound both externally, where sound transmits to the ear through the environment, and internally, via an ear canal derived from the respiratory system. As sound propagates through the ear canal it reduces in velocity, causing a time delay between the arrival of the internal and external input. The delay was suspected to arise as sound propagation changes from adiabatic to isothermal, caused by the ear canal geometry. If true, then the reduction in sound velocity should persist independently of the gas composition in the ear canal. This method was first simulated on a simplified plastic model of the ear canal, formed by a linear tube with an opening at one end for sound input, and a balloon membrane at the other for sound reception. A probe-loudspeaker was used to project a signal into the linear tube, and laser Doppler vibrometer recorded the arrival time of the signal at the membrane. A reduction in sound propagation velocity was observed in the linear tube. The sound propagation velocity through air and carbon dioxide was also quantified. Experiments were then conducted on specimens of Copiphora gorgonensis. By integrating laser Doppler vibrometry, micro-CT scanning, and numerical analysis on 3D geometries of each experimental animal ear, we demonstrate that the narrowing radius of the ear canal is the main factor reducing sound velocity. The numerical simulations of the sound propagation use the precise 3D geometry of the ear canal and take into account the viscous and thermal boundary layers formed near the wall of the ear canal; whose thickness also depend on the tube radius. Likewise, the ear canal is asymmetrically bifurcated at the tympana organ location (one branch for each tympanic membrane) creating two additional internal sound paths and imposing different sound velocities for each tympanic membrane. Therefore, external and internal inputs add up to four auditory paths for each ear (to compare, only one for humans). Implication of findings in avian directional hearing and potential applications in acoustic triangulation devices are discussed.