Abstract

Bush-crickets have tympanal ears located in the forelegs. Their ears are elaborated as they have outer, middle and
inner ear components. The outer ear comprises an air-filled tube derived from the respiratory trachea, the acoustic trachea (AT), which transfers sound from the prothoracic acoustic spiracle to the internal side of the ear drums in the legs. A key feature of the AT is its capacity to reduce the velocity of sound propagation and alter the acoustic driving forces of the tympanum (the ear drum), producing differences in sound pressure and time between the left and right sides, therefore aiding the directional hearing of the animal. It has been demonstrated experimentally that the tracheal sound transmission generates a gain of approximately 15 dB and a propagation velocity of 255 m/s, an approximately 25% reduction from free-field propagation. However the mechanism responsible for this change in sound pressure level and velocity remains elusive. In this study, we investigate the mechanical processes behind the sound pressure gain in the AT by numerically modelling the tracheal acoustic behaviour using the finite element method and real 3D geometries of the tracheae of the bush-cricket Copiphora gorgonensis. Taking into account the thermoviscous acoustic-shell interaction on the propagation of sound, we analyse the effects of the horn-shaped domain, material properties of the tracheal wall and the thermal processes on the change in sound pressure level in the AT. Through the numerical results obtained it is discerned that the tracheal geometry is the main factor contributing to the observed pressure gain.