The acoustic trachea of Tettigoniids as an exponential horn: Theoretical calculations and bioacoustical measurements

Abstract

The morphology and acoustic characteristics of the acoustic tracheal system were examined in certain tettigoniid species. Morphological measurements and statistical analysis reveal that in all species of bushcrickets investigated the shape of the acoustic trachea can be approximated by the equation of an exponential horn. Based on this approximation the transmission functions of the different tracheae were calculated theoretically. Because of its small size, the acoustic trachea must not be treated as an infinite exponential horn but its transmission function must be calculated by means of the equations for a finite-length horn. The finite horn amplifies sound from a certain frequency on (cutoff frequency) in a broad range of frequencies as the infinite horn does. But the broadbanded transmission is superposed by a few resonances which are caused by reflections inside the horn. Bioacoustical measurements were made with a probe microphone. As expected from the theoretical considerations the course of the measured transmission function corresponds much better with the one calculated for the finite exponential horn than with the one calculated for the infinite horn. The present results are discussed referring to the consequences of the transmission properties of the acoustic trachea for the hearing ability of Tettigoniids. Additionally, they are compared with other investigations concerning the function of the acoustic trachea in tettigoniid hearing.