Physics of directional hearing in the cricket Gryllus bimaculatus

In the cricket ear, sound acts on the external surface of the tympanum and also reaches the inner surface after travelling in at least three pathways in the tracheal system. We have determined the transmission gain of the three internal sound pathways; that is, the change of amplitude and phase angle from the entrances of the tracheal system to the inner surface of the tympanum. In addition, we have measured the diffraction and time of arrival of sound at the ear and at the three entrances at various directions of sound incidence. By combining these data we have calculated how the total driving force at the tympanum depends on the direction of sound. The results are in reasonable agreement with the directionality of the tympanal vibrations as determined with laser vibrometry.At the frequency of the calling song (4.7 kHz), the direction of the sound has little effect on the amplitudes of the sounds acting on the tympanum, but large effects on their phase angles, especially of the sound waves entering the tracheal system at the contralateral side of the body. The master parameter for causing the directionality of the ear in the forward direction is the sound wave entering the contralateral thoracic spiracle. The phase of this sound component may change by 130–140° with sound direction. The transmission of sound from the contralateral inputs is dominated by a very selective high-pass filter, and large changes in amplitude and phase are seen in the transmitted sounds when the sound frequency changes from 4 to 5 kHz. The directionality is therefore very dependent on sound frequency.The transmission gains vary considerably in different individuals, and much variation was also found in the directional patterns of the ears, especially in the effects of sounds from contralateral directions. However, the directional pattern in the frontal direction is quite robust (at least 5 dB difference between the 330° and 30° directions), so these variations have only little effect on how well the individual animals can approach singing conspecifics.