Range-dependent characteristics in the head-related transfer function of a bat-head cast

Kim, S.Y., Allen, R. and Rowan, D. (2009) Range-dependent characteristics in the head-related transfer function of a bat-head cast. British Society of Audiology Short Papers Meeting on Experimental Studies of Hearing and Deafness, , Southampton, United Kingdom. 16 - 17 Sep 2009. pp. 124-125 .

Abstract

It has been reported that a bat changes bandwidth and duration of its echolocating call as it approaches a target. Adaptive change in the echolocating calls has been well explained in previous studies that broadening the signal bandwidth enables better range accuracy and decreasing the signal duration prevents overlapping of the emitted and reflected signal in nearby distance from the target. However, the range-dependent adaptive change in emitted signals also implies that the reflected signals reaching the two ears (i.e. binaural hearing) change in gain and frequency properties as the distance between the bat and the target varies. This study aims to investigate the effect of the bat’s head on the change in received signals at the two ears at various distances by measuring the head-related transfer function (HRTF) of a bat-head cast implemented in experimental rig. It is to provide insights to range-dependent binaural information regarding the adaptive change of the echolocating calls. In this study, the HRTFs are measured for sources at distance from 0.0625 to 1 m using broadband noise. Based on the classic theory of HRTF, the transformation of the received signal at each ear is expected to be purely physical depending on the dimension of the bat-head cast and the distance between the head and sound source. The results of measured data show that relatively higher gain at low frequencies (below 10 kHz) is observed than that at high frequencies (above 10 kHz) as the bat-head cast approached the sound source. It is also noted that interaural level differences (ILDs) at a fixed distance have less sensitive changes at low frequencies than at high frequencies as the angle of the source direction changes in the frontal axis. However the sensitivity of the ILDs at low frequencies increase more that at high frequencies as the range gets smaller. Overall, it is concluded that the low frequency implies a more significant role during the target approaching stage in echolocation. Also, the systematic change in sensitivity of the ILDs in various ranges suggests that the bat might be able to calibrate the angular resolution by broadening the bandwidth at low frequencies.

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