Development and application of an echolocation model inspired by bats

This study explores the principles of echolocation in bats which can be potentially<br/>adopted for bio-inspired sonar systems. Using a biological signal processing technique<br/>which was developed based on bat’s hearing system, the effect of auditory processing<br/>on the object discrimination is investigated for both CF (constant frequency) and FM<br/>(frequency modulated) signals respectively. These signals are considered as two representative<br/>types of echolocating calls. This study has simulated returning echoes from<br/>target discs using different types of calls by applying measured impulse responses of<br/>the objects. The simulated echoes were then processed through auditory models. The<br/>results have shown that the auditory processing contributes not only to increase the<br/>gain but also to enhance the ability to discriminate the sizes of discs. The peak and<br/>notch characteristics appearing in the auditory spectrum also confirms the flexibility of<br/>designing auditory models to manipulate spectral and temporal characteristics of the<br/>echo signals. Secondly, the effect of the bat’s head on the received signals at the two<br/>ears for varying distances was investigated by measuring the head-related transfer function<br/>(HRTF) of a bat-head cast. It has been reported that a bat changes bandwidth<br/>and duration of its echolocating call as it approaches a target. Adaptive change in the<br/>echolocating calls has been well explained in previous studies in terms of characteristics<br/>of signal structure. However, the range-dependent adaptive change in emitted signals<br/>also implies that the reflected signals reaching the two ears (i.e. binaural hearing)<br/>change in gain and frequency as the distance between the bat and the target varies.<br/>The result of measured HRTF has provided insights to range-dependent binaural information<br/>regarding the adaptive change of the echolocating calls. The results of measured data show that relatively higher gain at low frequencies (below 10 kHz1) is observed<br/>than that at high frequencies (above 10 kHz) as the bat-head cast approaches the sound<br/>source. It is also noted that interaural level differences (ILDs) at a fixed distance have<br/>less sensitive changes at low frequencies than at high frequencies as the angle of the<br/>source direction changes in the frontal axis. However, the sensitivity of the ILDs at low<br/>frequencies increase more than at high frequencies as the range reduces. It is concluded<br/>that the low frequency implies a more significant role during the target approaching<br/>stage in echolocation including distance perception. Also, the systematic change in sensitivity<br/>of the ILDs in various ranges suggests that the bat might be able to calibrate<br/>the angular resolution by broadening the bandwidth at low frequencies. Furthermore,<br/>the HRTF results calculated from a computational sphere model confirms the potential<br/>function of low frequency to calibrate the ILDs sensitivity for varying distances.<br/>Overall, this study has shown that customised auditory processing of the echolocating<br/>signal improves the quality of sonar representation and the results of investigations using<br/>the HRTFs of the bat-head cast guide the future design of effective adaptive signals<br/>based on the range-dependent HRTFs, to potentially enhance the performance of sonar<br/>systems.<br/>1This study has defined the range of the low and the high frequencies based on the acoustical<br/>diffraction and reflection of the sound around the bat-head. The diffraction effect appeared to be<br/>prominent below 10 kHz.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2010

Author dc:creator

• Kim, Suyeon

Advisors dc:contributor.advisor

• Allen, R.
• Rowan, D.