Development and application of an echolocation model inspired by bats

This study explores the principles of echolocation in bats which can be potentially adopted for bio-inspired sonar systems. Using a biological signal processing technique which was developed based on bat’s hearing system, the effect of auditory processing on the object discrimination is investigated for both CF (constant frequency) and FM (frequency modulated) signals respectively. These signals are considered as two representative types of echolocating calls. This study has simulated returning echoes from target discs using different types of calls by applying measured impulse responses of the objects. The simulated echoes were then processed through auditory models. The results have shown that the auditory processing contributes not only to increase the gain but also to enhance the ability to discriminate the sizes of discs. The peak and notch characteristics appearing in the auditory spectrum also confirms the flexibility of designing auditory models to manipulate spectral and temporal characteristics of the echo signals. Secondly, the effect of the bat’s head on the received signals at the two ears for varying distances was investigated by measuring the head-related transfer function (HRTF) of a bat-head cast. 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 in terms of characteristics of signal structure. 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 as the distance between the bat and the target varies. The result of measured HRTF has provided insights to range-dependent binaural information 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 than that at high frequencies (above 10 kHz) as the bat-head cast approaches 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 than at high frequencies as the range reduces. It is concluded that the low frequency implies a more significant role during the target approaching stage in echolocation including distance perception. 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. Furthermore, the HRTF results calculated from a computational sphere model confirms the potential function of low frequency to calibrate the ILDs sensitivity for varying distances. Overall, this study has shown that customised auditory processing of the echolocating signal improves the quality of sonar representation and the results of investigations using the HRTFs of the bat-head cast guide the future design of effective adaptive signals based on the range-dependent HRTFs, to potentially enhance the performance of sonar systems. 1This study has defined the range of the low and the high frequencies based on the acoustical diffraction and reflection of the sound around the bat-head. The diffraction effect appeared to be prominent below 10 kHz.