Development and application of an echolocation model inspired by bats

Abstract

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.

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Identifiers

ORCID for D. Rowan: orcid.org/0000-0002-7190-9997

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