Wireless monitoring of in-flight bat calls and echoes from the echolocating fruit bat rousettus aegyptiacus

Whiteley, S. and Waters, D.A. and Farr, I. and Pierce, S.G. and Hayward, G. (2009) Wireless monitoring of in-flight bat calls and echoes from the echolocating fruit bat rousettus aegyptiacus. In: Fifth International Conference on Bio-Acoustics, 2009-03-31 - 2009-04-02. (Unpublished)

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Abstract

In order to learn more about how bats perceive their environment in such detail using sonar, it is imperative that their calls can be accurately recorded. By the nature of these calls and their production, this accuracy is difficult to achieve using the conventional method of a handheld or static microphone, coupled to some recording media. There are various factors which mean that this method is likely to produce an imprecise representation of the emitted call. Firstly, the frequency-dependent attenuation of ultrasound in air, coupled with the frequency-dependent directionality of both source and receiver, mean that the signal which arrives at the microphone is only a representation of the sound at that point in the field. Furthermore, with the bat moving in flight and also altering the direction in which sound is emitted, even two identical calls may be significantly altered from the viewpoint of the observer. All this makes it practically impossible to capture or recreate the call as emitted by the bat. One method which mitigates for these problems is the use of a miniature wireless sensor which can be mounted on a bat, thereby providing a microphone which is close to the emitter and does not move in relation to it. Such an approach has been successfully utilised to obtain echolocation recordings from bats while in flight in a laboratory situation. The work reported here aims to take this concept a stage further by introducing variable gain into the sensor such that echoes returning to the bat, as well as the emitted call, can be detected. The sensor is designed and built around a variable gain amplifier (VGA), with the emitted bat call used to trigger a gain-control circuit. Hence, with the emitted call as the point of reference, the circuit gradually increases from its low gain state to its high gain state, before being reset. The dynamic range of the gain is shown to be in the region of 70dB, with the frequency response and gain curve being characterised in the laboratory. Ideally, the system would have a flat response over the frequency range of interest (20kHz - 200kHz), however it is demonstrated that the system has a repeatable low-pass characteristic, which means the recorded signals can be compensated for in post processing. Furthermore, it is demonstrated that signals obtained from the system accurately recreate the ultrasonic signal, through comparison with signals captured using a calibrated microphone. The wireless system is then used to record emitted and reflected calls from the echolocating fruit bat Rousettus aegyptiacus while in flight in a captive environment. Five individuals were flown in a flight corridor, with trains of calls being obtained over three-second recording periods. These results indicate that the 'impulse-like' signals emitted by R. aegyptiacus have interesting, and repeatable, notches in the frequency domain, though their purpose is unknown. Furthermore, as would be expected, the calls recorded with this sensor can be shown to contain more high-frequency components in comparison with recordings in the literature made by conventional means. These experiments demonstrate that this technology is indeed capable of obtaining signals and echoes from a flying echolocating bat. However, the next stage of this work is to use such a sensor system on British bats, which are much smaller and lighter than R. aegyptiacus. For this, a smaller lightweight version of the sensor has been designed. At the same time a microphone is being designed which will have a flatter frequency response across the range of interest.

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