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Recording and mimicking the vocalization of bats using air-coupled BAT® transducers

 

   

Background:

Photograph of a real bat (closeup of head).the ultrasonic vision of bats - Bats are nocturnal mammals with poor eyesight who have evolved highly-sophisticated echolocation systems based on air-coupled ultrasound. Such echolocation systems are a form of ultrasonic vision that allow the bats to "see" without the use of light. Over 700 species of echo-locating bat exist and most feed themselves by locating and capturing insects in flight using ultrasonic techniques. Bats create and hear sound in much the same way as humans, namely using a larynx and vocal tract for transmission, and ears and an auditory system for reception. However, there are two significant differences: (i) bat vocalization and hearing occurs at much higher frequencies than our own; and (ii) bats probably concern themselves more with assembling spatial information from returning echoes than with bi-directional communication of ideas.
 
Photograph of a real bat (closeup of one wing).even to bats bandwidth matters - Because sophisticated tasks like navigation and hunting would not be possible with narrow band ultrasonic frequency responses, bats have naturally evolved with broadband ultrasonic systems. They typically employ a frequency-swept chirp-mode of operation in which both amplitude- and frequency- modulation are used during vocalization and subsequent neural processing. A typical bat in flight emits short  (~2msec long) bursts of frequency-swept ultrasound at regular intervals of ~100msec. By analyzing returning echoes for frequency content and time-of-flight, the bat is then able to determine the location and nature of objects in its immediate surroundings (at distances of up to 5-10 meters away). If any object of interest is detected, the bat increases both its pulse repetition rate and its ultrasonic chirp bandwidth while decreasing the duration of each chirp, so as to obtain more detailed information from the ultrasonic echoes. If an object happens to be a flying insect (e.g., a moth) the bat can determine from returning echoes both the insects' shape and wing-flutter frequency and so determine if it is a favourite snack. If deem
ed edible, the bat will initiate a final sequence of events known as the "pursuit manoeuvre" so as to secure the meal.

 

Experiment #1: (detecting vocalizations)

MicroAcoustic's BAT™ transducers can be used to detect the ultrasonic vocalizations of bats. In this example, typical vocalizations of two different bats of the same species (Eptesicus.fuscus) were detected using MicroAcoustic's BAT-1 transducer.
 
 
Ultrasonic emission from the first of two real bats, as detected by an air-coupled BAT detector.  

results for bat #1... A typical received waveform for the first bat is shown above (signal-a) and is seen to be a complicated amplitude- modulated chirp of approximately 5msec total duration. Expanding the beginning and end sections of signal-a (to create signals b and c respectively) shows more clearly the frequency modulation that also occurs in the vocalization from its beginning to its end. The overall frequency content in the vocalization is shown in figure-d, as obtained from a Fast-Fourier Transform (or FFT). Figure-d indicates that most of the energy in the vocalization falls within the frequency band from 25-50kHz, though energy is present all the way up to ~110kHz. Compared to the measured emissions from katydids by BAT™ transducers, the vocalization of bat#1 is seen to be of significantly broader bandwidth.
 

Ultrasonic emission from the second of two real bats, as detected by an air-coupled BAT detector.

results for bat #2... The second bat studied was from the same species as the first, and its vocalization is shown above (green) in the same format. Immediately obvious from a comparison of signal-e (bat #2) with signal-a (bat #1), is the individual nature of bat voices. Though the two vocalizations are of similar temporal length and spectral response, the amplitude modulation employed by each bat is quite different. This is not unlike our own human voices, where individual differences are always present though the general nature remains the same.

 

Experiment #2: (mimicking vocalizations)

The MicroAcoustic BAT™ can alternatively be used to mimic the vocalizations of bats. To demonstrate this, the vocalization of bat#1 (signal-a above) was reproduced and converted into a drive voltage using an arbitrary function generator and broadband power amplifier before being reapplied to a MicroAcoustic BAT™  source (model BAT-1). The ultrasound generated by the BAT™ source was then detected by a second BAT™ detector. The results are displayed in the waveforms i through l below, where similarities with the actual emission of bat#1 above are obvious. Particularly convincing is the spectral plot (figure-l), which bears striking resemblance to the actual frequency response of bat#1 (figure-d). Indeed, it could be said that the Micro Acoustic BAT™ has mimicked the vocalization of bat#1 better than a second bat of its own species (i.e., bat#2).

Ultrasonic emission from a MicroAcoustic BAT suitably driven so as to mimic the vocalization of one of the real bats studied above. This emssion was again detected by a MicroAcoustic BAT detector.

Conclusions:

1) These measurements show that MicroAcoustic's BAT® detectors can be employed to accurately record and study the ultrasonic emissions of actual bats. Because emitted sound pressure levels of bats are similar to those of other ultrasonic animals, the results also show that BAT® detectors will be of general use in the study of animal & insect bio-acoustic emissions.

2) Unlike available microphone technologies which are limited to frequencies below ~135kHz, the MicroAcoustic BAT® detectors have a much higher bandwidth (i.e., greater than 2MHz). This means that by employing a BAT® detector instead  you will be certain to capture ALL information present in your studies of bats and not overlook what may be present (though as yet undiscovered) at higher frequencies.

3) MicroAcoustic's BAT® transducers can be used as sources to reproduce (or mimic) the actual emissions of ultrasonic animals. This opens up new potential avenues for systematic behavioural research, in which BAT® sources are employed to mimic a wide variety of animal emissions under repeatable control of the experimenter.

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*Note:  The experimental results presented here were obtained by D.W. Schindel during his tenure at the NRC Institute for Aerospace Research. Contribution and reproduction of these results and figures occurs courtesy of the NRC Institute for Aerospace Research, Canada.

 
 
 

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