Bat Echolocation Researc h - Bat Conservation International

f tuned = f bat . As an example, with a setting of the oscillator
frequency at 45 kHz, an FM pulse sweeping from 70
kHz to 25 kHz would produce a V-shaped output sweeping
from 25 to 0 kHz and up again to 20 kHz. A low-pass
filter in the output (e.g., 5 or 8 kHz) functions as a ‘frequency
window,’ filtering out higher frequencies and providing
a clear, audible signal, sweeping down from 5 to 0
kHz and up to 5 kHz again (Table 1, Fig. 1). The same
output, however, will be produced at detector settings of
55 or 35 kHz, and there is no frequency information
available from the output signal (Fig. 1). The frequency at
which the bat detector is tuned provides the only information
on the frequency of the incoming signal.
REQUIREMENTS OF A DETECTOR
Table 1: An example of input and output frequencies when
heterodyned or mixed with an oscillator signal (i.e., the tuned
frequency) of 45 kHz. In different detectors, low pass filters may
be broader or narrower, resulting in a broader or narrower frequency
window.
be used to continuously monitor the activity of bats.
Limitations result from the process of counting the
time between the zero crossings because the sinusoidal
waveform is converted to a square wave in the output signal,
thus losing some of the physical information in the
call. Also, the FD detector converts zero crossings for
only the most intense frequency and does not provide
information on harmonics. The process of ‘averaging’
groups of 10 waves also makes it impossible to evaluate
rapid frequency changes. These typically occur at the
beginning or end of a pulse where rates of frequency
change tend to be relatively high in FM signals of bats. In
that respect, FD detectors give less reliable measures of
start and end frequencies in the steep FM parts of pulses.
The signal produced by FD detectors can be used for
sound analysis, but physical detail in the signal is lower
than from TE detectors. In some systems, the pulse envelope
is determined and superimposed on the output
wave. In this way, information on amplitude is preserved.
A Fast Fourier Transform analysis of a FD signal, e.g. to
calculate peak frequency, will however be much less
accurate than in a TE signal. A zero-crossing analysis,
through its nature, would be more accurate, but as a consequence
of the averaging of groups of 10 cycles is still
not as accurate as in TE.
Heterodyne detectors use a variable-frequency oscillator
and mix the incoming frequencies of the bat(f bat )
with the oscillator frequency tuned in the
detector(f tuned ). The resulting signal contains the sum
frequencies (f tuned + f bat ) as well as the difference frequencies
(f tuned - f bat ) as frequencies with high energy.
The sum frequency will be higher than the ultrasonic frequency
of the bat and will be filtered out. Where the frequency
of the pulse is near the tuned frequency, the difference
frequency will be low, reaching 0 kHz where
Section 2: Acoustic Inventories
The requirements that we place on detectors for
detecting bats closely resemble the requirements of a bat
trying to detect insect prey. As for a bat hunting an
insect, we wish to detect the presence of a bat and, with
a certain accuracy and resolution, know its position
(distance and angle) and possible flight course. In this
context. we discuss a number of different requirements
and features.
Detection Range and Sensitivity
Ideally, the range over which a bat is detected should
be as large as possible. This allows the sampled area to
be larger and the number of observations higher. Greater
range allows for an earlier detection of bats resulting in
a longer observation time, which enhances recording
and identification possibilities.
Detection range depends on several variables that
include the frequency range and strength (amplitude) of
the emitted signal, the sensitivity of the microphone to
different frequencies, the orientation of the bat to the
microphone, whether the bat is flying in open or cluttered
airspace, and atmospheric conditions. The amplitude
of calls and the signal type used by the species of
bat (Fig. 2) are
dependent on habitat
and the distance
a bat is from
surrounding obstacles
that produce
clutter (i.e., echoes
from objects other
than the target,
Figure 2: The range over
which a bat is detected is
dependent on call intensity
and the type of signal
(frequency, bandwidth)
used by the bat.
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