Bat Echolocation Researc h - Bat Conservation International

ent species. But in the field, it is difficult to audibly recognize
all individual species using real-time broadband
output from FD detectors.
Harmonics. The ability to identify harmonics in the
field enhances the likelihood of the heterodyne detector
to be able to discriminate among species. When the frequencies
of narrowband calls of different species are similar,
the frequency ‘gap’ is doubled or tripled in harmonics,
which can enhance discrimination. Due to the processing
of the signal, harmonics are no longer present in
the output of a FD system. Where multiple harmonics are
present, the system divides only the strongest frequency.
In time expansion, harmonics are not audible in the field,
but are available for analysis in the output. With its
capacity to process harmonics, this system better enables
the recognition of pulses of different species overlapping
in time but separated in frequency.
Frequency range. A tuned heterodyne system allows measurement
of the peak frequency (F max ) and the minimum
frequency (F min ) in the field, whereas TE and FD detectors
do not. A computer and analysis software are needed
for this to be possible. For signal analysis, one must be
wary of the difficulties that a frequency-division system
has in capturing rapid frequency changes, which can
occur at the beginning or end of a pulse. Maximum frequency,
especially, will be underestimated (Fenton 2000).
Shape or curvature. Information about pulse shape is
available through analysis of TE and FD recordings, but
only large differences like FM, FM-QCF-FM, FM-QCF,
QCF, QCF-FM, and FM-CF-FM can be heard in timeexpanded
pulses. Fine details of pulse shape require postrecording
analysis. The output of a frequency-division
detector does not allow any measure of shape aside from
the recognition of tonal quality. Working with heterodyne
detectors in the field leads to an ability to interpret
aspects of shape indirectly through other parameters,
like tonal quality, change of frequency while tuning, or
position of QCF or shallow FM in the pulse relative to
steeper parts.
recorded sound, there is little possibility to account for
how distance and behavior affect the loudness of a call.
TIME-RELATED PARAMETERS
Pulse duration, interpulse interval, and repetition rate. Both
frequency-division and heterodyne systems allow
observers to use interpulse interval and repetition rate
features in the field. The ‘slowed down’ aspect of the
time-expansion system enhances the possibility of measuring
pulse lengths. However, due to the auditory discrimination
performance of human hearing, interpretation
of pulse intervals and repetition rate become difficult
if not impossible because they become too long and
too slow.
Recordings from time-expansion and frequency-division
detectors can be used to make relatively accurate
measurements of pulse duration, interpulse interval, and
repetition rate. To define the beginning and end of a signal,
a good signal-to-noise ratio and no overlap between
signal and echo are required (Kalko and Schnitzler
1989). Because the tuning window of the heterodyne
system cuts out part of the pulse, these detectors cannot
accurately measure duration. However, for low-dutycycle
bats with long interpulse intervals, errors in the
measurement of interpulse intervals and repetition rates
will be slight.
Rhythm. The tonal quality in the real-time output of
the heterodyne detector enhances the potential to interpret
the rhythm of call sequences. The real-time audible
output of a frequency-division detector enables an interpretation
of the rhythm, but differences in tonal quality
between alternating pulses are hard to perceive. As with
repetition rate, interpretation of rhythm is difficult with
a time-expansion system, because of the greatly slowed
reproduction of the call sequences.
Whereas the human brain is good at interpreting patterns
and thus an excellent tool to assess rhythm, quali-
AMPLITUDE-RELATED PARAMETERS
Loudness, maximum sound pressure, and energy. Sensitivity
and detection range differ between detector systems and
brands. To interpret differences in perceived loudness,
the observer must accumulate substantial experience
with the particular detector (Limpens 1994, 1999;
Waters and Walsh 1994).
To appreciate loudness, a heterodyne detector should
be tuned to the best listening frequency, or QCF frequency,
when present. Both heterodyne and frequencydivision
systems, being real-time systems, allow the
observer to interpret what is heard while the distance to
and behavior of the bat is observed directly. With a
time-expansion system, observers must integrate data
about the distance and behavior of the bat with the playback
of the time-expansion signal at some subsequent
time. In automated, stationary set ups, or in analysis of
Section 2: Acoustic Inventories
Figure 8: Scale and resolution in space and time.
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