Advantages And Limitations Of Analysis; Quality Of Recordings - Batbox Duet User Manual

ADVANTAGES AND LIMITATIONS OF ANALYSIS

It is possible to glean a lot of information from recorded bat calls :
Peak-frequency
•
Pulse length
•
Repetition rate
•
Social calls
•
Even with the heterodyne function alone, all of these factors can be
assimilated, by experienced bat workers in the field, in real time, to
produce accurate species identification. However, if the detector is tuned
to a 55kHz bat and a noctule, which is echolocating at 20kHz flies past,
the calls may be missed on the heterodyne. Frequency division, though,
will pick up both frequencies and render the calls as 5.5kHz and 2kHz,
respectively, so that later computer analysis would reveal the noctule pass.
An important thing to remember is that some species will remain very
difficult to identify, even with good recordings containing all of the above
features. This is because some bats will produce the same echolocation
calls in similar environments, in order to glean the same SONAR picture
for their navigating and feeding requirements. The best way to identify
these bats is by looking at their commuting calls.

QUALITY OF RECORDINGS

You will need a recorder with an input level control. This can sometimes
be in the form of an AGC (Automatic Gain Control) in the cheaper
recorders, but far better, is to use one with a manual level control,
which does not interfere with the recorded pulses and produces less
background noise.
Poor recordings, i.e. record levels too low or high, will not give much
useful information, so a single pass by a distant bat will, almost certainly,
remain unidentifiable. It is, therefore, important to make sure that your
recorder is set up correctly. The loudest pulses should not go above 0dB
(preferably –3dB) to allow for very close bat passes. Recordings that go
into 'clipping' (over-recorded) will add unwanted harmonics to the signal,
so do not automatically choose the strongest pulses from your recordings
for analysis. Tape recorders (cassette) are kinder to levels that are too high
than digital recorders (MP3, minidisc or DAT). However, digital recorders
have a much greater dynamic range and less inherent noise, so that you
can afford to record at slightly lower levels.
Although recording levels can be easily brought up to 0dB level
(normalised) with computer software, it is important to get onto the
recording the strongest signal possible without distortion so as to create
the biggest difference between the wanted signal and any
background noise.