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Lesser Horseshoe bat Rhinolophus hipposideros
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Mp3 mono recording at 107 KHz, length 7KB. Warm evening in September.
A lesser horseshoe bat is flying around in a barn. This bat has an
almost constant frequency call, but its movement produces a doppler
shift in frequency which is exaggerated by the heterodyne detector.
The frequency rises as it swoops towards the microphone and falls
in frequency and intensity as it passes.
The "nose leaf" of the Horseshoe bats focuses the call making it quite
directional.
Photo J Kaczanow
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The start of evening activity. The bat has warmed up and is
scanning the barn before flying. The pitch it uses while static
is higher than the flying echolocation call but of course has no
doppler shift.
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The static call slowed down 4 times. The frequency after converting
and slowing is 1 KHz giving a bat frequency of 110KHZ (4 KHz plus
the detector base frequency of 107 KHz).
Almost all of each call is at a constant frequency, only the start
and end having a small sweep.
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A look at a single call gives the duration as about 50 milliseconds
in which time the call would travel about 16 metres. We can assume
that echoes are being heard while calls are being made and this is
likely to be the reason for the wavy envelope of the call showing
a "beat frequency". We can derive from this the velocity of the bat
with respect to the reflecting surface of about 0.5 met/sec.
Measured high and low frequencies of 3.5KHz to 700Hz give a speed
change of approximately 8 met/sec, which would indicate a swoop
speed of 4 met/sec or 9 mph.
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The sharp eared may have questioned how many bats were flying in
the barn, as there seemed to be some fainter calls on the recording.
A further examination showed that there was a echo of some of the
calls and this is more noticeable when the recording is slowed down.
As the bat makes its circuits, the pitch falls sharply as it heads
towards the microphone, but its echo from the slate roof follows a
different path and does not change much in pitch.
The following recording is of the first flight calls slowed down 8
times. You can hear the echo quite clearly following each call with
a delay of about 20 milliseconds equivalent to a distance of about 7 metres.
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