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World of Ultrasound

It was as recent as 1940 that it was first stated that bats used ultrasound for location and hunting. Even then the idea was dismissed as a fanciful idea in some learned quarters.

Handheld bat detectors have only been available and affordable for a decade or two, but now many bat enthusiasts have discovered the World of Ultrasound that has been unknown to the human race for millennia.

Bat detectors are indispensable for amateur and professional alike and are an indispensable aid to identifying bat species. However they are only an aid as other clues are often needed for a positive identification.

The following bat sounds were recorded with the most common and simplest type of detector the heterodyne detector

Click on the images for the sounds.

Lesser Horseshoe bat Rhinolophus hipposideros

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

 
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.
 
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.
 
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.
Lesser horseshoe call
 
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.