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As dusk advances you may be lucky enough to see bats, but as darkness falls you usually don't even know they are there.
There are lots of silly ideas about bats - like them getting tangled in your hair in the dark. This just doesn't happen - they are wonderful navigators in the dark, and manage this not with super-sensitive eyes (like owls) but by echo location - seeing by sound. Unlike birds, they do not fly into window glass because it is opaque in their sound world.
Basically they emit a 'chirp', and listen to the echoes. (Usually it is
described as a 'click', but the slowed down recordings will persuade you that it
is a sliding tone.)
From these echoes bats can build a rich 'picture' of the world about them. However impossible this seems, dolphins obviously share the skill. Try to explain vision to a person who has never had sight, and the difficulties of understanding a completely alien form of perception become clear.
But you hear dolphins calls, but you can't hear bats. Why not?
Humans can only hear sound up to about 16KHz (that's 16,000 cycles per second). Bats emit sounds at typically 2 or 3 times higher pitch than we can hear. Sounds above human hearing are usually called ultrasonic.
Whales and elephants can make sounds below what we can hear, about 16Hz (16 cycles per second), and these are called infrasonic.
So how can we make sound files for you to play?
The answer is the Bat Detector, an electronic box that has an special ultrasonic microphone (ordinary microphones only just manage the human range of sound) and electronics to make the sound audible. The recordings below were all made on warm evenings at dusk in April 1998.
In the bat detector we have there are two ways to make the sound audible:-
What you hear is in 'real time' i.e. the sounds you hear are the same
length and repetition rate, but the pitch is arbitrary, and varies more than
the bat might perceive.
Why? Say the bat goes from 45KHz to 46KHz, then it has changed the note by about 2%. If our steady tone is 43KHz then we hear a change from the two differences - 2KHz to 3KHz - a change of 50%. If our frequency was above the note then what we hear would go lower rather than higher in pitch! So the calls are audible but the sound is not directly meaningful - the frequency you need to hear the bat helps identify it though.
So this scheme is great for detecting and identifying bats, but doesn't produce any real idea of the calls themselves.
To hear a recording made this way Click Here (72Kb). The bat finished an attack about 3 seconds into the sample and then resumed normal navigation - more on this in a moment. The frequency set was about 43KHz.
To hear a recording of this sort Click Here (109Kb). Most of the time you hear normal navigational calls - nearly regular squeaks, in this case interspersed with a few at higher rate, presumable to identify and avoid some obstacle.
If you are lucky you can catch an attack sequence as the bat locates a suitable flying insect, and homes in on it. The calls get quicker and quicker until there are fast even at 10 times reduction, and the intensity also drops - the moth or whatever is close! This is the same sequence (not the same event though) that become a 'buzz' in the Heterodyned recording.
To hear a recording of this sort Click Here (81Kb). It was made using the same settings as the above.
A second or so after the climax normal navigation resumes - obviously a mouthful of moth doesn't stop the calls - and it must not or the bat could fly into something while not 'looking' where it is going.
Some people think bats are ugly or grotesque. Look at those facial features with the eye of an amazingly designed audio transmitter and analyser that can paint a picture of the world, and maybe your view will subtly alter.
In the UK bats are heavily protected species in decline through loss of both natural roosting habitat such as undisturbed caves, and changes in housing design that make loft spaces unsuitable or inaccessible. If these calls from the dark make you feel that bats have a place in the world, then they were worth making!