Stuart,
> Would Graham care to comment?
I suspect there may be people here who know more about bat sound than I do,
and this is very much a UK perspective, but...
Bats generate the sounds in the larynx, just as humans do, but the sounds
can be emitted through the mouth or through the nose. Frequency is
controlled by the size of the larynx and adjusting the tension in the vocal
chords, and for those species that emit the sounds through the nose, the
focus of the sound is altered by the design of the nose leaf.
The sounds are high energy with even small bats (5g) producing calls of
120dB at 1m. Given that the bats also have very sensitive ears, this gives
them a bit a problem and on each sound pulse the Stapedius muscle in the ea=
r
contracts and results which results in the passage to cochlea, being
blocked, thus avoiding the bat deafening itself. Given that the pulse rate
can be 200Hz or more, this is pretty impressive, but bats "are" impressive.
To generate the energy to keep this going, the bats wing beats are
synchronised with its breathing, and the down stroke drives the air through
the larynx. This also means that bats have a very high food demand.
The actual sounds are usually broken down into CF and FM sounds, Continuous
Frequency and Frequency Modulation, with some bat species considered as
primarily CF species and others as primarily FM species. The peak
frequencies vary with species, and the energy levels vary with species, but
the calls are complex and vary with environmental conditions, such as the
habitat the bats are flying in. Having said, while being conscious that you
could be getting it wrong, the different species do produce different
sonograms and you can get reasonably confident IDs.
In the UK, lesser horseshoe bats are at the top end of the frequency range
with highest energy being measured at around 110kHz, at the lower end are
noctule bats with highest energy being measured at around 20kHz. This means
that Children/young adults, without the aid of a bat detector, can hear som=
e
bat species echo locating.
For the rest of us there are three main types of bat detector:
heterodyne, which adds or subtracts sound to the bat calls to bring the
ultrasound down into a frequency ranges we can hear. This gives very
distinctive sounds that have become characteristic of different bat species=
,
but doesn't give a very good representation of the real sound. You also nee=
d
to tune the detector into a particularly frequency, so you can miss hearing
a high frequency bat, if you are tuned into a low frequency setting on the
detector.
Frequency division detectors, record the entire spectrum of bat frequencies=
,
and the resultant signal is then divided by 10 or 20 to drop the frequencie=
s
down to an audible range. This keeps the structure of the original bat call=
,
but loses the harmonics and adds some spurious noise.
Time expansion recorders also record the entire spectrum of bat frequencies
and then play them back slowed down by 10 times. This method is meant to
retain all the features of the original call, and is the preferred method
for bat sound analysis.
However, Heterodyne detectors are about =A3150, Frequency Division detector=
s
are about =A3300, and Time expansion detectors start at =A31000.
As well as echo locating in ultrasound, bats use contact calls in the range
audible by all humans without some specific hearing problem. I have found
several roosts simply by hearing bats chattering, while I have been walking
along a town street, or along a woodland path.
On one occasion, when looking after a sick bat, I took it out into the
garden, to give it an opportunity to fly off. It sat on my hand and started
audibly calling. Within a few minutes, I had several bats flying around my
head and the hand that my bat was sitting on, and they too were audible
calling. The behaviour of the flying bats had me convinced that eventually
one was going to land on my hand or perch on my head, but they never did.
Bats often have breeding roosts in the roof space above children's bedrooms
(usually warmer than the rest of the house) and on several occasions, after
parents have discovered that they have bats in their roof and want rid of
them, it is the children who have spoken up for the bats, because they like
having them there, even though they didn't know the sounds were being made
by bats.
There is a lot to know about echolocation, and I know I have wandered into
bat anecdotes, but they are such fascinating and extraordinary animals.
Graham
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