Listening to Bats
© Brooke Clarke 2007
Background
When outside at twilight on a summer
evening, like to watch satellites, there were either
bats or fast flying birds in the area. There are local stores
selling "bat houses" the idea is to encourage bats to be around since
they eat insects. So I suspect there are bats here. They
are flying too fast for me to focus on them.
Around January I got a "CSE Batdetector". It's a small hand held
unit that runs from a standard 9 volt battery. When you plug in
stereo headphones that turns on the power so you need to unplug the
headphones when it's not in use to save the battery. On the front
is a knob for tuning into the frequency of the bat.
Look near the photo's lower right corner of the box and you can see the two holes for the stereo microphones.
This unit
uses two condenser microphones and processes the signals as stereo so
you get a feel if the bat is flying right to left or left to
right. In twilight conditions that's a help when trying to see
them (more like seeing a fuzzy blob moving fast). If you rub your
index finger on your thumb it generates a lot of ultrasonic
noise. When near a computer there's ultrasonic noise that's
strongest around 25 kHz. The IMP2 Slave Clock Pulser has a 32,768
Hz crystal that can be heard when the bat detector is right next to it and the knob is peaked around where 32 kHz would be.
When I first got the bat detector I went out at twilight and later that evening but
could not hear anything. Some Googling gave me the idea that bats
are only active when it's warm and January is not too warm. But
about mid May the bat detector was hearing bats when tuned to about 26
kHz.
Listening to Bats
Bats make a ultrasonic "chip" and listen for an echo (very similar in
concept to RADAR) to hunt and to sense what's happening around
them. There are many species of bats and so the possible frequency of
these "chirps" can vary over a wide range, say from just above human
hearing at 20 kHz all the way up to maybe 140 kHz. Notice that
the range or bandwidth of human hearing is less than 20 kHz for a young person and
more like 10 kHz for most people and the possible range of bat
frequencies is 12 times wider, so with a single down conversion
you can only hear 1/12 of the possible band of frequencies.
So how to listen to bats? There are a number of ways.
Hetrodyne
By mixing a local oscillator with the
amplified output from a microphone the bat's ultrasound is changed to a
frequency that you can hear. The mixing process preserves
the amplitude modulation on the chip so close bats are louder than far
away bats. Once you know the frequency for your local bats you
probably don't need to change it. The bandwidth of a single bat
species is typically less than 10 kHz.
Frequency Division
The idea is to amplify the bat ultrasound
then use it to clock a digital counter chip. The output can be
taken from some divisor that brings the ultrasonic frequency down to
into human hearing range. The two disadvantages are that you
loose the amplitude information so can't tell if a bat is close or far
away and the width of the "chirp" gets compressed so the fidelity is
not as good. But the good news is that there's no tuning required.
There are some ideas floating around that would keep track of the
amplitude informatin and use it to modulate the frequency divider
output, but I don't know if this has been done or is available.
SDR-IQ
If
some wide band of frequencies are recorded like 15 to 150 kHz for
example then it can be played back and all viewed on a spectrum
analyzer. You can see subtile details on the spectrum analyzer
that your ears can not distinguish. The Software Defined Radio
called the
SDR-IQ has
this capbility. I.e. it can record 500 Hz to 190 kHz directly and
can show a real time frequency spectrum for that band or any smaller
band. While it's displaying the wide band you can put a cursor on a bat
frequency and it will demodulate that down to baseband. I haven't
yetused the SDR-IQ for bats so don't have details about modulation
type, bandwidth, etc.
It's powered by the USB2 cable and has
a BNC input jack. The DB-9 connector is so that the SDR-IQ can
control radios that may be acting as RF front ends for frequencies
above 30 MHz. For frequencies of 30 MHz and lower the SDR-IQ can
receive them directly. The hardware on the board mixes the input
and a Direct Digital Synthesizer supplied Local Oscillator the I &
Q channels of the mixer output get sampled and the digital data stream
gets decimated down to a bandwidth that the USB2 port on a computer can
handle. The Spectraview software runs inside the PC to further
tune and demodulate the digital data. The sound card might act as
the audio output with the SDR-IQ, but for the stock setup is not
involved with the reception.
Recording
A normal audio recorder can be used on the headphone output from the heterodyne or frequency divider type bat detectors.
An ultrasonic microphone could be amplified and recorded on a recorder
that could handle the bandwidth (not a normal Hi-Fi audio
recorder). Such as an instrumentation recorder or a video
recorder
with an attachment would work. Modern Instrumentation recorders
are based on digital techonlogy related to digital TV recording and
have total bandwidths similar to a TV channel. For example the
Kinetic Systems DAQ848 has 48 channels each just uder 100 kHz wide or about 4.8 MHz total bandwidth.
A more advanced approach uses direct to hard drive recording, such as done by
Wide Band Systems. The bandwith is extended by using RAID.
The SDR-IQ (see above) can record a very wide band (up to 190 kHz) directly to a PC USB2 port.
Microphones
The normal audible range electret type condenser microphone has some
response above the 20 - 20,000 Hz audio range. There are also
ultrasonic speakers and sensors made to work with remote control
applications but these are usually optimized to operate over a narrow
band and are not suitable for the 15 to 150 kHz bat band. There
are commercial products that work very much like a bat detector called
"ultrasonic leak detectors" but unless you can find some specification
about frequency coverage it's hard to know what they cover. There
are also hydropones with the needed frequency coverage, but they are
designed to work under water and their performance in air is rather
poor.
The other type of microphone is the condenser mike specified to work in
the ultrasonic range. It would be nice if there was an electret
type made for ultrasonic since then you would not need a DC bias
supply, but as far as I know they don't exist or are very
expensive. That leaves the standard condenser microphone.
Probably more properly called a transducer since it can make ultrasonic
sound as well as it can detect it.
The one that's probably been made in the highest quantity is the one
used on a number of Polaroid cameras for the auto focus function. Details on the
Polaroid Sonar One Step Camera are on a seperate web page.
Wavelength
The speed of sound in air is about 1124 feet/second, at 50 kHz a wavelength is 0.27 inches.
If a sound wave impinges on a microphone diaphragm on it's central axis
then the diaphragm can be many wavelengths in size and will work better
because of the large diameter. This is the case with the Polaroid
Sonar type transducers. They are used in an application where the
outgoing and incoming wave fronts are on their central axis.
That's why in this particular application the diaphragm size can be
many wavelengths in diameter, but for general purpose microphones it's
much smaller.
If an omni directional microphone is used to pickup sounds from a
random location and it's diaphragm is comparable to a wavelength the
average sound pressure can vary because a peak may cancel out a
valley. In this case the diaphragm needs to be small compared to
a wavelength to get flat response.
I expect that when the Polaroid Sonar transducer is used as a bat
microphone it's response will have peaks and valleys that are quite
deep (more than 10 dB) and spaced close to each other. But as the
bat moves it will be moving through a number of these and the peaks
will have a much higher output level than you would get using one
of the much smaller professional type microphones.
Links
00530 hits since June 02 2007created 2 June 2007
If
some wide band of frequencies are recorded like 15 to 150 kHz for
example then it can be played back and all viewed on a spectrum
analyzer. You can see subtile details on the spectrum analyzer
that your ears can not distinguish. The Software Defined Radio
called the SDR-IQ has
this capbility. I.e. it can record 500 Hz to 190 kHz directly and
can show a real time frequency spectrum for that band or any smaller
band. While it's displaying the wide band you can put a cursor on a bat
frequency and it will demodulate that down to baseband. I haven't
yetused the SDR-IQ for bats so don't have details about modulation
type, bandwidth, etc.