By Jonathan Berber
Back in the 18th century, it was suspected that bats navigated using
sound, but it took until 1938 to conclusively demonstrate
echolocation. The mechanism almost exclusively utilizes ultrasonic
frequencies which are inaudible to humans. In brief, bats emit high
frequency, short duration energetic pulses of sound and interpret
the echoes that return to their ears. The sounds have been shown
to be quite complex, featuring swept frequency chirps and
constant frequency components. Some people (including me) can
hear a limited portion of the sounds emitted by bats. This may be
the lowest parts of their frequency modulated content. Certainly,
it’s perfectly possible to be surrounded by bats alongside some
woodland at dusk and hear nothing but their wings fluttering.
That’s where the bat detector comes in. By down-converting the ultrasound into our audible range, we can
indirectly detect it and at least enjoy a window into the
hidden world of echolocation. It has been suggested that
bats utilize delay times, arrival time differences, and the
Doppler effect to interpret echoes. Of course, we can
only guess at what bats actually perceive. Perhaps the end
result is somewhat analogous to vision, with an image that
is refreshed as new information comes in.
This bat detector is a heterodyne design. This means
that realistic sounding calls are heard, but bear in mind
that they have been frequency shifted and are
representative artifacts of the original sounds.
Nevertheless, the sounds heard with this detector are
highly informative with real scientific value. It is perfectly
possible to identify bat species by careful observation
using this detector as their call patterns are species-specific and can be quite distinctive. I personally (along
with a knowledgeable assistant) have identified two
species locally, namely brown long eared bats and
common pipistrelle bats.
What is Heterodyne?
The incoming ultrasonic signal is mixed with a locally
generated signal that is close to the same principal
frequency. The mixer output contains a signal which is the
difference between these two. So, for example, if the
incoming sound is based at 45 kHz and the local oscillator
is running at 42 kHz, the
output will be around 3 kHz
— nicely within the range of
human hearing. Interestingly,
the same output would also
be produced if the local
oscillator was set to 48 kHz
as the mixer produces
difference signals in both
senses, positive and negative.
The block diagram in
Figure 1 is pretty much as it
FIGURE 1. Block Diagram.