>
> How far away are those bats? In theory one may think that some of
> them drown into the increased noise level of the mic, but I wouldn't think so.
>
> Klas.
>
Klas, likely a naive question, but can you clarify whether you are suggesting
that
microphone noise floor actually rises when microphones are at -20C or whether
the
sensitivity declines as the membrane stiffens?
====
>Given the attenuation with distance for these small energies, don't
>most of the ultrasound sources have to be fairly close to be
>recorded? How far away can you record a bat call in a very quiet
>location? Rob D.
You've both asked one of the basic questions that people have been exploring
for some
years, Mostly you can't see flying bats, so you don't know from visual
observation. There
is a large variation in body size (3-80+ grams). More importantly, from the
point of view
of attenuation, species call at quite different frequencies.
In western North America there are two that have most of their call energy in
the audible
(peak energy near 6 KHz, 8-10 KHz for the other). There are also a couple of
species
with most of their energy in the audible in Europe (and other temperate to
tropical bat
faunas as well).Eyeball maximum distance estimates from spotlighting and
recording big
audible frequency bats are in the 50m range. The sounds they make are
aesthetically quite
interesting, but some of us have to artificially drop the frequency to hear
them.
There aren't a lot of measured call intensities for bats, but values are around
120-135 dB
(see
http://en.wikisource.org/wiki/Echolocating_Bats_Cry_Out_Loud_to_Detect_Their_Prey).
Modal call frequencies for most echolocating bats are in the 20-60 KHz range,
but a few
are much higher (e.g., 160 KHz). Practical distances for recording the latter
species are
quite small and very dependent on frequency sensitivity of the microphone. From
flash
photography combined with call recording, prey detection distances for smaller
ultrasonic
bats are also small, perhaps only a meter or two. Echolocation for most bats is
a short
range system.
To get more information on how far away you can record various species a number
of
people have assembled microphone arrays coupled to multichannel recording
systems and
used time of arrival to estimate 2D or 3D position of bats flying within the
array. There are
many challenges in this, especially when multiple bats of the same species are
recorded
simultaneously. I have no experience with it, but what looks like one of the
nicest systems
for this is produced by Raimund Specht. With this kind of system it would be
possible to
explore how detection distances differ with environmental conditions, though
there is the
problem of steep gradients in humidity, etc. near the ground.
We have noticed that ultrasonic range condenser mics with a fritted backplate
unfortunately begin to pop extensively as they drop through the dew point while
mics with
a grooved solid backplate (and different circuitry) don't generate a lot of
noise until ice
starts to form on the membrane. At that point some quite startling oscillatory
artifacts are
generated.
Bill R.
|