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Re: Condenser mics and extreme cold...

Subject: Re: Condenser mics and extreme cold...
From: "werainey" werainey
Date: Sat Jun 7, 2008 12:00 pm ((PDT))
>
> 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.




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