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Re: Bat detector

Subject: Re: Bat detector
From: "Rich Peet" richpeet
Date: Tue Nov 21, 2006 9:10 pm ((PST))
I am not that concerned about the polar pattern but more concerned
about the ultrasonic freqs the mic can capture.  My job is to keep the
mic on the bat and I have enough experience with narrow capture mics.

Have you seen a lot of mics that can perform above 50,000?
Is the piezo with a freq generator the best way to test a mic?

What we need here is a way to figure out what we can point at a bat
and have it sound good.  Likely we will be wanting to go over a 192
sample rate as well as soon as that can be done without breaking the
bank. ie. a new a/d mass produced chip.


--- In  Eric Benjamin <> wrote:
> One point that I haven' seen discussed so far is the
> fact that most ordinary microphones are extremely
> directional at ultrasonic frequencies.  You would have
> to point them directly at the bat, and bats have an
> inconvenient habbit of flying around.
> Because of the cylindrical shape, most microphones
> have similar polar patterns at high frequencies.
> Using the Br=FCel and Kj=E6r microphones as a reference, a
> 1" microphone that is flat on axis will be down 9 dB
> at 10 kHz at 90 degrees off axis, and down more than
> 20 dB at 20 kHz at 90 degrees off axis.  A 1/2 inch
> microphone has similar behavior, but scaled up in
> frequency.  It will be down 10 dB at 20 kHz at 90
> degrees off axis, and down 16 dB at 30 kHz at 90
> degrees off axis.  A 1/4 inch microphone will be down
> 8 dB at 30 kHz and 11 dB at 40 kHz at 90 degrees off
> axis.
> So even if the microphone is equalized to be flat on
> axis, it will be down a lot for sources off axis.
> For this reason I would see a 1/4 inch microphone as
> being a maximum size for these recordings.  The only
> recording microphone that I'm aware of at that size is
> the Earthworks QTC series, one of which, the QTC50, is
> specified as being flat to 50 kHz.
> At a smaller size, there is the Knowles FG series
> capsule (2.5 mm diameter), which, although it is not
> particularly flat at ultrasonic frequencies would at
> least have good off-axis behavior, as long as it isn't
> stuck into anything that makes it larger than its
> natural size.
> Eric

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