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
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