Dear Lang:
The usual problem with automatic acoustical sound cancellation is the
distance between the two mic diaphragms, in wavelengths, and the effect
this has on the phase arrivals. To put it mildly, it destroys any
cancellation, per se.
If the mics are 12 inches apart, the results will be not at all the
expected ones. This distance is one full wavelength at about 1 kHz. The
cancellation effects will only be apparent for such frequencies as the
travel distance DIFFERENCE, to the two mics, is much less than a
wavelength. At about 100 hz, for example, wavelength 10 ft., for sounds
hitting one mic one foot before the other mic, will generate a phase
difference of 360/10 = 36 degrees. Soldering the mic inputs together in
reverse polarity, (180 degrees) the effective cancellation will still be
spoiled by having them from 144 to 216 degrees apart, and all in between,
exacerbated by having inexact level matches as well as phase variation.
For tones lower than 50 Hz, you may achieve some useful attenuation, and we
all await your results. But these are hardly the frequency domain handled
by any parabola anyway, so typically are edited out afterward to isolate
bird sounds. In the range from 500 Hz upward, the phase shifts and
cancellations will be highly unpredictable and possibly even nasty
sounding, but I do confess more than a meagre curiosity, and do not let my
grousing ruin the tests.
my very best,
Marty Michener
MIST Software Associates
75 Hannah Drive, Hollis, NH 03049
coming soon : EnjoyBirds bird identification software.
At 09:43 AM 2/11/02 -0500, Lang wrote:
>Klas and others:
>
>I am preparing to test the dual-mike, reversed polarity technique using my
>Telinga parabola and two MKH 20 mikes. I've mounted a second MKH twenty on
>the edge of the parabola, aiming upward. This should receive most all
>background sound. I'm in the process of soldering a special short cable with
>two female XLR plugs, to reverse the polarity coming from the edge
>microphone. I'll then connect both mikes with using a "Y" connector and
>route the mixed signal to my recorder.
>
>This should be very interesting. I hadn't thought of using this technique,
>but it should drastically reduce background rumble and it possibly could
>have minimal effect on the song of a bird, as long as the bird is
>well-focused using the parabola. Basically, the only attenuation of a
>"focused" bird song should be whatever the level would be at the mike
>without using the parabola.
>
>Klas, have you actually tried using this technique in the field? If you
>have, then what were your results?
>
>Lang
>
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