Thanks that answers my questions on how far the mics can be apart
from the source of the wave. I am hoping if my cancelling mic is
directly above the parabolic mic and I have a line of site noise
source the waves will be similar (ie the barking dogs). But I don't
know that yet.
I will confine myself to post production cancelling in cool edit so
that I have the gain of the unwanted noise matched before cancelling.
Not to mention determination of what else was cancelled or added. I
worry that by having it real time that you can actually inject more
noise and reduce the dynamic range of the desired source. Possibly
because the cancelling mic heard sources that the parabolic does not,
as well as the target which it did.
I too will play.
--- In Marty Michener <> wrote:
> 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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