The second mike is mounted so that the element is at the bottom of the dish
"well below the focal point", about 5" below to be more exact.
Further testing does verify Marty's observation that the technique will not
work at higher frequencies unless the mike elements are placed right next to
one another, which defeats the purpose by eliminating the main signal too.
It's pretty much a wavelength thing. At 500 Hz, the wavelength is 27". At 1
kHz that drops to 14".
Thus, I imagine that a 5" separation (as described above) will still give
good pretty good elimination up to maybe 500 Hz or thereabouts, but the
effect will be reduced above that. Another possibility would be to keep the
second mike element parallel to that of the first, maybe by mounting it
along the edge of the parabola, but slightly within the dish. Should the
second mike be mounted along the edge of the dish, then one should insert a
volume control somewhere to allow you to reduce the effect of the second
mike in order to correctly balance the input levels to optimize noise
reduction (otherwise the second mike signal will overpower the first mike
signal with regard to the background noise, and it's louder signal would
then become audible . . . in other words, each mike is, in effect, noise
cancelling the other . . . ot all depends on how you look at it).
One other problem is the "sound" of the resulting signal. Because off-axis
low-pitched sounds are enormously reduced, the resulting recording might
sound very "filtered", similar to what would happen if you rolled things off
below 1 kHz later in the studio. A careful comparison of the two approaches
should be performed, but I'm not set up well to do this.
Lang
> Lang,
> How did you have the second mic mounted so that it was in the bottom of the
> dish at the focal point?
> Wil
>
> -----Original Message-----
> From: Lang Elliott
> Sent: Monday, February 11, 2002 10:57 AM
> To:
> Subject: Re: [Nature Recordists] Re: Noise Cancelling (was Pollution)
>
>
> I just did a second test in which I tried to greatly reduce or eliminate
> "white noise" coming from behind the parabola (from a point source speaker).
> I immediately realized that I get a much better result if the second,
> polarity reversed mike is placed somewhere on the "inside" of the reflector,
> so that it is receiving the same sound level (from the white noise) as the
> main mike. I got good results by scrunching the head the the second mike
> down to the bottom of the parabola, along the axis, well below the focal
> point (= at the bottom of the dish).
>
> I also tested this arrangement using the on-axis playback of my bird song
> and found the same 7db reduction in signal level when using the noise
> cancelling mike.
>
> Encouraging! If this kind of setup allows for, let's conjecture, a 15-20 db
> reduction in background hiss due to wind or stream noise as well as a nearly
> complete elimination of distant rumble, while only reducing the on-axis bird
> sound (or other animal sound) around 7 db, then this has definite
> application, at least for parabolic-type recording.
>
> Lang
>> Well, my first test is done and the results are encouraging.
>>
>> I did the test in a room by playing a high pitched bird song over a single
>> speaker and aiming my parabola directly at the sound source, focusing it
>> carefully before taking measurements. The room is a conventional one with
>> hard walls and hence some reverberation, which might interfere slightly
> with
>> the results.
>>
>> I used the meter on my DAT recorder and adjusted the input level peak of
> the
>> repeated bird song to -2 db when using the Telinga parabola with a single
>> MKH 20 microphone mounted at the focal point of the parabola.
>>
>> As I mentioned earlier, I mounted a second microphone on the edge of my
>> parabola, pointing it upward and equipping it with a short cable that I
>> wired to reverse the polarity.
>>
>> When I added the second mike to the recorder input using a "Y" cable
>> connector, I immediately noticed two things:
>>
>> 1) background rumble from traffic outside completely disappeared
>>
>> 2) the bird song itself was reduced in intensity around 7 db
>>
>> This is encouraging. The 7 db reduction in the bird song seems minimal in
>> comparison to the extreme reduction of rumble. In non-reverberant outdoor
>> situations, this reduction might be even less, perhaps only 5 db or so.
>>
>> As Marty just mentions, this is to be expected for sounds at the low end,
>> which you could filter away later in the studio. However, will this work
>> with more broadband sounds such as hiss from wind or stream noise.
>>
>> I propose that it might very well allow for drastic reduction of things
> like
>> stream noise, as long as you orient yourself so that the stream is
> directly
>> behind you. Thus, if the noise-cancelling mike is mounted along the edge
> of
>> the parabola, in the same plane as the mike mounted in the parabola, then
>> arrival time will be virtually the same and the annoying stream noise
> could
>> be almost completely eliminated. Sure, there would be a lot of complicated
>> things happening as it concerns other off-axis nature sounds (such as
>> another bird singing thirty degrees off-axis), but this might not matter
>> unless such off-axis sounds are loud. It seems it would be far more
>> important to render your main subject cleanly while completely eliminating
>> both rumble and broadband sound such as a distant stream or waterfall
> placed
>> directly behind you.
>>
>>
>> Lang
>>
>>> 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
>>>>>
>>>
>>>
>>>
>>> To unsubscribe from this group, send an email to:
>>>
>>>
>>>
>>>
>>> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>>>
>>>
>>>
>>
>>
>>
>> To unsubscribe from this group, send an email to:
>>
>>
>>
>>
>> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>>
>>
>>
>
>
>
> To unsubscribe from this group, send an email to:
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
> To unsubscribe from this group, send an email to:
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
________________________________________________________________________
________________________________________________________________________
|