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Re: comb filtering and boundary mics

Subject: Re: comb filtering and boundary mics
From: "Rob Danielson" danielson_audio
Date: Wed Jul 28, 2010 12:42 pm ((PDT))
At 3:43 PM -0700 7/27/10, Dan Dugan wrote:
>
>Rob wrote:
>  > I'm noticing a dropping chord with a fundamental around 407 Hz that
>>  changes to about 347 Hz in all samples.
>
>Exactly.... Any reflective surface other than the ground nearby?
>Effect of the ground should be constant unless you actually changed
>elevation as you moved...comb filter effects are very sensitive to
>small differences.

A 24" high retaining wall about 20 feet behind the mics? As the low
mid-range tone happens in all three tests, I agree the source is
likely environmental.


>  > Can one tell if the steep, downward sloped bands in the last 2/3rds
>>  of the A samples and the less steep upwards sloped bands in the last
>>  half of C samples is comb filtering?
>
>That's how I interpret them.
>
>Me, too, after high-passing 12dB/oct at 8K I hear a little something
>just at the end of #1, and swishing in no. 3.

Here are the ID's in a sonogram still: http://tinyurl.com/34drwpa
and in a movie: http://tinyurl.com/3ay2r25 (.zip download)

Do you think the comb filtering in C could have been from the 3/4" X
5" wood plane located 4" below the free air mic (see photo
http://tinyurl.com/356cmzg) ?  Sound waves arrived parallel to that
tiny surface.  (The PBB2 array you see in the back was positioned
down one foot and angled so reflections would not bounce back at the
other mics. I'm pretty sure that surface was not responsible.)

For me, it significant to see comb filtering appear in very subtle
forms but not in the Perp 2 Boundary array. Crown calls their array
using this basic design, "Phase-Coherent."

A group of about 6-8 recordists has been working at trying to
understand what is behind the impressive performance characteristics
of certain boundary rigs-- chiefly by building upon positive results
and testing our assumptions as well.

Peoples guesses are in the blog:
http://diystereoboundarymics.blogspot.com/  Rob D.



>-Dan
>
>


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