At 4:39 PM +1000 7/16/10, Paul Jacobson wrote:
>
>Dan's comments are actually quite interesting, and you response
>prompted me to try a comparison between a free air AT3032 and one
>which was boundary mounted.
>
>I used the same mic for all tests, and un-altered gain (56.8 or
>roughly 36.8dB gain) settings on the SD722 for all tests. The mics
>were 50cm from the speaker in all tests to ensure the sound field as
>nearly identical as possible. There is a lot of passing traffic so
>I've endeavoured to find the quietest section of each take. The
>sound file was 16/44.1 full bandwidth pink noise played back at
>60dBSPL measured at the mic position using Faber Acoustical's Sound
>Meter iPhone app.
>
>The result was that there are definite dips in response with the PBA
>which aren't present with the "free" AT3032. I also observed that
>with a 5/8" set back very little boost in gain over the free mic. In
>fact the boundary mounted mic only shows a higher output between
>1khz-4.5khz and a significant roll off above 9.5khz. The worst case
>seems to be at 11.5khz where the output of the boundary mounted mic
>is 10dB down on the free mic. There are also a series of dips in
>response of the boundary mounted mic between 5-7khz which could well
>be a comb filtering effect.
>
Paul--
Here's the outdoor ambience blind test with insects, raccoons and
distant tractor recorded with and without the same boundary I used in
the clock/indoor test (AT4022 mics and an SD744T at full gain)
http://tinyurl.com/2daj45d
Here's the same sound file ID'd with a sonogram:
http://tinyurl.com/2796eq2
I see definite differences from 6K Hz up but, inexplicably, the
boundary mic appears to have better response at 10.8K Hz in the last
clip. The disappearance of the "fuzziness" in the strain at ~6.1K Hz
in the first four boundary mic clips is interesting.
Is this a significant array performance detriment? Let me put it this
way: If someone said I have two pairs of very low noise stereo mics
you can rent to record ambience in a quiet natural location and sent
me these samples to listen to, I'd pick the one labeled "boundary."
Its more important to me to have considerably less "jumble" from
500Hz down and the enhanced upper mid-range presence than it is to
have that dip in response from 6K-20KHz.
>What surprised me the most was the degree to which the boundary
>mounted mic's hf rolled off compared with the free mic. This is
>probably fairly significant when looking at the SASS/HSPBA
>comparison at
>
><http://diystereoboundarymics.blogspot.com/2010/03/jacobson-skeoch-sass-an=
d-diy-parallel.html>http://diystereoboundarymics.blogspot.com/2010/03/jacob=
son-skeoch-sass-and-diy-parallel.html
Yes, its very possible. Maybe I can squeeze in a PBB2 vs PBA vs Open
Air test when I do the test Dan suggested. Rob D.
>cheers
>Paul
>
--
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause
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