On 16/07/2010, at 2:36 AM, Rob Danielson wrote:
> Which is the boundary mic arrangement and which is open air? Is
> there a significant difference in self-noise performance? Is there
> "awful comb filtering" in one sample that is not in the other?
Hi Rob,
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.
The follow file is a comparison between free followed by boundary mounted. No
adjustments or filtering have been applied to the clips.
http://demo.mactrix.com.au/at3032/freevsboundary.mp3
and a screenshot of the frequency response of the two clips. The free mic is
the green trace and the boundary mounted mic is the red:
http://demo.mactrix.com.au/at3032/3032green_free-red_boundary.png
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-and-diy-parallel.html
cheers
Paul
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