At 6:21 AM -0600 11/16/10, Rick Munday wrote:
>
>
> > >and the yoga block I attached them as "cheeks" on each side of the
>> >block.
>
>> Not sure what you're describing exactly, but as long as the boundary
>> planes are unobstructed it, should be fine.
>
>I'll post some pictures and audio later today. Not many places open
>around us due to hunting season. Will try one of the parks near us.
>
>> Using closed-cell high density foam as a boundary tends to
>> concentrate more of the boundary effect lift in the 1K-3K Hz range at
>> the expense of 400-1K Hz and 3-6 K Hz response. Comments on Vicki
>> Powys last pink noise test in the DIY Boundary Mics blog outlines
>> some of this. Its not a huge deal, but you might want to try finished
>> wood or another harder boundary material in your next experiment.
>
>The boundaries are wood. I guess I should've described it better. :)
>
>It's built per the plan, some fudges here and there. The angle is
>listed as 35 degrees and yet a 35 degree cut didn't meet the boundary
>length of 3 5/8" with a 4 1/2" opening between the boundaries, oddly
>enough a 45 degree angle worked exactly and left the mics angled at
>roughly 110 degrees. Typo?
Sorry. I might have mixed tweaks from two constructions. The drawing
is to scale, the text measurements might not have been updated since
the last tweaks. I'll see if I can track the exact measurements
down. An 1/8" change in the opening or width measurements of the
top/bottom changes the boundary face angle considerably. Aiming for
the 35 degree corners first is a good way to go (180 - 110 =3D 70/2 =3D
35). Another important dimension I'd try to hit is the capsule
set-back distance. Mic capsule center to center spacing can be from
6-9". We are experimenting with almost all of them though.
If you used 2-45 degree angles, the faces should be angled at 90
degrees which will make for less center imaging. If you get too big
of a hole in the middle (particularly in high frequencies), the 90
degree angle is a probable source. Also, narrowing the foam baffle
in the center would probably help some. The 10mm capsules will help
off set this as they have better high frequency response to bring out
more treble in the center of the stereo field compared to larger
diameter flush-mounted mics like AT4022's. Who knows, you may like
your mistake better than Crown specs-- a perk of experimentation.
> > >Now I just have to fill the voids in the back and we're set. Has
>> >anyone used "Great Stuff"? Or does it need to be a denser material?
>
>> The low expansion formula for doors and window will work and is
>> easier to control. Maybe put a piece of wax paper so it doesn't fuse
>> to the foam baffle? You just need to close-up the open area but it
>> doesn't have to be air-tight.
>
>Heh, good idea. I will do that when I have a chance. For now I stuck a
>couple old socks in there (clean socks!). :-)
Fabric/open cell foam typically won't block under ~1500Hz. Optimally,
use something denser like leather gloves if clothing is the preferred
theme. The mice will chew up the foam baffle first and then the
gloves. :-)
>
>> There are lots of people to thank, actually, including Michael
>> Billlingsley, Bruce Bartlett, Crown International, Walter Knapp and
>> Lang Elliot and Mike Rooke. Vicki Powys has also utilized SASS-P
>> dimensions in her array using two 172's per side that's impressive.
>> I'll be posting a comparison with her SASS-P MKH-20 mod on the DIY
>> Boundary Mics blog in a short while. Rob D.
>
>I stand corrected, credit where credit is due. Thanks everyone!
And a number of other key folks who are following- up on the
invention, fine-tuning the principles for diffuse field recording.
If you want to join-in on the conversation, we have an email
listserve now that is in the "Links" section of the blog:
http://diystereoboundarymics.blogspot.com/ Rob D.
--
|