# Panel absorber and SBIR



## jpo (Mar 8, 2008)

Hello,
short and simple:
is a tuned panel absorber good cure for SBIR <100Hz ?
Thanks
JUho


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## bpape (Sep 14, 2006)

Potentially yes if the 100Hz +/- maybe 15-20Hz is the only SBIR problem. Most of the time, there are a series of peaks and dips.

Bryan


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## jpo (Mar 8, 2008)

Thanks Brian,
I use formula 344m/s / (4*0,86m)=100Hz to calculate SBIR dip. Next dip should be ( in theory ) 200Hz, 400Hz etc?
What is the formula for calculating SBIR peaks from front wall ?

And how to calculate SBIR dip from side walls?

I do have a measuring setup and RoomEQwizard but instead of random speaker movements I'd like a little more understanding of what is going on:gulp:


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## bpape (Sep 14, 2006)

Calculating from front wall is a function of the distance from the driver to the wall behind it. Side wall si the same but distance is side wall to center of driver.

You'll have peaks at 1/4, 5/4, 9/4, etc. You'll have dips at 3/4. 7/4, 11/4, etc.

Bryan


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## jpo (Mar 8, 2008)

So, for my current setup speaker to front wall is 0,7m. That gives first dip at 123Hz and peak 369Hz. Second dip 614Hz and peak 860Hz.
Distance from woofer center to side wall is 1,09m.
Woofer to listener is 2,5m and reflect path sidewall to listener is 3,96m.
3,96-2,5=1,46m difference direct vs. reflected
From that first dip 117Hz and first peak 352Hz.
:gulp: Almost same frequencys...
Does SBIR occur straight from side wall to speaker ( in my case 1,09m-> 79Hz dib )
I'm thinking the "rule" speaker distance from sidewalls and front wall shouldn't be the same.
We should add that (reflected-straight)/2 should also not be same than dist. to front wall.
So this is in theory.
I read somewhere that these distances should be in 1,3 ratio minimum.
Luckily I have line array speakers so floor reflections don't count. At least not much.


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## bpape (Sep 14, 2006)

You're likely not going to get a 1:3 ratio.

What I personally try to do is use prime fractions (1/7, 2/11, 1/5, etc.) and keep them different from each other. Now, if you run into a situation where your seating position is compromised, then you may WANT to introduce some deliberate SBIR to try to compensate if you can't move the seating.

SBIR is an interaction between a direct wave and a reflected one from the close boundaries.

Bryan


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## atledreier (Mar 2, 2007)

Old thread, but I'll dig it up anyway.

Is there a formula to use to calculate the gain from a boundary at a given frequency?

I'd like to make a spreadsheet to play around with, and the ones I find online are not detailed enough. Besides, I like to make my own and understand the physics behind it.


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## DanTheMan (Oct 12, 2009)

To calculate how much gain/loss might be a bit tricky if accuracy is desired. Bryan will certainly know more about that.

For the 'what frequency' part, this will do ya:
http://www.hometheatershack.com/forums/home-audio-acoustics/37641-i-did-little-work-sbir.html

http://mehlau.net/audio/floorbounce/

Dan


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