Building an Hemholtz Resonator with "oz board"

AK-69 · Post by **AK-69** » Tue Oct 18, 2005 11:05 pm

I'm looking into building a few (differently tuned) HR's using oz board. I don't know if there is any official name for this, but I'll give a description: thin wood/mdf sheet with a lot of holes in it. You might call it perforated wood.
I was in a studio recently where they used only these as broadband absorbers and the room sounded great. No problems from very low to high frequencies!
Does anyone have plans for building these? Any feedback is welcome!

AK-69 · Post by **AK-69** » Sun Oct 30, 2005 9:57 pm

Someone must know what i'm talking about?! Maybe my explanation isn't clear.. It's a regular panel absorber made out of rigid fiberglass. The only difference is that it's in a frame and the side facing the wall is made out of a panel with square holes in it. I just looked at one recently and it might be a lot thicker wood than I first thought. I guess there is an air gap between the panel and the fiberglass.
Please share any knowledge, I'm very determined to build one!!

knightfly · Post by **knightfly** » Mon Oct 31, 2005 2:57 am

A short excerpt from F. Alton Everest's Master Handbook of Acoustics - "In commonly available materials, such as pegboard, the holes are so numerous that resonances at only the higher frequencies can be obtained with practical air spaces. "

There is a lot more on this subject but I'm not comfortable posting entire sections of material that's for sale, any more than I pass around copies of CD's I've purchased - you need to buy a copy of this book anyway, it's the bare minimum if you're doing anything with acoustics. If you can't find a copy locally, perhaps Amazon.com - they have both the paper book and the electronic version (better searching) for around $USD 25.

BTW, common pegboard has holes on 1" centers (25mm) - some are smaller, like 3mm, some are larger, around 5 or 6mm.

There is a formula in the book for calculating the frequency of a perforated panel resonator, but it's stated that it's only for ROUND holes - not sure, but I think that changing the multiplier (outside the radical) to = the multiplier used for slat resonators might be more accurate. Here's the formula as it stands, I'd change the number 200 to a value of 216 for square holes, but this is just a wild guess -

Bottom line here is that if you want accurate results, you'll need to spend more time/study, and experiment to find what REALLY happens. This would require something like ETF software to be able to quantify what you actually accomplished

http://www.etfacoustic.com/

This is possibly the most reasonable cost software of its kind... Steve

AK-69 · Post by **AK-69** » Mon Oct 31, 2005 7:07 am

Thanks knightfly! This looks like enough to get me started

Is there anyone who has actually built one of these?

AVare · Post by **AVare** » Mon Oct 31, 2005 9:54 am

AK-69 wrote:Thanks knightfly! This looks like enough to get me started Is there anyone who has actually built one of these?

Built what? A midrange tuned absorber?

Andre

AK-69 · Post by **AK-69** » Mon Oct 31, 2005 8:13 pm

Built a perforated panel absorber.. in general. I understand that I'll have to figure out what my problem fequencies are, and tune them according to that. I would like to know the details on construction.. plans maybe? I don't have a clear idea of how it should be constructed efficiently.