Is a helmholtz resonator with resonance at 125Hz possible?

[Schwerdy]

Hi all!

First I want to excuse me for my very bad english. Second I have problem in my wannabe-studio. At 125 Hz everything gets insufferable loud! So I would like to build a corner absorber as described here:

http://www.johnlsayers.com/HR/index1.htm

The document says, that the corner unit is for the 150Hz - 500Hz range, but with the Helmholtz Resonator Calculation Sheet, I can build one for 125 Hz. EG:

Slot width.: 3
Slat width.: 420
Depth from wall.: 500
Slat Depth.: 20

So it should be possible, or not?

The Third question: What is the "depth from wall" for a corner unit? Is this the mathematically "height" of the triangle?

Thanks!

[lovecow]

Schwerdy,

The document says, that the corner unit is for the 150Hz - 500Hz range, but with the Helmholtz Resonator Calculation Sheet, I can build one for 125 Hz. EG:

Slot width.: 3
Slat width.: 420
Depth from wall.: 500
Slat Depth.: 20

So it should be possible, or not?

If your measurements/dimensions above are in millimeters, then I think you meant "50" for "Depth from wall." Otherwise, 500mm deep will put the resonance around 40 Hz.

Other than that, this design should work for the range you specified.

The Third question: What is the "depth from wall" for a corner unit? Is this the mathematically "height" of the triangle?

According to Everest, an average depth is used for calculation purposes.

[knightfly]

Schwerdy, Jeff is right on the money - it appears to me that you used the old calculator on the SAE site, which has the same error in the formula that is all over the web; if you check here

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=1363

there are two newer versions of that calculator that are correct in their calculations - for your example, if you were to use 6 mm slot width and 90 mm slat width with your 500 mm average depth from wall and 20 mm slot depth, you would be right at 125 hZ.

If you don't have that deep an AVERAGE depth, you could adjust things - for example, if your average depth is only 250 mm, you could use 3mm slot width/90mm slat width/250mm depth/20mm slat depth, which calculates at 127 hZ -

Please note that in the corrected version I posted, I left in the OLD, INCORRECT calculation - this is highlighted in green, and is there for those who had already calculated and built these units just for an idea of what those units might be doing - if you're building these for the first time, just IGNORE the GREEN highlighted value and use the one in the BROWN highlighted area.

Hope that helps... Steve

[lovecow]

Schwerdy,

FWIW: Steve's specs for both scenarios (500mm and 250mm avg. depth) give you a better absorber than what you originally posted. Note that this assumes you'll fill the cavity about 20-40% with "fuzz."

[Schwerdy]

Thank you very much for helping!

But I still don't really understand what's the "depth from wall" or "average depth". Maybe my stupidity in English is the cause...

I'll attach a drawing. Is the "depth from wall" (what i need for the calculation - and the building!) the small or the long red arrow?

Thanks!

[lovecow]

For this type of triangle (right, isosceles), the average height is half the height. If the height from the center of the face into the corner (long arrow) is 500mm, the average height is 250mm.

[Schwerdy]

Thank you!