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Discussion Starter · #1 ·
Hello,


I am new in the Room Acoustics. My room is 15x9 ft (3ft H) and the fundmental Bass resonance of the room is 37hz. I am having a large peak at 35 - 40hz, around +12db. I seat near the rear walls , where the Sofa is located (I can not move the Sofa). Also here in Brazil there are only 12" TubeTraps (Not the recomended 16" for 40hz), and also I can only have two TubeTraps in the front walls and one at rear corner, because the other corner is where the "L" shaped sofa is located.

I went in the economical way and assembled an Helmholtz Absobver following instructions from a Internet site, but got zero results, with no absorption at 40hz.

The Absorver is (all metric units) 0.6m2 area panel , with 12cm depth, 35 5mm holes, and the plywood is 10mm thick.

There is a 7.5mm Fiber glass foam Inside the absorver.

Any help will be wellcome.


Many thanks


Antonio
 

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Hello Antonio,


From your room dimensions, it sounds like the problem is the first axial lengthwise mode.


Helmholtz resonators have a narrow band over which they absorb. They need to be built for a very specific frequency.


The one you describe has a resonant frequency well above 100 Hz, so it's not surprising that it had no effect for you. It is too small, as well. Ethan is right about the size of bass traps. To attenuate a 12 dB peak, you may have to cover as much as 1/3 of your front or rear wall with bass traps!


Assuming the problem frequency is directed entirely to your subwoofer, a cheaper solution is to move the subwoofer away from front or rear walls. There is a null for this mode midway between front and rear walls. If you put your subwoofer there, you won't excite the room mode to begin with.


Regards,

Terry
 

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Discussion Starter · #5 ·
Hello,


Thank you. Just to explain with more correct information, I assembled TWO Absorbers and put it behind the Front Speakers. I AM NOT USING SUBWOOFERS FOR THIS TEST, ONLY FRONT SPEAKERS.


Dimensions:

- 1m h

- 0.6m w

- 12cm d

- 7.5cm fiber foam inside (7.5cm , not 7.5mm)

- 35 perforations , 5mm hole


I think I made a mistake. I assembled the abssorbers with the same Plywood thickness (front, rear, sides , all 10mm plywood). Don't the front panel be thinner (4mm)?


Regarding the panel abssorber frequency, the site bellow has a formula which i got teh dimentions:
http://www.mhsoft.nl/Helmholtzabsorber.asp


Any help is wellcome.


Thanks again


Antonio
 

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Hmm. The calculator at this web site appears to work correctly, according to the simple formula typically used for Helmholtz resonators. Unfortunately, such resonators can be difficult to tune because of the precision of the holes, and something called the "end correction." This is an approximation which adds a little plug of vibrating air that extends beyond the length of the air tubes created by the drilled holes.


The best way to find out what you have is to put a microphone inside, play some pink noise or a sine wave sweep, and see where the resonance reaches its maximum.


It shouldn't matter that the sides and back are the same thickness.


Make sure that the porous absorption is placed at the front (just behind the holes), not the back of the box.


Hope this helps. Good Luck!
 

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Discussion Starter · #7 ·
Hi Terry,


Many thanks.

- Remember that the Rear, Sides AND THE FRONT PANEL (the perforated one) are the same thickness (all 10mm plywood). Is this right? Or I have to replace the FRONT PANEL for a 4mm plywood?

I think this is where is the problem.


- >Make sure that the porous absorption is placed at the front (just behind the holes), not the back of the box.
 

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Quote:
I put the fiber glass panel in the back of the box. Is this make big diference?
Yes. It will not work correctly with the fiberglass in the back.
 

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Discussion Starter · #9 ·
Hi Terry,


Regarding the front Panel:

- Remember that the Rear, Sides AND THE FRONT PANEL (the perforated one) are the same thickness (all 10mm plywood). Is this right? Or I have to replace the FRONT PANEL for a 4mm plywood?

I think this is where is the problem.


Thanks


Antonio
 
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