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Old 14th August 2006, 06:47 PM   #1
Bazukaz is offline Bazukaz  Lithuania
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Default Low frequency absorbtion techniques

Hi,
I would like to ask about low-frequency sound absorbtion in a small room. The stone wool , placed on walls and ceiling does not work in this range. So i am considering to try a helmholtz resonator. However , i have difficulties finding some information on the web.
The room is 6.5 x 3.2 x 2.2 m dimensions , which is not good for sound recording.Very low frequency ringing lasts for seconds if you beat a door with something. It is also seems that there are not many(maybe 2 or 3 ) frequencies , at which the resonances are very irritating.They are probably in the range of 25-100 Hz.

Has anybody had this problem ? What do you think about helmholtz absorber ?

Regards,
Lukas.
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Old 15th August 2006, 04:01 AM   #2
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Helmholtz resonators will work, you will have to make an array of them at the one end of the room, and make each slightly tunable, to get the best level of absorption, across the widest band. You can chase after the fundamental with just one, but that's always tricky. The energies wil continue to build on the areas where there are none. This technique has been used in some studios, but it takes time and persistence to get it right.

It also sounds like it is a rigidity problem of the walls or floor.
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Old 15th August 2006, 08:59 AM   #3
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Hi,
You are right , one wall is made of only one layer of brick, and seems to resonate at specific frequency, as well as concrete ceiling.We have tried putting various layers of damping , but it helped little. It is even possible that wall's resonant frequency is close to room mode resonance , which would make the things even worse.
By the way , how large the helmholtz absorber's volume should be it to be effective ? Is it possible to use 20 cm(8") diameter PVC pipe for enclosure ? What is the best to use for port/volume wadding ?

Regards,
Lukas.
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Old 15th August 2006, 12:35 PM   #4
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Hi Bazukaz,

Here's an excellent link to resonant absorbers of all types.
http://www.mhsoft.nl/Helmholtzabsorber.asp

Cheers, Ralph
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Old 15th August 2006, 01:58 PM   #5
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what you will be doing, if you do this thing, is building a bunch of pipes of either 1/2 or 1/4 wavelength of the resonant modes. Full size damped pipes, in size, like those of a pipe organ.

One simple way to check, is to 'pound' (by hand, or rubber hammer) the suspected wall and cieling, in their most flexable point. This is generally in the center of the panel.

If it is all hard surfces, it could even be ventalation systems that are causing this issue. Finding the souce of your woes is the first step. Neutralizing it becomes far easier at that point, as you have an actual target for your efforts. right now, you are in a world of unknowns, so the problem seems far more difficult than it actually is.

Think of the room as a speaker cabinet with serious problems,and the door as the driver mounting point. Then fix the panel and box leakage issues, as well as going after internal damping to kill cross reflections.

Same issues, it is merely a larger scale.
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Old 15th August 2006, 08:45 PM   #6
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Hi,
Thanks for your help.
Actually , we are sure that the brick wall resonates. I pound at various places to it , and the low-frequency sound can be heard easily. For sure , the best solution would be to lay another layer of brick , and fill the gap with sand , but i am seeking for a simpler solution , if possible.

Best regards,
Lukas.
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Old 15th August 2006, 09:23 PM   #7
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If the wall is external, the brace it with a 45 degree angle piece of wood, anchored into the ground.
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Old 20th August 2006, 10:13 PM   #8
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you can use tube traps, wich are effective low frequency absorbers in the range of 30-150Hz. You must put them in the corners behind the speakers, two pieces superposing. Ther are diy tube projects, wich are useful
www.teresaudio.com/haven/traps/traps.html

http://www.dougploss.com/tubetrap.htm
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Old 22nd August 2006, 05:30 AM   #9
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Smack the brick wall a bit harder and put some cracks in it. That should help with the wall resonance.


I'm being serious here!
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Old 22nd August 2006, 07:44 PM   #10
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maybe 5% higher or lower the resonance, and the high frequency reflections less above 8 kHz, but nothing seriously may happen.
But there are big differences between men's sensibility...
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