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In a recent Home Theater Builder article about column construction by Dennis E., there was a discussion of how to turn a column into a Helmholtz resonator tuned to 200Hz and 100Hz. The person doing my room analysis recommended absorption tuned to 50Hz near the primary viewing position where, conveniently enough, I just happen to have columns.


Unfortunately, my column dimensions are different than those in the article and I didn't find any additional information searching on this site. Can anyone help me (or point me in the right direction) with the calculations on creating a Helmhotz resonator tuned to 50Hz? My columns are 10" deep by 16" wide (so 9 1/4" x 14.5" interior dimensions), and I'd like to use pegboard rather than slats to do the tuning.


Any help would be greatly appreciated!
 

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Quote:
Originally posted by ChadA
In a recent Home Theater Builder article about column construction by Dennis E., there was a discussion of how to turn a column into a Helmholtz resonator tuned to 200Hz and 100Hz. The person doing my room analysis recommended absorption tuned to 50Hz near the primary viewing position where, conveniently enough, I just happen to have columns.


Unfortunately, my column dimensions are different than those in the article and I didn't find any additional information searching on this site. Can anyone help me (or point me in the right direction) with the calculations on creating a Helmhotz resonator tuned to 50Hz? My columns are 10" deep by 16" wide (so 9 1/4" x 14.5" interior dimensions), and I'd like to use pegboard rather than slats to do the tuning.


Any help would be greatly appreciated!
The absorber in that article is actually a perforated panel absorber. The thread Ethan is referring to is,

http://www.avsforum.com/avs-vb/showt...hreadid=278701


The tuning of a perforated panel absorber depends on the depth of the space between the panel and the wall behind it, the percentage of the panel that is perforated, and the thickness of the panel. I don't have handy the equation to calculate the tuned frequency of the absorber. However, the above thread has a link in it to a site I found with a calculator. One thing, there is a relationship between absorber size and the amount of energy absorbed. Ethan alluded to that. I posed that very question in the above thread, which Terry Montlick answered. But I didn't quite grasp his answer. I'm still working on trying to figure it out.


Tim
 

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The formulae can be found in the "Master HandBook of Acoustics" for pegboard helmholtz resonators. Unless you have read that book, I strongly suggest not wasting your time with DIY.
 

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Quote:
Originally posted by ChadA
In a recent Home Theater Builder article about column construction by Dennis E., there was a discussion of how to turn a column into a Helmholtz resonator tuned to 200Hz and 100Hz. The person doing my room analysis recommended absorption tuned to 50Hz near the primary viewing position where, conveniently enough, I just happen to have columns.

Hi Chad,


At 50Hz it would seem that your peak is due to an axial length mode. You mention that you have columns near your primary viewing position, are they on the rear wall, or side walls? I'm guessing that they are on the side walls.


The reason I ask is that in the Home Theater Builder article that you reference Dennis Erskine states:

Quote:
A word to the wise, however, if you have a length mode, and you place your aborber on a side wall, you've missed the target.
It would seem that if your columns are on the side walls they won't be effective for length mode peaks. If your columns are in the rear and you are seated on the rear wall then you probably have other acoustic problems to deal with since you are seated in a room boundary.


The way I interprete Dennis' comment is if you are seated in the middle of the room and you convert columns mounted on the rear wall to Helmholtz resonators, then this would best address your 50Hz peak. (Again assuming the 50Hz peak is due to a length mode.)


Perhaps Dennis or Terry can elaborate for us.


Larry
 

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Yup, Larry is right.


A room mode can only be reduced by placing an absorber at its pressure maximum (Assuming it's the type of absorber that works on pressure and not particle velocity - true for both a Helmholtz and resonant panel absorber). If the mode is a lengthwise axial one, a "high pressure area" will be found for sure at the front and rear walls.


However it could possibly be a widthwise axial mode, in which case a pressure maximum would be at the side wall surfaces.


A 50 Hz wave has a wavelength of 22.6 feet. Axial modes happen at 1/2 wavelength, 1 wavelength, 1 1/2 wavelength, etc. This translates to 11.3 feet, 22.6 feet, 33.9 feet, etc. If any of the room dimensions match any of these, then Bingo! You've found your axial room mode, and know where to place your absorbers.


BTW, the 1st axial mode has pressure maxima at the wall surfaces only. The 2nd axial mode has pressure maxima at these wall surfaces PLUS at a single plane midway between them. The 3rd axial mode has pressure maxima at the wall surfaces, plus at two planes spaced equally between them. Get the idea? ;)


Hope this helps. Sometimes it's hard to explain a simple concept in words. Pictures can work better. Gesticulating at the computer screen doesn't seem to work at all, in my experience. :)


Regards,

Terry
 
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