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Author Topic: second octave trap design  (Read 2469 times)

djwaudio

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second octave trap design
« on: January 10, 2007, 12:59:07 am »

Hi everyone,
Nice forum you have here Fran!

I would like to build a set of corner traps that would work on the second octave. Pretty low frequency stuff.  I'm thinking these will go behind the speakers. Any recommendations for a trap design? Should I be thinking a 1/4 wavelength hemholtz style resonator, or a diaphramatic absorber, etc.?

I'd appreciate being pointed in the right direction for reading material on the subject or possibly design criteria information.

Thanks.
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Dana

Dana J. White
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franman

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Re: second octave trap design
« Reply #1 on: January 12, 2007, 11:03:54 pm »

EIther Helmholtz cavity or membrane traps can work for this application.. between 40-80Hz you would need some pretty deep membrane traps.. We try to avoid letting the membrane materials get much heavier than 1lb / SF as I don't feel the membrane affect is easily activated on heavier materials...

Helmholtz cavity are very easy to design in this frequency range and you could stack 4-5 tuned boxes on top of each other in each corner, and stretch fabric over the whole rig.. I think that might be how I would go (without any additional information)...
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djwaudio

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Re: second octave trap design
« Reply #2 on: January 18, 2007, 03:31:17 am »

Thanks for the ideas Fran. I'm thinking the Hemholtz stack in the corner as being viable. I wonder if this would be preferable to filling the corners with rigid fiberglass?
Are there any resonator designs floating around that are reasonable to build?

In this situation the null is centered around 55Hz and then next is around 220Hz.

The other challenge here is the side walls are not the same. Interestingly the imaging is still quite good, but I am thinking about adding a sort of acoustical shell (large gobo) extending from the rear of the speakers to the sides. Perhaps I could incorporate some trapping into that as well.  
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Dana

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J.F.Oros

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Re: second octave trap design
« Reply #3 on: January 18, 2007, 04:59:41 pm »

If you are going the resonator way (instead of broadband absorbtion) try to build your tuned devices (whatever type they will be) so that you can adjust a little their resonant frequency. That way you can tune them to the exact problem frequency on the room, because there is a big chance that your fresh constructed and installed in place device will resonate at a frequency a little different than the one predicted by your device calculator formula.

A verry complex calculator for panel resonators and absorbers you can find here : http://www.whealy.com/acoustics/Porous.html

And I would greatly recommend to take acoustic measurements while you are doing the tunning, so you can be sure that you are fixing things instead of making them worse  Smile

Good luck !
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franman

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Re: second octave trap design
« Reply #4 on: January 28, 2007, 03:31:45 pm »

designs for these types of tuned traps are available online or in the master handbook of acoustics by Alton Everest.. Good reading folks!
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djwaudio

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Re: second octave trap design
« Reply #5 on: January 29, 2007, 02:27:42 pm »

franman wrote on Sun, 28 January 2007 12:31

designs for these types of tuned traps are available online or in the master handbook of acoustics by Alton Everest.. Good reading folks!


Hi Fran, The Everest book sounds like a definitive resource. I was just about to pull the trigger on a book called "Recording Studio Design" by Newell. Can you suggest a design available online  as well?

If I were to go the broadband route, I was considering the use of this product over
OC 703.

http://www.bondedlogic.com/

Any comments on this stuff? They appear to be repurposed blue jeans!
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Dana

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franman

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Re: second octave trap design
« Reply #6 on: February 04, 2007, 01:19:25 pm »

djwaudio wrote on Mon, 29 January 2007 14:27
If I were to go the broadband route, I was considering the use of this product over
OC 703.

[url

http://www.bondedlogic.com/[/url]

Any comments on this stuff? They appear to be repurposed blue jeans!


I'm sorry but this doesn't look like a suitable substitute for 703... The acoustic specs are weak and it's too damn thin.. With the foil backing needing to be removed you'd have a wicked time trying to build a suitable bass trap out of this stuff..

BUT, it is a great use for old Jeans!! (I love this LEED crap sometimes!).... Cool
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David Glasser

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Re: second octave trap design
« Reply #7 on: February 04, 2007, 07:47:13 pm »

We've used this with great success for bass traps (not sure of the brand, but it is recycled jeans). The product we found does not have a foil backing - just batts, available in several thicknesses. It's so much easier to work with than fiberglass, and there's no lingering chance of environmental contamination from glass fibers.
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jfrigo

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Re: second octave trap design
« Reply #8 on: February 05, 2007, 02:41:33 am »

franman wrote on Fri, 12 January 2007 23:03

Helmholtz cavity are very easy to design in this frequency range and you could stack 4-5 tuned boxes on top of each other in each corner, and stretch fabric over the whole rig.


Perforated panel traps are great for this as well. It's still basically Helmholtz, but you can build it as one large box, or build it into appropriate architectural features (like existing window cavities on renovation projects) rather than stacking several individual resonators. Some notes and info are in Everest's bible along with everything else. It's based primarily on Mankovsky's work if I recall.
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