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Author Topic: Do Poly's Need to be sealed?  (Read 1536 times)

djwaudio

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Do Poly's Need to be sealed?
« on: January 07, 2009, 06:41:35 pm »

I'm reading Everest and am considering using a Poly, but the surface I want to join to is not smooth. So sealing the sides with bulkheads isn't an option. By not sealing the sides, does this make it a basic membrane absorber with a curve?

What are the absorptive differences if so? I'm hoping to deal with some 70Hz null that speaker placement hasn't been able to address in an open floor-plan home.

Thanks!




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Thomas Jouanjean

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Re: Do Poly's Need to be sealed?
« Reply #1 on: January 08, 2009, 07:11:20 am »

djwaudio wrote on Wed, 07 January 2009 17:41

I'm reading Everest and am considering using a Poly, but the surface I want to join to is not smooth. So sealing the sides with bulkheads isn't an option. By not sealing the sides, does this make it a basic membrane absorber with a curve?


If closed, the cylinder will resonate at it's nominal frequency + membrane (wood) frequency, and you will have heavy re-emission at those frequencies. Also, curved wood doesn't behave like a flat wood panel - as there are specific tensions in the material. So wood re-emission freq will be hard to tell, and also vary with how the wood is maintained in an arc form.

If your cylinder is not air tight, it will behave like a tuned Helmholtz resonator, so you can tune it - but practical effective/optimum tuning can only happen on a rather short bandwidth and is linked to the size of the cylinder and size of the "port".

djwaudio wrote on Wed, 07 January 2009 17:41

 What are the absorptive differences if so? I'm hoping to deal with some 70Hz null that speaker placement hasn't been able to address in an open floor-plan home.


The difference is that one will be fixed and decided by the M
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djwaudio

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Re: Do Poly's Need to be sealed?
« Reply #2 on: January 08, 2009, 01:48:40 pm »

Thanks for the enlightening discussion Thomas.

I'm thinking of not enclosing the sides at all. So, essentially, I'm imagining a curved 4'x8' plywood panel attached to the wall only on the short sides. From your explanations, the properties of the wood will change as the panel is curved, which is reasonable to assume. I imagine this becomes stiffer and drives the resonant frequency down. My hope is that it will be fairly broad, but at least be largely active in the second octave area. Would it be more effective/predictable to not bend the wood and simply make it a large flat-panel absorber?

The room is presently treated with ASC style Tube Traps which are in the front corners and between the speakers on the front wall. I have been experimenting with their placement by finding the points where I feel the most bass in the room, and putting a trap there. This helped a good deal to bring the fullness in the sound back.
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