# OC 703 Panel Specs



## drummer9 (Jun 26, 2011)

I have several OC 703 panels: 2' x 4' x 4" thickness which are spaced about 9" from the wall. What is the lowest freq. they can absorb?

Thanks.


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## bpape (Sep 14, 2006)

That's really hard to say. Realistically though, you're going to start to get filtering rather than smooth absorption with that much space behind it. 

If you're talking direct waves, probably 100Hz or so. For random angle of incidence, it can have some impact down into the high 40Hz range - though it's not going to generate a very high coefficient down there.

Bryan


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## drummer9 (Jun 26, 2011)

Thanks Bryan,

I built them on 1 x 2 frames. Think I should just put them back without any air gap?

g


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## bpape (Sep 14, 2006)

Not at all. I'm just saying that normal rule of thumb to maintain a smooth absorption curve is that the gap is a maximum of the thickness of the absorption. So, a 4" panel would optimally have a 4" gap behind it.


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## tubamark (Jun 25, 2009)

bpape said:


> Not at all. I'm just saying that normal rule of thumb to maintain a smooth absorption curve is that the gap is a maximum of the thickness of the absorption. So, a 4" panel would optimally have a 4" gap behind it.


Another factor rarely mentioned on this topic is the proportion of the panel L x W dimensions to the gap itsef. The smaller the panel, the less useful that gap becomes. For example, a 12" x 48" inch panel with a 4" gap will not have the same absorption coefficients as a 4' x 8' panel with the same gap.

Right at the surface of a rigid boundary, the speed of sound is literally zero, and sound energy only exists as pressure. Fiber absorbers can't function under this condition. The purpose of the air gap is to keep otherwise useful material away from the wall, where it is least effective. The air gap gives any sound that has survived the trip thru the material to the wall a chance to get moving again before passing thru the absorber a second time. Essentially, the rearmost fibers of any on-wall panel are not absorbing sound at lower frequencies.

If panel dimensions are not big (relative to the gap), the energy buildup at the wall just releases _around_ the panel, rather than bouncing sound back into/thru it.

Low frequencies are most affected by gap change, as the highs get absorbed pretty well before ever reaching the gap.

--Mark


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