# Home theater/music room cloud absorber positioning



## patchesj (Jun 17, 2009)

I am going to install a 4x8 sheet of OC705 as a cloud absorber. My ceiling height is 89" (7'5"), room is 158" (13'2") by 214" (17'10") deep. My screen/false wall is 24" forward from the front wall. My concern is that my first reflections on the ceiling are about 7' wide, but very narrow front to back. This would lend itself to positioning the 4x8 sheet with the 8' wide side across the width of the room. However... The reflections start 5' off the front wall, my seating position is roughly 11' from the front wall so the 4x8 panel will stop 2' in front of my main seating position. This will cover 100% of the front row reflection points, but I've always heard that you want a cloud absorber directly overhead to help take care of any vertical modes in the room. I could turn the absorber 90 degrees and then have it cover the area directly over head, but then I'm only getting 60% of the reflections... Thoughts?

The obvious answer I guess would be to get more material and make the absorber larger...

I am seeing some room mode issues near 64Hz and 94Hz, but I think these are mainly related to room length so perhaps the cloud directly overhead isn't as necessary? 

Thanks.


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## SAC (Dec 3, 2009)

Not sure where you have heard the recommendation for the cloud to be directly overhead. That is incorrect. Positioning it directly overhead will do nothing to mitigate the reflections with which you are concerned with respect to the listening position relative to the speakers. (Just _what_ kind of spurious noises _are_ you emitting???? :laugh: )

Remember that the angle of incidence equals the angle of reflection. 

Simple ray tracing can help locate the cloud, while the use of the ETC response can absolutely insure its positioning to control the actual reflections as well as verify the effectiveness once installed and facilitate any adjustments necessary.

Note, while you did not mention the thickness of the panel, whether 2" or an optimal minimum of 4" (of optimal ~3lb/ft^3 Fiberglass or ~4 lb/ft^3 mineral wool), you will ideally want to suspend it with a minimum of a 4" gap from the ceiling for optimal broadband effectiveness for the specular energy. Note a 2" thickness will reduce the lower frequency extension effectiveness and allow those frequencies to still be reflected and impact imaging, localization, intelligibility and the coloration of the direct signal as the incomplete absorption will have the effect of acting as a high pass filter and effectively 'EQing' rather than eliminating the indirect energy.

The cloud is for the control of broadband (meaning above modal frequencies, hopefully extending low enough to incorporate the low-mid frequencies) specular reflections. Thin porous absorption placed near boundaries (in regions of low particle velocity) will have minimal effect on LF modal behavior. In fact, the aforementioned 'requirements' for an optimal minimum panel thickness of 4" with a 4" gap is required simply to extend the effectiveness down to the lower regions of the specular behavior - behavior that by definition begins after the modal behavior ends...


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## bpape (Sep 14, 2006)

I would agree that the reflection zone would be addressed first with something relatively thick. 

The problem with over your head is that you'd need something really thick (more than just 4") to address any purely axial height modes (rather than problems from reflections and phase interactions) and given the low ceiling height already and any obstruction of a projector beam, it may simply not be feasible.

Bryan


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## patchesj (Jun 17, 2009)

I'm using 2" material, with about 1.5" air gap behind. Due to low ceiling height I can't go much more.


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## bpape (Sep 14, 2006)

Then it won't likely completely address some of the lower frequency problems that could exist from those reflections. That's the big problem with low ceilings. They are problematic to place appropriate treatment - yet that's exactly why they need to be treated with thicker material.

Bryan


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## patchesj (Jun 17, 2009)

bpape said:


> Then it won't likely completely address some of the lower frequency problems that could exist from those reflections. That's the big problem with low ceilings. They are problematic to place appropriate treatment - yet that's exactly why they need to be treated with thicker material.
> 
> Bryan


Agreed... Trying to do the best with what I've got. Next house we build will have 12' ceilings. 

I've installed the cloud. Ended up with a 2" air gap on the back side. I've taken some REW measurements. Improvements in the waterfall graphs, mainly above 60Hz. Also see improvements in RT60, but I know that isn't as telling in smaller spaces. I can post some before and after graphs in a bit.

Beyond the measurements, there is a very noticable improvement in mid range clarity, a slight widening of the soundstage, but a very drastic improvement in the depth of the soundstage. Well worth the effort.


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## patchesj (Jun 17, 2009)

rt60. Blue is before, gold after








Waterfall before and after















I haven't rerun audyssey after the change. Probably need to do that again.


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