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Originally Posted by
BasementBob 
For a 'grain of salt' remember that I've never built one of these. I just surf.
Your knowledge, detailed posts and willingness to help those that lack the knowledge (read complete novices like me) is appreciated . People like you and Bryan (bpape) are a great help to us all.
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The "d = 28900 / (M * f^2)" formula is 1DOF (one degree of freedom). It assumes that whatever's on the back is infinitely solid. The closer you get to that (like 6" of solid concete slab floor) the closer to the predicted results you'll get. Perhaps 2 more layers of 3/4" MDF. A flexable back gives two resonant frequencies, one of which is probably lower than the predicted.
I don't really have the room to add another 1.5". There will be cabinets with granite top along that side. I may be able to add sections of MDF between the cabinet frames to make the back more solid.
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It also assumes there's the structure stiffness is negligable. For wood that tends to be true for larger panels, and less true for smaller ones. A 4x8 sheet is certainly large enough. A 12"x12" sheet is too small (too stiff due to the frame). In between it's not obvious to me. Yours is 32"x31". Stiffer should be a higher frequency.
My sheet (between studs) will be 34" x 31"
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You've added insulation, which should lower the frequency according to between 1.2 to 1.4 times for the depth of the insulation. So, you have a 2x4, that's 3.5" depth. But 1" of that is 703 and 2.5" of that is air. So I'll pick a middle point and suggest that we multiply that 1" by 1.3, giving a new depth of 3.8".
d = 28900 / (M * f^2)
becomes
f = sqrt(28900 / ( d * m) )
e.g
f = sqrt (28900 / 3.5 * 0.6866 ) = 109hz
f = sqrt (28900 / 3.8 * 0.6866 ) = 105hz
Have you weighed your 1/4" sheet - is 0.6866 the actual surface density ?
No not weighed yet (still to purchase). I used the weights in your archive thread as initial reference points.
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A lot of designs try to be tunable. By
a) Allowing for the addition/removal of mass (adding mass by adding some decorative trim, removal of mass by drilling some holes and covering with something lighter).
b) Changing the depth
c) Changing the mechanical stiffness
d) Changing the air stiffness (sealing the edges to increase the air spring, or opening the edges to weaken the air spring. In yours you could weaken the air spring by drilling holes through the 1.5" of MDF)
And there was me using sealer to make the interior air tight
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There's also a ringing problem that may come up, although the insulation will likely take care of that. Sometimes insulation damps the resonance so much the panel doesn't absorb any more.
I assume that 100hz is an axial mode, that means your room is about 22.5 feet long, and 8 feet tall. Further assuming it's about 14' wide gives a room surface area of 1214 ft^2. Your absorber is 6.8 ft^2, or 0.5% of the surface. That room would have an optimal RT60 if it had about 552 sabins of absorption at all frequencies.
The room is 24'10" long, 11'1" wide at the front/screen, 8'10" at the rear with 7'8" ceiling (some of the area is soffetted). At the moment I've treated the front and 3' of sides with 2" 703 with cotton in the side corners. At moment the screen area is not covered as I'm using painted wall. Will change that once I get my screen.
Along the sides of the main HT area there will be (still to add) 1" 703 extending 8' along the wall between 7" & 55" in height. They will be panels in the region of 48"x20" framed with 2.5" Oak. The remaining wall surfaces will be wood panels(in the rear) to 57" then drywall.
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Assuming your absorber gives a peak resonant absorption of 0.2 (wild guess -- not based on anything), then at 6.8 ft^2 that's 1.3 sabins. That's 0.2% of the required absorption at that frequency.
Resonant absorbers work best at high pressure areas, such as at the wall. If this is at a modal pressure null, then it will do nothing to absorb that mode.
I'm not able, mainly for aesthetic constraints, to place against the wall. The bar was not specifically placed using any science.
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The good news is that even worst case, there's so little surface area here that even if the target frequency is different than your resonant peak frequency, well I don't think its going to hurt much.
Will you be doing an ETF5/RplusD/etc waterfall chart to look for modal ringing before/after differences?
I don't have any software to perform detailed tests. I was thinking of using the test tones from Ethans site and my SPL meter to plot a graph. Is this a worth while thing to do?
Or are there any NoVA folks that know how to do this?
Again appreciate your comments.
Cheers,
Mark
Was a bit of a gusher at the time.
If I really listen I can just about tell the funice is on but think that is the air movement through the HVAC register.
Guess I'll just have to keep looking.
