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Large/Wide Poly on Back Wall
Old 13th August 2019
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Large/Wide Poly on Back Wall


I have an idea to build a poly roughly 7' 3" wide and 4' tall on the back wall of my room, which is roughly 18' behind the listening position. It would basically be the simple design outlined by Malcolm Chisholm here but with the sheet oriented the other direction. While general scattering/diffusion would be welcome, I'm mainly interested in trying this to see if it could help with bass problems/modal behavior. To explain why I'm thinking about this, here are a few data points I've found:

In this thread John Brandt shares some poly info:
Originally Posted by jhbrandt View Post
IF the poly is 24" wide it will scatter sound down to about 283Hz...
fc=((speed of sound)/(unit width))/2

The absorption will basically be the same as a broadband absorber of the same size - except you must subtract the reflections... so at panel resonance, the absorption may be as much as 0.8 if the poly is deep enough. Note; for our purposes 0.8 is nearly as good as 1.0

At 1kHz and above it may be 0.2 to 0.0
Always test to confirm. Plywood behaves differently from Masonite.

As Malcolm Chisholm says in his construction notes about polys: "A full 4x8 sheet, wall mounted, calculates to 63 Hz...12x12 contiguous will absorb to 33 Hz, 12x14 to 21..."

Doesn't say how he calculates the absorption numbers. Unfortunately I haven't been able to find anything specific about how to calculate absorption for different sized/depth polys, but as demonstrated in this thread they seem to offer some good LF absorption.

According to John's equation, the poly I want to build should scatter down to around 78hz, which I would think might attenuate some of those upper bass frequencies back at the mix position? My understanding is that these can be effective at LF absorption and could perhaps even be tuned to a specific frequency that would be most helpful to my space?

Here's a sketch of my room in its current state and another with the proposed poly (if I use a full 4x8 sheet bent to 7' 3" it will likely be a little deeper than it looks here.):

Ceilings are 7'8" to the subfloor (or rather to the drywall I installed between the joists)

I have attached measurement graphs as well as IR files in both .wav and REW formats.

For additional context about the space, other treatments at this point include floor to ceiling fluffy 12"x24" in the corners of the front wall:

Joist cavities in the mixing area (the smaller part of room's L shape) filled with fluffy 8" deep (which I stuffed into body pillow cases and suspended with tension rods:

6" of OC703 straddling the corner, which I designed to fold out into a modular gobo:

4" thick OC703 panels, 7 in total, two of which could possibly be repurposed to go behind the poly:

And this large, untreated, flat surface, which probably ought to get something later on (another poly?):

So my measurements show quite a bit of modal ringing in the waterfalls and a very deep null at 100hz. I feel like perhaps a large poly could help with those problems, but to what extent I'm unsure. Also I'd perhaps like to tune it to around 100hz if that's where it will absorb the most? If so, not sure how to achieve that. Any way about it, it's a cheap experiment and at worst I might end up with some holes to spackle. If I go through with it, I'll definitely share some before/after measurements.

Thoughts, opinions, advice appreciated, thanks in advance.

Attached Thumbnails
Large/Wide Poly on Back Wall-etcleft8-12-19.jpg   Large/Wide Poly on Back Wall-etcright8-12-19.jpg   Large/Wide Poly on Back Wall-frleftandsub1-48.jpg   Large/Wide Poly on Back Wall-frlrandsubnosmoothing.jpg   Large/Wide Poly on Back Wall-frrightandsub1-48.jpg  

Large/Wide Poly on Back Wall-waterfallbass8-12-19.jpg   Large/Wide Poly on Back Wall-waterfallfr8-12-19.jpg  
Attached Files

IRLeft8-12-19.wav (256.0 KB, 75 views)

IRRight8-12-19.wav (256.0 KB, 70 views)

File Type: mdat RoomMeasurements8-12-19.mdat (8.28 MB, 8 views)
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