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Long Awaited Diffusor Recipe

Here it is, the intial version, diagrams and possibly photo's to come later at the web site. Please post your questions, as I may have left out some detail due to working on it so recently, that I overlooked it.

DIY PolyCylindrical MultiAxis MultiRadial Diffussor Note
by Jon M. Risch, jrisch@strato.net Initial Post 5-23-99

This is the set of instructions on how to construct your own inexpensive sound diffusor, similar in performance to the retail diffusors out there, but at a fraction of the cost. It is an original design, and is made available to all free of charge. It is designed to work well in larger spaces, very small rooms should consider getting the retail solutions, as this diffusor will not function as well in close quarters. Rooms smaller than 14 by 12 feet or so may need a more aggressive diffusion, even the best commercial diffusors need some space and room to work properly.

Read the entire note first, then get the materials and plan out the construction.

Construction materials:
one 4X8 sheet paneling (or 1/8" plywood or hardboard),
$4 for paneling
3 ten foot corrugated 4" diameter plastic drainage pipe
(the non-perforated type), $2 each
6 1X2 by 8 foot long spruce/fir/pine boards, 66 cents each
construction staples: $2 for 1,000, 9/16"
3 1/2 yards of 45" wide 1/2" to 3/4" thick polyester batting, $2.17 yard
7-8 twelve oz. cans foam expanding insulation sealant, cheapest you can find, low expansion is best, may need 2-3 more cans for that type. DO NOT use Triple expanding foams! Approx. $3 per can for REDDY Insulation brand foam.
Liquid Nails adhesive (or silicone rubber sealant/adhesive),
29 OZ, $4
handful of nails, $1
One can black spray paint, $3
Total Rough Cost: $50

black grille cloth, RS has 32" wide by 3 yrds black for $20
wood dowels

Cut the ten foot corrugated 4" diameter plastic drainage pipes in half, so that you have two 5 foot pieces for each original 10 fopopt piece. (They are actually about 4 1/2" in diameter for the corrugated portion)
Then cut 4 of the 6 pieces in half lengthwise, so they form a half cylinder. Since these are flexible, it would be wise to immobilize them up against a junction of a floor and wall in order to cut them lengthwise. A sharp box knife with a blade that can be extended far enough will do the job, or a reciprocating saw with a coarse toothed blade. It is also possible that a solder gun tip designed for melting through plastic to "cut" it would work Try to make the cuts as straight and smooth as possible, so the two halves do not have ragged edges or uneven edges, but are substantially exact half cylinders. For the two 5 foot pieces that were not cut in half lengthwise, cut a straight cut down the side without spiraling or wandering sideways. Use of one of the 1X2's as a straight edge with a marker or paint pen will help keep you on track better, I would not recommend doing it by eye. The project only needs 6 of the 8 halves, so if you mess one up real bad, you still will have enough to do the project.

Cut the end off the sheet of paneling, leaving the length along the 8 foot side as now 5 feet. You should now have a 4 foot by 5 foot sheet of paneling.

You will now construct a frame to hold the 4 X 5 paneling sheet. The frame will be shorter than 5 feet wide, to cause the sheet of paneling to bow outward and form a curved surface. The frame will also be slightly wider at the bottom than at the top, which will cause the curved surface to have a different effective diameter from the top to the bottom of the panel.

The frame needs to be constructed to hold the sheet of paneling at the side edges, so the cross members, the ones running horizontally, will be behind the side members. The paneling will tuck into the side of the side members, and initially at least, held by tension. A single cross-brace will run diagonally, and will also be behind the side members, in the same plane as the horizontal frame pieces.

Select two of the 1X2's for straightness and freedom from knots or burrs in the wood, so they will be sturdy, and cut the good end to 6 feet in length. These will be the frame's side pieces. Cut one of the 1X2's to 51 inches, and another to 55 inches. These will be the horizontal cross pieces, with the shorter one at the top. Cut the diagonal cross-brace to approx. 68 inches long, preferably with angled ends to match the frame. Now lay everything out on the ground/floor, so as to check the fit and dimensions of all the pieces. If the pieces need adjusting, now is the time to do it.

The horizontal cross pieces will be located approx. 3 inches from the top, and 45 inches from the top, so they are about 3 inches from the top and bottom edges of the curved paneling. The vertical pieces outer edge line up with the outer edge of the horizontal cross pieces.

I used small finishing nails to tack the boards together after smearing some Liquid Nails adhesive at the joints. Be sure to lay out newspaper or a drop sheet to catch any dripping or mistakes/ Either the Liquid Nails or the silicone rubber adhesive will give off fumes, so the adhesives should be used where there is adequate ventilation, or outdoors.
Do not get messy on the inside where the sheet will tuck into the side members, wipe away any excess adhesive immediately. The diagonal cross-brace will determine the squareness of the frame, so adjust the trapezoid to be even, and then tack the cross-brace down into the final alignment. At this point, you could either let the frame adhesive set-up overnight, or continue with the assembly.

Continuing, the sheet of paneling is bowed and placed on the frame, best done while the frame is on it's back. Tuck the sides of the panel into the frame side pieces sticking up. Once the edges of the panel are "tucked" into the frame edges, and the sheet has been adjusted to be as squared up as the different bowing will allow, drive a small finishing nail at an angle through each corner of the sheet at the side edges into the frame where the horizontal frame pieces are. This is just to tack the sheet to the frame. Run a bead of adhesive along the side where the panel meets the side frame pieces. Push the nozzle of the adhesive tube down into the crease a little, to get the adhesive into the seam between the frame and panel. make sure that there is a good bead of glue along this seam. This can get messy, so I recommend newspaper or a drop sheet underneath the frame and panel to prevent a mess. Have lots of paper towels or disposable wiping cloths available too.

If the frame can be left to set up while laying on its back, then this is a good place to stop, or you can continue on. If left to set up, it should be undisturbed for about 18 to 24 hours to allow the adhesive to cure.

Continuing, left over pieces of the 1X2's are used to brace the panel from behind, the first one being cut to catch the diagonal cross-brace in the approx. center of the panel, where the cross-brace is under the panel. Measure and cut a slightly oversize piece, and trim to fit. DO NOT push the panel out with this brace, let the natural curve of the panel define the radius. Use adhesive, and drive a finishing nail in the brace from behind through the cross-brace piece, and through the front surface of the panel into the new center brace on end. When the nails are driven, be sure to back stop the other end so that you are not stressing the frame when hammering.

Brace the ends at the horizontal cross brace pieces, with at least three braces in a radiating pattern or a "W" or "M" pattern of 4 pieces. Cut to fit by cutting a little long initaly, there should be plenty of 1X2 left overs. Once again, do not distort the natural curvature of the panel sheet. Glue and nail the braces into place, and you may need to use nail stops nailed into the horizontal cross piece to keep the braces from sliding and moving. At this point, the adhesive must be left to cure for at least 18 to 24 hours.

Now this structure might seem rather flimsy, and prone to be resonant, but we will be addressing that in the final assembly stage. At this point, it only needs to be strong enough to support itself, and a little weight attached to it.

After the frame and panel structure has dried/cured enough, assembly can continue. Now the two 5 foot sections of corrugated pipe that were split down one side will be affixed to the curved panel. They go at the top and bottom of the panel. The corrugated pipe is slipped over the end of the panel where the slit is, and the pipe is stapled (or nailed or screwed, what ever works best for you) to the rear of the curved panel, with about 1 1/2 to 2" of the pipe lapping over onto the back of the panel. This will make the majority of the pipe stick forward from the panel surface, so that almost the full depth of the pipe will be sticking forward from the panel, and the width of the pipe will be extending the height of the panel. Once the pipe has been stapled in place, a bead of the construction adhesive will be used to secure the pipe to the panel, both on the back, and on the front side. I would do the front first, as it is harder to get to once the other pipe sections are in place, and it should be underneath the pipe so it won't get all over as easily as for the other pipes. Once the adhesive has been placed, try not to move or slide the pipe around any more. Some will want to let this dry/cure before proceeding, others can elect to proceed with the rest of the pipes. If you proceed, do your best to avoid moving the glue bead.

The other half-cylindrical sections of corrugated drainage pipe will now be placed in a pattern on the remaining surface of the curved panel.
I will be posting diagrams at the web site, but a sufficient description should allow those eager to proceed to figure the pattern out OK. The best way to describe the pattern would be to think in terms of pipe section zones that are about 7 inches tall in a strip across the curved panel surface. Measuring from the outermost diameter of the top pipe, define a zone that is 7 inches tall, and start the end of the corrugated pipe half section at the very top, so that the top portion of the zone has the pipe halve in it, and the bottom portion is bare panel. Then curve the pipe half section downward gradually until at the one quarter point across the panel, it is all the way to the bottom of the 7 inch pipe zone. Now curve it back up toward the top until it is all the way at the top again in the middle, then all the way down at the 3/4 point, and all the way up at the other side. The end result should look like a sinewave rather than a triangle.

Once this pipe half section has been stapled into place, by stapling along the edges (about 10 staples along one edge, for a total of 20), then the next pipe zone can be defined just below the first one. Now this half section of pipe will be offset in it's "sine wave" shape, so that instead of starting out all the way to the top of the 2nd pipe zone, it starts in the middle, and curves down to full down within it's zone at the 1/8 point, swings back up to full top of it's zone at the 3/8's point, and back down to full down at the 5/8ths point etc.

The next zone down of one half section pipe would start out full down, and curve up and down exactly the opposite as the first curved pipe section in the first zone.

The fourth zone would be the exact opposite curvature as the 2nd zone, and then the overall pattern would repeat until the bottom most zone was reached.

The spacing works out to the following:
7 inches per zone, so that by the time you reach the bottom you have six zones.

Once all the half pipe sections are stapled down, then run a bead of adhesive at both edges of the section of pipe, use lots to assure adhesion of the pipe half to the curved panel. The staples are just to hold them in place until the adhesive sets.

You should now let the adhesive cure overnight, and should now have a curved panel with a series of snakelike curving sections of corrugated drainage pipes running horizontally across the front, with the top and bottom section straight, and sticking out a few more inches than the curved ones in the middle.

This structure is still rather flimsy and resonant, both the panel and the pipes are quite noticeably resonant and colored in their sound when you tap on them hard, although the panel should be better than it was.

Now for the fun part! Heh heh!

Take the expanding foam insulation in a can, and read and follow the directions for same, with the idea that you are going to try and squirt the foam into the middle of the half pipe sections, so that it expands to the edges. It is possible to inject it from one end, and then the other, but a lot easier and less problematic and messy to cut/drill a small hole in the center of the pipe sections to insert the foam nozzle/tube. The first one will be the messiest, so try to anticipate that the foam will expand quite a bit. Some foams are three to one, expansion, most are 2 to 1, and some are low expansion formulas. I prefer the low expansion, but you need a few more cans to do the job. The 2 to 1 expanding version will need 7-8 cans to do all the tubes, and the low expansion 2-3 more. The low expansion foam is denser, and will tend to damp the panel and the pipes more. It also makes the whole assembly heavier to lift and carry. Be sure to read and follow all directions on the cans of expanding foam, thefoam is a skin irritant, and dangerous to get in the eyes or other sensitive membranes. I strongly urge you to follow all common safety rules and wear eyeguards, gloves and protective clothing.

You should have some paper plates ready, along with some packing tape, etc. to close off the ends of the pipes as best as you can, with any excess foam wiped up in a short time period once you are sure it is done expanding. Tape the plates over the ends of the pipes, you can trim it all down later for a little neatness. At this stage you must let the foam set up completely, and cure completely. I would recommend several days before bringing it into the house, just to be on the safe side for any fumes or gassing off. On the last day you let it air out and cure, you can paint the whole diffusor assmebly flat black (or glossy if that is your thing), and let the paint dry for that last day too. Before painting, trim any excess foam from the edges and trim the paper plates, removing the tape as much as is practical.

The rear of the diffusor has the potential for resonant cavity behavior, so to dampen this tendency, stretch the polyester batting across the back, and the top and bottom frames, and staple into place. If your room situation is such it this material would be seen, then cover it with burlap or grille cloth.

At this point, you now have an acoustically completed diffusor, one capable of achieving about 75-80% of the performance of a retail 2-D diffusor, and at least as good as the 1-D versions. There will be a mild amount of lower midbass absorption, but not very much, and not enough to unbalance a room, in most cases, it will be a positive thing anyway. If the midbass absorbtion is a problem, then you can either make the assembly stiffer (more braces, thicker wood) or stuff the entire hollow area behing the curved panel with polyfill or fiberglass, etc.

If you want something that HAS to look better than a Buck Rogers set, then get out the adhesives, nails and screws one more time. The easiest way to hide the bizarre profile, is to create a small minimal frame to stretch grille cloth over. Add on some 3/4 inch dowels with adhesive and a nail through the back, at the extreme four corners of the frame where the panel corners are, and at the peak of the panel (which will be through the edge pipe), making the dowels stick out about 8 inches from the base of the frame and the base of the center of the panel. Once these are in place, and the adhesive has set and cured, and they have been painted black, they can have black grille cloth stretched over them, and stapled, glued, hot-melted to the frame and the other grille cloth. I estimate that a 3 foot wide grille cloth would require up to 4 running yards.

The grille should not affect the operation and action of the diffusor at all, and will hide the multiple curves and pipes. The end result of doing a stretched frame would be a large black wedge looking thing up against the wall.

As stated at the beginning of the note, it is designed to work well in larger rooms/spaces, if you are in a small room or close quarters ( smaller than 12 X 14 feet, or listening location within 5 to 6 feet of the rear wall), then one of the retail 2-D diffusors might work better for you. The RPG SkyLine PRD and SysDevGroup Art Fusor develop's their diffusion quicker than most other commercial diffusors, and are more suitable for a smaller room.

See my website at:
for more details of acoustic treatments, and how to treat a room, and where to place acoustic treatments.

Jon Risch

Copyright Jon M. Risch 1999, all rights reserved,except
transmission by USENET and like facilities granted. Any use
or inclusion in print or other media are specifically prohibited. The informational content is not warrantied in any way or form, and any use of said content are at the reader's own risk, the author shall not be held responsible in any way for any damages
or injuries arising from the content of this post. Common
safety practices are encouraged at all times. Do not fold, spindle or mutilate.

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Topic - Long Awaited Diffusor Recipe - Jon Risch 18:36:56 05/23/99 (4)

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