Cheap DIY Diffuser Panel Idea

[PaulP]

Originally Posted by d_pert

So it's basically a bunch of different size half-cylinders all glued onto a
board side-by-side. Could paint it, too.

I've been thinking along the same lines, instead using larger sonotubes for
the semi-cylinders, or bending thin plywood or hardboard for larger profiles.
Adding your smaller pipes would give me a wider range. I have a feeling
small pipes won't do much for low frequencies.

In my fifth edition Master Handbook of Acoustics most of the real-world
examples suggest using polys on the ceilings and walls. Polys have the
avantage that they can be stuffed with absorbant to get some absorption
as well as diffusion.

I've been reading quite a bit lately on diffusion and there isn't much in the
way of agreement as to what's best. One argument in favour of rpg's
(not the company) is their ability to diffuse temporally in addition to
spacially, something polys aren't supposed to do. A question I have is if
a poly disperses spacially, won't the sound be sent back into the room
along different vectors so that for a single person in one spot only one
thin slice of the reflection will be received while the rest of the sound will
end up being reflected at different times when it encounters the room's
surfaces in different spots. So wouldn't the sound end up getting dispersed
in time ?

I find myself attracted to polys for their ease of construction and light
weight, especially for the ceiling. I'm still trying to figure out a good
pattern. One rule is to have the polys run in different directions on the
various surfaces of the room.

Paul P

[d_pert]

Will reply more thoroughly tomorrow.

Can you post a link to a good photo of a "poly".

Thanks.

[PaulP]

Originally Posted by d_pert

Can you post a link to a good photo of a "poly".

The term poly is short for polycylindrical which Everest/Pohlmann use for
describing "a convex section of a cylinder". So your pipes cut in half are
polys.

Yesterday evening I came across these posts that show some nice (big)
ones by matucha :

How I built my bass traps...
How I built my bass traps...
How I built my bass traps...

I've been thinking of polys mainly for my ceiling which are 9' (2.7m) high.
Full absoption across the ceiling with (smaller) polys underneath spread
out in some yet to be determined pattern.

Paul P

[Jesse Eastman]

This will do something, but there's no way to tell exactly what it will diffuse at. It would be probably easier and more functional to build standard poly diffusers.

[d_pert]

I have a feeling small pipes won't do much for low frequencies.

Indeed ... however it's now my understanding that bass TRAPS (diffusion) are the best for our typical home studio applications. A bass diffuser would have to be HUGE to do anything. I'm viewing the general, broadband diffuser as something that would work along with bass traps – as a system.

A question I have is if a poly disperses spacially, won't the sound be sent back into the room along different vectors so that for a single person in one spot only one thin slice of the reflection will be received while the rest of the sound will end up being reflected at different times when it encounters the room's surfaces in different spots. So wouldn't the sound end up getting dispersed in time ?

Makes perfect sense to me - yes. Without getting too hung up on specific diffuser design ideas, I think the general idea is to DISORGANIZE the sound. As long as we’re accomplishing that – we’ve done tons for the cause. I remember "retro-reflection" in the optics part of physics class .

I find myself attracted to polys for their ease of construction and light
weight, especially for the ceiling. I'm still trying to figure out a good
pattern. One rule is to have the polys run in different directions on the
various surfaces of the room.

Cool. Here’s another idea: What do you think of a collection of small, attractive window shutters (you know, the kind with angled slats all over it)? There are room-divider, standing accordion types, too. Geez, that could be a good start for a back wall, eh?

[d_pert]

Originally Posted by Jesse Eastman

This will do something, but there's no way to tell exactly what it will diffuse at. It would be probably easier and more functional to build standard poly diffusers.

Thanks. What would you call a "standard" poly diffuser? Are there rule-of-thumb measurments? Can you refer to a photo link?

[Ethan Winer]

Originally Posted by d_pert

I buy a variety of different diameter PVC tubes

That might help if the tubes are 2 feet in diameter.

--Ethan

[d_pert]

Thanks Ethan. What effective bandwidth would a bunch of them, say 2ft and 1ft alternating produce - realizing this is a really crude estimate?

Is your 2ft spec for a broadband-type application? I'm not intending this proposal be for bass improvement - that would be handled by traps.

Actually, there are 2ft diametre PVC available ... could still be a neat design ... bunch of these 8ft long...

Thanks.

[d_pert]

I really need to review what actually happens when different shaped wave fronts (straight, circle-section) encounter planes, curvatures, cylinders and polyhedra in general which present sizes relevant to the frequency of the wave front.

Have you ever seen a downloadable simulator or illustration that shows (possibly in more than one projection) how various wave fronts are effected and reflect in/out of:

-half-cylinders
-RPG style QRD units
-Helmholtz resonators
-any polyhedron

I mean a computerized version of a simple wave-pool. Did you ever do wave interference labs with lights and a large tray of water and mechanical point or line wave generator? Totally illuminating!

I'd love to see a software modeling app to mess around with.

[PaulP]

Originally Posted by d_pert

Have you ever seen a downloadable simulator or illustration that shows
(possibly in more than one projection) how various wave fronts are
effected and reflect in/out of:

-half-cylinders
-RPG style QRD units
-Helmholtz resonators
-any polyhedron

Well, there's this guy :

Ripple Tank Simulation

though it won't be easy to similate a Helmhotlz resonator. It would be
nice if frequency and dimensions were shown.

Paul P

[d_pert]

Awsome. Awsome. Awsome. Feels like Christmas.

(Forgive my exuberance.)

[Ethan Winer]

Originally Posted by d_pert

What effective bandwidth would a bunch of them, say 2ft and 1ft alternating produce - realizing this is a really crude stimate?

I have no idea. The only poly I ever built was the one I used in my All About Diffusion video. Glenn or Frank (GIK guys) might be able to tell you how to determine frequencies directly based on diameter, though I'm sure the shape has an affect too.

--Ethan

[PaulP]

Here's a quote from p.133 of the Master Handbook of Acoustics :

The polycylindrical diffuser (poly) is an effective diffusing element, and
one relatively easy to construct; it presents a convex section of a
cylinder. Three things can happen to sound falling on such a cylindrical
surface made of plywood or hardboarmd: The sound can be reflected
and thereby dispersed as in Fig. 9-6B; the sound can be absorbed; or the
sound can be reradiated. Such cylindrical elements serve as absorbers in
the low frequency range where absorption and diffusion are often needed
in small rooms. The reradiated portion, because of the diaphragm action, is
radiated almost equally throughout an angle of roughly 120°,as shown
in Fig. 9-7A. A similar flat element reradiates sound in a much narrower
angle, about 20°, as shown in Fig. 9-76. Therefore, reflection, absorption,
and reradiation characteristics favor the use of the cylindrical surface.
Some very practical polys and their absorption characteristics are
presented in Chap. 12. The dimensions of such diffusers are not critical,
although to be effective their size must be comparable to the wavelength
of the sound being considered. The wavelength of sound at 1,000 Hz is a
bit over 1 ft, at 100 Hz about 11 ft. A poly element 3 or 4 ft across would
be effective at 1,000 Hz, much less so at 100 Hz. In general, a poly base
or chord length of 2 to 6 ft with depths of 6 to 18 in meet most needs.
Axes of symmetry of the polys on different room surfaces should be
mutually perpendicular.

It is important that diffusing elements be characterized by randomness. A
wall lined with polys, all of 2-ft chord and of the same depth, might be
beautiful to behold, but is not very effective for diffusion. The regularity of
the structure would cause it to act as a diffraction grating, affecting one
particular frequency in a much different way than other frequencies, which
is counter to the ideal of wide-frequency diffusion.

[d_pert]

Thanks. That really covers it.

[Weasel9992]

Good post Paul.

Frank

[bobbybovine]

Did anyone ever construct any of these? Curious to know if how well they work.

Bob

[darkbuddha]

I'm probably gonna catch crap for this, but I thought I'd mention it...

I built some poorman's polys by cutting a 12" diameter tubular cardboard concrete form into thirds. This results in a 12.5" arc that is 10.4" wide and 3" high. I filled the back with SAFB insulation, faced them with some kraft paper (not that I think they really needed the paper now), and covered 'em in fabric. And surprisingly, they actually worked a bit to even out some of the upper-mid and upper ranges and even helped with some peaks and nulls down low, especially for the rear speakers. They did add to a couple other problems, but it's kinda a "6 of one, half dozen of another" kinda thing. Most importantly, they solved some phasing issues I was having when I moved too far out of the "sweet spot" between the front and rear monitors.

There it is for whatever it's worth.