DIY Diffusors to the Max

[G. E.]

After having built and used quite a few DIY diffusors for the last 2 years I'd like to share some thoughts on my newest (and most challenging) project to complement the acoustic design of my project studio: a set of four equal 17x14 diffusors based on a primitive root sequence, built to fit into an existing 60cm x 60cm [1.97' x 1.97'] framework (actually the acoustic "cloud" in the studio). DIY, ain't come for cheap though -- let's see what we've got so far...

First rendering single diffusor (238 step wells):

[G. E.]

The construction is based around the following numbers -- each step acounts for a chosen 1mm [~0.04"] height in the previous rendering (colors indicating the rule how to expand the primitive root sequence to a 2-dimensional field starting at the top-left corner):

[G. E.]

I thought I should comment on the idea to use a 17x14 grid instead of some more common quadratic grid: based on the underlying number-theory one cannot use a NxN grid without dropping parts of the primitive root sequence and sacrificing the design right from the begin (hm, I'm sure I'll do that later in the design proccess...).

To stay anywhere near quadratic for the whole grid I changed the wells from being quadratic to a rectangular shape instead (30mm x 40mm [~1.18" x 1.57"]) which means that there is a slightly preferred direction with a higher upper bandwidth-limit.

[G. E.]

While being in the design-process I developed a simple diffusor calculator with variable depth-rounding based on an Excel-sheet. At the moment its 17x14 only but may be adapted for other sizes with some Excel knowledge. I plan to use the depth-rounding to reduce the number of different heights (optimal: 238) to a more reasonable number regarding the build efforts.

Personally I feel I'm not going (again) for only 4 different well depths as in the famous BBC design but more like lets say 17, 19 or 23 different well depths. Any comments on this?

17x14 fields primitive root diffusor calculator with variable depth-rounding:

[G. E.]

To further enhance the diffusion-qualities (probably for sound-waves coming from a bigger angle as is the case in a ceiling application) at least in one direction I may put thin panels of acrylic glas in between the rows of wells as shown in the 3D-modell. This is different from other designs that use either no panels at all or panels in both directions. Comments?

Diffusor 3D-modell (Google Sketchup file):

[G. E.]

Just found a similar two-dimensional primitive root diffusor calculator for different grid-sizes. It may be all you need to design your own PRD... so let's move this thread to another question: where to place diffusors in a ceiling application? Indeed very open to your suggestions!

[G. E.]

Time to bring up details of the room I'm working on:

• lenght: max 751cm [24.64'], min 535cm [17.55']
• width: 396cm [12.99'], smaller in the back
• height: 234cm [7.68'], 221cm [7.25'] in the back
• odd shaped in the back
• some extra "chimney wall" on the left side
• all walls, floor, and ceiling 30cm [~1'] solid concrete
• PVC-flooring
• RT60: 250ms

You see it's quite narrow and low-ceilinged nevertheless I went for a mid-field speaker-setting (plus a mono auratone in the nearfield).

For about half a year now I've been examining a lot of variations of speaker position, listening position and wall treatments (reflection, diffusion, absorption). What I show here is my best guess for now from a listening experience. I'm already working in the room and enjoying solid bass response, strong center image and huge depth of the soundfield. Some listeners asked for wider stereo-image (but I couldn't find it without loosing the tight bass...).

What I'm looking for right now is a more open "feeling" from above (it's quite dead because of the cloud) and maybe a slightly longer RT60.

[G. E.]

The portable diffusor/absorber elements in the back of my room already got some use as advanced gobos in a lively recording-room (RT60 ~450ms) some time ago -- hm, very good results at mixdown time, my "sluttiest snare ever"...

[mikahanau]

Sorry i don`t know anything usefull about diffusion, so i stay with reading your ideas.
In the next weeks i have to build a big diffusor for the backwall of my new studio, so this is the right time to learn as much as possible about diffusion.

I like your ideas, and maybee i go also your way ;-)
cheers
Mika

[G. E.]

Originally Posted by mikahanau

... In the next weeks i have to build a big diffusor for the backwall of my new studio, ...

Hallo Mika, I'm sure there are lots of (much better) threads around on Gearslutz whether and how to treat the BACK wall with diffusion. I'll be happy to focus on the ceiling-application because my room has very strange "back" walls...

Generally speaking, what I learned the hard way is that one might easily be disappointed by the results of "some" diffusion. Better focus on diffusion later when the bass response and first reflection problems have already been solved -- these are the most obvious problems of new rooms.

Thanks for chiming in anyway!

[G. E.]

Found an old pic of my PRD while being manufactured. Its based on a 19x12 design with three rows repeated which makes up for the final 19x15 grid. Got very heavy in the end, hard to move around...

[Loyd Mongul]

G. E. whats the dimensions of your portable diffusors? Those look really killer! thumbsup

[G. E.]

Originally Posted by Loyd Mongul

G. E. whats the dimensions of your portable diffusors? Those look really killer! thumbsup

Actually I've been inspired by a photo of Skywalker Sound! Each of the two portable diffusors/absorbers is 2' x 4'. I've included a second mount for a different height (~1.5' higher). Image shows the "beta" version with small "feet". They broke after 2 minutes of moving, I redesigned the feet afterwards and used much stronger curlings that now survive everyday use.

[Loyd Mongul]

I dig the design. Thanks G. E.

[Nordenstam]

Hi there!

Good looking diffusers! =)

Originally Posted by G. E.

To further enhance the diffusion-qualities (probably for sound-waves coming from a bigger angle as is the case in a ceiling application) at least in one direction I may put thin panels of acrylic glas in between the rows of wells as shown in the 3D-modell. This is different from other designs that use either no panels at all or panels in both directions. Comments?

Hmm.. That's an interesting idea! Don't really know this stuff, but it does seem intuitively wrong to my limited understanding. The single directional well dividers seems like they would transform it into a 1D diffuser..?

Especially when thinking about it with Huygens principle in mind. Normally, each well (or the inverse, as in the usual case with well-less 2D diffusers, protrusions) acts as a wave source that radiates into the room. The superposition of all the small wave sources in the array creates the desired diffuse sound field at a distance from the array. With the glass wells in there, the waves would bounce back and forth between the glass walls instead of propagating into space, doing weird things to the intended scattering and recombination of waves.

It seems the bigger wells will be the new wave source that emits into the room. With the effective depth of each of the wells between the glass plates being a complex sum of the various smaller depths within the bigger wells. If your design is random (with a flat spectra frequency transform) across all direction, each complex-sum-well will also be random in respect to each other. If I'm not totally mistaken about those devices, the end result should still be wide area scattering, but not as spatially uniform as a full 2D grid would give. It also seems that the scattering will be better without a grid..?

Lastly, if the sides/ends of the glass plate wells are left open, as shown in your drawing, impedance in the bigger well will probably also be quite strongly affected by the size of the wooden pieces at the ends. The variations in size there will probably skew the impedance response of each well, changing the frequency and spatial response of the array. Enclosing the ends would probably give more uniform response if you go for the dividers.

Or..?

My brain doesn't like raytracing.. Have a bit of a hard time imagining what the response would actually be from your array. Have read Cox and D'Antonio "diffuser bible" but haven't done the hard math or built any of these devices. It may very well be that I'm all wrong about this! Would love to learn more about this subject. Looking forward to responses!


Best regards,

Andreas Nordenstam

[G. E.]

I think one has to remember that in theory a perfect 2D-diffusor is meant to have dividers between the wells in both dimensions anyway. Ideal dividers should be both infinitely thin and infinitely stiff. The panels I've been thinking about are 3mm [~0.12"] thick, made of acrylic glass and glued to the surrounding wells of massive wood to get as close to this specification as possible. See: their thickness is way smaller than the wavelength even @ 20kHz so they basically won't reflect sound hitting their outer surface. They're not added to the theoretical design -- they're removed from the design in just one dimension (whereas most diffusors remove them in both dimensions)...

Question still is: does this compromise the diffusor design less than removing all dividers?

[jhbrandt]

Thumbs up Dude.

I am happy to see someone doing the math. Your diffusors look great and I'll bet you are getting some fantastic recordings.

Please keep up the good work. WOW. Good job!

[jhbrandt]

"does this compromise the diffusor design less than removing all dividers?"

Not really, IMHO, divider-less diffusors are not as efficient as divided ones. I have actually had more luck with the 2D QRDs. The hemi-disc or omnifusors are ideal for ceilings.. other than that.. I have always used the 2D.

[G. E.]

Originally Posted by jhbrandt

"does this compromise the diffusor design less than removing all dividers?"

Not really, IMHO, divider-less diffusors are not as efficient as divided ones. I have actually had more luck with the 2D QRDs. The hemi-disc or omnifusors are ideal for ceilings.. other than that.. I have always used the 2D.

Hi John, I'm not sure if I get you right so I've uploaded another rendering of the diffusors I plan to build. They're going to the ceiling (probably 4 pieces), they're 2D, they're PRDs (Primitive Root Diffusors), and as I said I may drop the dividers in one dimension... (to drop or not to drop?)

[G. E.]

Any knowledge out there about where to put diffusors on the ceiling in regard to the listening position? I think I remember someone saying NOT to use reflection points (which seems reasonable to me since I never had good results when I used diffusors on strong reflection points on side walls). So question is where...

[G. E.]

Originally Posted by G. E.

Time to bring up details of the room I'm working on: ...

Here's a rendering of the room -- started to draw it with Google Sketchup:

[G. E.]

... some may see the acoustics are partly related to a "Controlled Image Design" because of the tilted reflection points on both sides.

[G. E.]

Ready to show the possible position of two new diffusors mounted on the "cloud"...

[G. E.]

This is a speaker-/listening-position that I've found to be very usefull in my room... new in the Sketchup-file are two perforated triangular panel Helmholtz aborbers in the upper front wall corners (lots of absorption behind the panel, working "broadband" around 200Hz).

[G. E.]

Up next finishing what is in the room right now is the front wall diffusor, based on a primitive root sequence or 19x12, partly repeated to give a 19x15 grid.

[Schaap]

You have one big basstrap in the 'sharp' backcorner. I think you get a build-up of basstones there. I would do more basstrapping there, an extra chunk behind the trap maybe, and in general I guess you should do more absorption in the rear wall.

Henk

[G. E.]

Originally Posted by Schaap

You have one big basstrap in the 'sharp' backcorner. I think you get a build-up of basstones there. I would do more basstrapping there, an extra chunk behind the trap maybe, and in general I guess you should do more absorption in the rear wall.

Henk

Great idea, Henk! Actually right now its still being used for storage (what a waste). Your suggestion made me thinking that I may install big moveable absorbers that make up for a nice tracking corner!? I have another room for tracking but why not use the highly diffuse acoustics of the control room for recording...

[G. E.]

Completed the 3D-modell with two CD-shelfs (each for 1.000 regular CDs) that I use for CD storage (collection & archive) on two walls. They are 80% filled with various CD (jewel-cases, digipaks, slipcovers, etc.) and may provide some absorption and maybe minor diffusion. Anyway, better than a blank wall I guess...

[G. E.]

... decided to go for 19cm [~7.28"] well depth rounded to 1cm [~0.39"] and ordered the wood-pieces (cut in the right length) for a test-run of four diffusors. As shown in the last rendering I think I'll be using two of them in the "cloud" and the other pair for a mobile diffusor (to fit the tracking idea).

[asagaai]

Hey GE

I have built diffusors like yours- outa pine- very heavy but good for diffusion instead of heavy foam etc, and have mounted on ceiling.

If you go to my homepage (Myspace) are photographs of the diffusors in my studio, mounted to ceiling.

I have found it is best to avoid placing drums under diffusors - but if the drum kit is placed under roof absorption, and you put room mikes under roof diffusors with portable wall diffusors set up around- you get a stunning rich sound.

I also have about 4 panels of diffusors that I move around when tracking- great to place to get 1st point diffusion when tracking acounstic instruments etc.

My studio is very similar in some ways to what you are doing in respect of diffusors on ceilings- be very careful how you mount to ceiling-are very very heavy and if they fall they would crush skulls and kill people.

Good luck

GJ
Newcastle/OZ