The design I would like to share here is a simple QRD-7 diffuser with rather narrow wells, enabling diffusion far into the high mid region and the treble. Consequently the maximum well depth is limited to 14cm, as this parameter is proportional with the width of the wells. In this particular design the "productive usage" of the width/depth sequence is 100%, meaning that there is no way to increase the well depth or DEcrease the well width without either compromising the primary design target, which is to provide homogenous diffusion over the entire working range, or just reducing the working range itself.

The drawing should be self-explanatory. Nevertheless it would be good to visit my DIY-qrd page to get an idea of how this device should be built (http://www.soundscapes.nu/rb_dif.htm or http://www.soundscapes.nu/rb_qrd.htm)

Even though it is in Dutch you may find the illustrations useful and if it raises questions I'll just answer them here.

You may also want to visit http://www.soundscapes.nu/!gallery/dif/index.htm , a gallery of photos with diy- and commercial diffusers to get an idea about how to physically build a QRD panel.

The diffuser plan:

A few important additional remarks on this diffuser design are useful.

You may change the following parameters without corrupting the design:

a. Width of outer wells, which are actually the sidewalls of the panel and should measure at least half the well width in a symmetrical design. I propose 18mm, being a standard thickness here, but you may use any panel thickness between 17 and 22mm. The panel width of the end-product will of course vary when changing this parameter.

b. The panels used for the fins (the separations between the wells) are 4mm in this design, but they can be anything between 1 and 6mm, 8mm maximum. Of course the panel width will also vary slightly with this parameter.

c. Thickness of the panels for back-wall and top/bottom. Anything more than 8mm would be overkill because, when assembled correctly, this panel is rigid enough as it is.

You may NOT change the following parameters without severely compromising the design:
Well width and well depth. These must be as accurate as possible (within 1mm or less) to maintain diffusion integrity throughout the range mentioned in the specs. Avoid gaps in the corners as well! Work very accurate and the normal use of glue will close any gaps that occur.

The height of the panel is entirely free. If you can build a panel like this 180cm high it's okay.
The working range of this diffuser is theoretically 700-4900Hz, but in practice the panel will work outside of this range as well. The actual range will be something like 500--6000Hz making this the best simple diffuser design I can think of.

Well width determines the highest frequency that receives spectral and spatial diffusion. Well depth determines the lowest boundary of diffusion the panel is capable of.
The panel width is therefore the direct result of the chosen well width, resulting in a rather narrow panel in this case. I would suggest using at least two panels next to each other behind every speaker -- a third will surely follow then.....
The height of the configuration would at least have to equal the height of the ribbon, but of course a higher panel would be no problem, nor would the use of three panels next to each other.

Have a nice Christmas everybody!
Toine.

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Edit:
Width of fins might be 8mm, a standard thickness of european wood.
Gaps must be avoided through accurate construction.

That's a wonderful christmas present. I think even I might be able to build that...

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Originally posted by joseph:
Thank you Toine!

That's a wonderful christmas present. I think even I might be able to build that...

Joseph

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Build it - YES. Place it in the living room? - NO CHANCE!

Magman.

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Originally posted by Magman:
Build it - YES. Place it in the living room? - NO CHANCE!

Magman.

[This message has been edited by Magman (edited December 25, 2005).]

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Thanks for sharing!

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Originally posted by geddie:
Thanks, Toine! Can't do it right now, but I'm saving this info for future reference!

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Originally posted by Magman:
Build it - YES. Place it in the living room? - NO CHANCE!

Magman.

[This message has been edited by Magman (edited December 25, 2005).]

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Originally posted by wineslob:

LOL What.... WAF too low???

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She banished my big subs from the room. You think I would get away with diffusors?

By the way - I heard an amazing demonstration of the effectiveness of RPG diffusors; the 2' x 4' models. I heard a room that had been treated with 26 RPG's (amazing!). They removed 2, and the soundstage shifted dramatically. Very effective devices. The best imaging system I have ever heard was a studio control room that was equipped with a full set of diffusive and absorptive surfaces of various types. It is difficult to explain just how dramatically these devices can improve the sound of a small to medium sized room. Hearing is believing.

Joseph

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Originally posted by joseph:
I heard a room that had been treated with 26 RPG's (amazing!). They removed 2, and the soundstage shifted dramatically. Very effective devices.
Joseph

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Originally posted by Magman:
Up by MV's with the Duntech Soverigns, right?

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Matching diffusers come later

Now I am totally confused about WAF

Perhaps if you showed your SO's the pictures from Toine's Web site ???

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Originally posted by Ralph:
Go figure - I showed my wife the design and then the pictures from Toine's web page. She liked them and said that would look good in the new room.
Now I am totally confused about WAF
Perhaps if you showed your SO's the pictures from Toine's Web site ???
Ralph

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Now if all else fails some of you guys might get permission to apply diffusers when they're invisble behind some fabric. Although covering diffusers with fabric will have the unexpected side-effect of highly selective absorption (in a narrow band, that is), it is still better than nothing. I designed a few rooms that are literally full of diffusion, but the eye sees none.

Link to plans for the P1312 Quadratic Residue Diffuser and at bottom of page link to kit of the larger version P1324 Quadratic Residue Diffuser.

http://www.decware.com/p1312.htm

Link shows installation of 12x12 diffusers.
Note at bottom of page another link to the 30 diffusers being made and assembled.

http://www.angelfire.com/ultra/hifi/

I see advantages in using such diffusers with Apogees, as noted by many forum members, because I've had good results by placing half-round, half-reflective, Tube Trap at the first-reflection point on the wall between the speakers - about a foot further toward center than each speaker. The sound stage widened quite a bit, and gave more distinct instrumental placement. The 9" Tube Traps, however, also dried up the lower midrange, in their absorptive range, so I'd like to try something less absorbent.

While the $120 kit seems like a reasonable buy, the others seem a bit overpriced, IMHO, so I'm considering experimenting with some inexpensive diffusers made of PVC pipe, as was mentioned in another post recently.

Has anyone tried their hand at designing such a thing? I can see two approaches that seem to make sense:

- Cut some PVC pipe the long way to make semi-cylindrical half-rounds, and mount them on the wall. This is kind of traditional in many recording studios - the only downside I can see is that they need to be pretty large pipe to be effective down into the midrange - a 1KHz wavelength is about a foot, and the pipes would need to be pretty near that in their diameter to be effective, I expect.

- Mount sections of pipe as cylinders a few inches in front of the wall, or use differing sizes of pipe flat against a back-panel. This should allow for QRD-like design, but I haven't a clue what the pipe diameter (or selection of diameters), or the spacing between, or to the wall, would need to be.

Can anyone enlighten me or provide a design tool that would give some guidance on QRD-type diffuser design using pipes instead of rectangular wells? Any comments on the diameter of a cylinder and how it relates to reflective diffusion over a range of frequencies?

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Originally posted by OldRick:
I've been contemplating diffusers for the "front" wall, behind my Duetta Sigs, While the $120 kit seems like a reasonable buy, the others seem a bit overpriced....

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You could always buy one such kit and build the remaining units you would need yourself by copying the design. I guess one of the reasons for the price is the fact the designer realises his product will be copied by many, so if he manages to sell at least one kit, he is still making money. As a designer of my own line of diffusers (WaveWeavers) I'm getting very original mails from people trying to persuade me to sent drawings. "Just buy one and copy it" is my usual answer to diy-people. I won't give the design away just like that.

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Originally posted by OldRick:
Has anyone tried their hand at designing such a thing? I can see two approaches that seem to make sense:
- Cut some PVC pipe the long way to make semi-cylindrical half-rounds, and mount them on the wall. This is kind of traditional in many recording studios - the only downside I can see is that they need to be pretty large pipe to be effective down into the midrange - a 1KHz wavelength is about a foot, and the pipes would need to be pretty near that in their diameter to be effective, I expect.

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True, although the actual working range may be as much as a half octave below and above the calculated frequency in the case of serious designs. However, a half circle or cylinder does have a narrow (insufficient) working range by default. With regard to the working range a cylinder is not nearly as effective as the simplest QRD-7, that spans about 4 octaves (higher N-numbers are more complex to build, but do have a wider working range).

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Originally posted by OldRick:
- Mount sections of pipe as cylinders a few inches in front of the wall, or use differing sizes of pipe flat against a back-panel. This should allow for QRD-like design, but I haven't a clue what the pipe diameter (or selection of diameters), or the spacing between, or to the wall, would need to be.

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That is an interesting approach, Rick! Have never tried it though.... The trick with a QRD are the fins -- the separation walls between the wells. Building a QRD without fins would be very easy indeed (some do this commercially and claim it is as good as one with fins) but is not nearly as effective as a complete design with fins.

Toine.

The QRD design is really just a way of pseudo-random-number generation for the repetitive depths of the wells, so random spacing and sizes of pipes should be reasonably effective. The "wells" would be the spaces between pipes, and the round pipes should be decent diffusers for higher frequencies, as in the Fractal version of the QRD.

An issue to be explored would be whether having the inside sides of the wells convex because of the round pipes would spread their reflectivity across a broader range of spectrum than constant-width rectangular wells.

Perhaps glue two smaller pipes together to "stack" them to keep the well depth constant, or alternatively use different-size pipes and just leave the frontal plane of the diffuser at random depths.

This would be pretty easy to evaluate if I had a measurement room - perhaps I'll broach the subject with a local sound-studio vendor.

If anyone near the Portland Oregon area wants to play with this notion, let me know. A cursory look at the RPG patents makes me think that it might be patentable...

How much of these should be built, does it have to cover the same size on the wall as the size of the speaker ?

Should they be placed just behind the speakers ?

Is it possible to paint them ? if yes what kind of paint should be selected or avoided ?

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Originally posted by adol:
Questions from an acoustic treatment newby :
How much of these should be built, does it have to cover the same size on the wall as the size of the speaker ?
Should they be placed just behind the speakers ?
Is it possible to paint them ? if yes what kind of paint should be selected or avoided ?
I really think this should help a lot on my system that is a tad lost in my new big listening room, so thanks you all for the answers

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I think covering the area of the mrtw-ribbons behind the speakers against the wall is the least you can do. I would experiment with exact placement of the dif-configuration before actually fixing the panels to the wall though. Covering a somewhat larger area wouldn't hurt at all and there's also the wall behind the listening seat if you want to treat more surfaces.
However there's an unwritten rule that states that when using diffusers, you must apply a minimum of 4 or 5 periods of the design (that is: 4 complete panels wide). This is the reason you sometimes want to use a narrow panel when space is restricted: better to use 4 narrow panels than 2 wider ones to cover the same surface.

They should not be placed "just behind the speaker" -- at least not if you have enough space between the speakers and the wall. If the distance between the speaker and the diffuser is too small, the panel doesn't work as a diffuser at all and you may want to use another approach instead.
The same goes for the distance between a diffuser panel and the listener. This distance is determined by the depth of the deepest wells, and therefore the deepest diffuser possible is NOT an advantage in every case. A diffuser width a maximum well depth of 30cm requires the listener to keep a minimum distance of appr. 3meters. It's a bit like the distance one has to take into account for a large loudspeaker to integrate the sound of its units into one integrated field of sound. Too close, and you do not hear the system as a whole. Too close to a diffuser and it will not meet the specs it was designed for.

There's no problem at all painting a diffuser. Its efficiency will not depend on a layer of paint. Any material that is acoustically hard will do, even styrofoam (RPG's skyline is made of that). Paint it any way you like.
You can even use stone (brick) to make diffusers that integrate into the wall itself.

Toine.

[This message has been edited by ThingMan (edited August 11, 2006).]

A beautiful 3-D image full with music is the result - the audition happens in my livingroom

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Originally posted by rsjm80:
Thank you again to you and all for your disscussions that always help to educate me. By the way I owe you pics of my room and the mounted fish I use as diffusers, if you send your email address I promise I'll send some to you. Ray

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Good to hear you made progress; it wasn't until you mentioned the fish that I remembered our conversation.... :-)
My email-address is thingman at xs4all.nl
Looking forward to some photos!
I'm on a holiday as of tonight so I will not check my mail before the end of August!

There are a number of seminal articles on room tuning at http://www.acousticsciences.com/articles.htm and http://www.asc-hifi.com/articles.htm and while the a number of the reviews are self-serving, much of the research has become the background work from which a lot of other firms have developed their products. J. Peter Moncrief's articles, in particular, are very instructive.

It's worth noting that Tube Traps are frequently available on the used market, and these are often a bargain compared to other alternatives.

i have now build these diffusors. More are being build. Mine are as tall as the main speakers which is 2,35m. Here is a picture, sorry about the quality.

Cheers

Flo

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Originally posted by TinyTim:
Thank you Toine,

i have now build these diffusors. More are being build. Mine are as tall as the main speakers which is 2,35m. Here is a picture, sorry about the quality.

Cheers

Flo

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They look great, Florian!!

A few questions: how wide are these, what is the distance to the speakers, how many other diffusors are built and at what place are they planned to be located?

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Originally posted by TinyTim:
Thank you Toine,

i have now build these diffusors. More are being build. Mine are as tall as the main speakers which is 2,35m. Here is a picture, sorry about the quality.

Cheers

Flo

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Hi Flo,

thank you for your very much appreciated input toine.

as the thread title says these diffusors should be placed behind the speakers; what should one place on the sides and in the back of the listening place?

These are placed 1.88m behind the main panel. They are just like the plan from Toine shows. They are 2,34m tall also.

Currently its just two, but tomorrow it will be 2 more. Planed in total are 12.

I learned from reading various acoustics sites that the main reflection point is behind the listener since this will bounce back the sound to the front wall. So by killing the direct wave from behind you, it cuts down on the reflections from the front wall.

Firts i will deaden the sound in this room untill all the echoes etc... are killed off. Then i will introduce reflective and diffusive objects to bring back the liveliness into the room.

Cheers

I have a couple of questions about your interesting design.

Does the type of wood make a difference?
Can the panel be turned on it's side so the the longest portion goes up and down instead of left to right?

I have Auralex diffusors but I want to upgrade to these, as suggested by Thingman.

You need on the side walls too. They make a huge difference and are in fact essential when working with Apogees.

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Originally posted by christophs:
thank you for your very much appreciated input toine.
as the thread title says these diffusors should be placed behind the speakers; what should one place on the sides and in the back of the listening place?

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No problem to place diffusers in those locations as well!
I think the most important surface for an Apogee is the wall behind the speaker. Next important is the wall behind the listener. As these speakers do not really radiate a lot of energy sideways, the sidewalls are probably the least effective locations (I notice GD's remarks and he's probably right: they will have an effect on the sidewalls, but the biggest advantage will probably come from the front wall).
Exactly the opposite of a dynamic speaker -- if you have a dynamic speaker, then the sidewalls and rear-wall are probably most effective.

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Originally posted by rgodin:
Thingman,
I have a couple of questions about your interesting design.
Does the type of wood make a difference?
Can the panel be turned on it's side so the the longest portion goes up and down instead of left to right?
Thanks,
Rick

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Rick, the type of wood doesn't really make a big difference. The purpose of a diffuser is to provide as hard a surface as possible. Any type of wood is suitable -- painting isn't a problem either.

Thank you, I thought that changing the orientation of the panel would a bad thing. Know I know for sure.