quote:

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Originally posted by Jose Manuel:
IMHO rear wall must be reflecting with dipolar speakers.

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Reflecting is exactly what it is when solid and acoustically hard diffusion is implemented there (Skylines are indeed not the most suitable solution for diffusion of the front-wall behind an Ap, as they are not really solid and sonically neutral).

The difference with or without solid QRD-diffusion on the wall behind the Ap's is quite noticable. Diffusion does not significantly affect the quality of the sound itself, nor the tonal balance, although I noticed improvements in the lower midrange on several occasions with Magnepans and Apogees.
However it deepens and widens the stage with several meters, up to the point where the physical walls disappear and make way for the recorded space, if available -- sometimes hugely wide and deep as is the case when you sit in row 12 in a concert hall.

Without diffusion everything is still excellent, no doubt, though less involving and convincing when it comes to portraying the recorded spatial clues. Without diffusion stage depth is less dependent on the recording and more dependent on the room boundaries themselves -- in other words: depth is usually the same and much more reliant upon distance to that wall.

The only aim to implement diffusion is to make the physical walls of your listening environment disappear. A flat wall will never do this, due to certain principles that can easily be explained. Unless, of course, you have a very large room, which unfortunately I do not have...

It is not a matter of good versus bad; it is merely a matter of real versus more real, especially if the room is not as big as you want it to be...

Toine.

Building these diffusors is my next project. Two questions for you:

1. Are the 35 cm and 70 cm wells hollow or stuffed?

2. If they are hollow, then what is the width of the sheet? 8cm?

I guess I need 18cm, 4cm, and 8cm sheets,right?

Thanks

the "empty" spaces in my diffusors are stuffed with wool.

Cheers

quote:

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Originally posted by gallant_diva:
Toine:
Building these diffusors is my next project. Two questions for you:
1. Are the 35 cm and 70 cm wells hollow or stuffed?
2. If they are hollow, then what is the width of the sheet? 8cm?
I guess I need 18cm, 4cm, and 8cm sheets,right?
Thanks
GD

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Hello Ish,

What do you mean by 'sheets'? Is that panel thickness?
All measurements are in millimeters, not centimeters.
Well width is 35mm.

No problem to leave the hollow spaces empty. No problem to stuff them with (mineral) wool either.

Please have a look as well at the photos mentioned in the first message of this topic. They might answer many questions that can arise.

Toine.

quote:

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Originally posted by gallant_diva:
Oppps, yes mm, not cm. Thanks TT and thingman.
I was refering to panel and well thickness., which would be cut from sheets. I am thinking of making at least 4 units, each 6 feet or so tall.

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Periodicity indeed is a condition for homogeneous diffusion. The minimum number to be used would be 4 or 5 periods (full sequences) for a given surface to be appropriately diffused; more is always better in order to suppress the 'lobing effect' of a single or double period even more.
Toine.

quote:

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Originally posted by gallant_diva:
Toine:
I have a question. What is the difference in the function of the Skyline diffusor and the QRD diffusor that you posted? Can you please list their pros and cons? Thank you.

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The Skyline (also called a PRD or primitive root diffuser) is a so-called 2D-diffuser, which means diffusion of sound occurs in all directions after striking such a surface.
A PRD has a relatively small working range of about 2,5 to 3 octaves at best.
A PRD-surface takes a lot of time to construct and will be very heavy if made of wood (the harder the surface of a diffuser, the better it will work, so wood is crucial!).
The original Skyline diffuser is made of lightweight polystyrene, not very neutral for sound, quite absorptive and not very reflective by nature. Made of wood the weight would be very high, as stated, but the diffusive effect would be 'better'.
A PRD-diffuser is not a quiet surface to look at and hiding a diffuserpanel behind fabric is not an option. It must be directly "visible to sound"...

A QRD (quadratic residue diffuser)is a so-called 1D-diffuser, which means that diffusion of sound occurs at right angles to the segments. One has to chose by experiment which angle is the best.
The simplest, well designed QRD-7 has a wider working range than a similar PRD (> 3,5 octaves). A higher number QRD (11, 13 or 17) has an even wider working range and than there's also the possibility of fractal diffuser design (embedded diffusion in each segment), stretching the working range to almost cover the entire audiospectrum (400-20000Hz is no problem with a two-way fractal design).
A QRD diffuser with specs comparable to its PRD-equivalent is far easier to construct and even the higher numbered QRD's are still relatively easy to build.
Wood and other hard surfaces are much easier to shape into a QRD and the hardness of surface is crucial for the working of a diffuser.
QRD-diffusion is easy on the eye, especially when the colour is chosen with care.

So the fundamental question arises whether or not one needs 2D-diffusion over 1D-diffusion to obtain the best possible results.
Theoretically 2D-diffusion seems to be the best option, but in practice this is not the case.

My experiments with 2D-diffusion combined with Apogee speakers were not that positive, mainly because of the limited working range. One gets a lot of diffusion in too small a section of the audio spectrum, usually somewhere in the midrange. Not good in the long run, though sometimes very impressive in the beginning.

If fractal QRD-diffusion is beyond building capability, then a well designed QRD-11 or 13 is a fine compromise, as it can have a working range of 5 octaves.
Having said this I must admit that for several years I listened with a lot of satisfaction to a front-wall covered with simple QRD-7 panels with a working range of almost 4 octaves (500Hz to 7kHz), which is enough for anything but the most demanding circumstances.

PRD-diffusers tend to have an effect that's usually 'too much'. Depending on the location where they're applied, they either force the soundstage to become too upfront or too pressed down.
The best place to apply PRD diffusion in combination with Apogees (in my experience!) is on the wall behind the listener, where they are also invisible to the eye. I have tried to use them as floor-diffusers in my less-than-ideally-shaped-room and this was not very satisfying. Now all of them are on the backwall, where their enveloping characteristics are well in place.

Hope this helps,
Toine.

Fantastic post - thanks. One of the most useful I've seen in a long time.

I interpret what you're saying to mean that the RPG Diffractal+ is the kind of diffuser you like. Are there other comparable (or better) commercial products on the market? Or is the alternative to RPG to make such a diffuser oneself?

best,

quote:

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Originally posted by sea-speak:
hi Toine,
Fantastic post - thanks. One of the most useful I've seen in a long time.
I interpret what you're saying to mean that the RPG Diffractal+ is the kind of diffuser you like. Are there other comparable (or better) commercial products on the market? Or is the alternative to RPG to make such a diffuser oneself?
best,
Brandt

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Hello Brandt,

RPG's diffractals indeed represent today's pinnacle in affordable diffusion, even though it's still a very expensive module. I was able to lay my hands on 8 large panels last autumn and they are simply terrific behind an Apogee. The whole spectrum is diffused, which is clearly audible.
The problem with building your own diffractal are the embedded diffusers, which is essentially a massive length of wood (about 3cm thick) with segments cut out and very thin fins left over. I have a set of knives that must be put in a machine that rotates them at very high speed (don't know the english word) but I haven't found a good place yet to have those inlays cut. If mass production is not an issue, then of course there are ways to produce those small embedded diffusers yourself.

As I mentioned in my earlier post today, very satisfying results can also be obtained with a well designed QRD-11 or 13 diffuser, to cover the most important part of the spectrum (500-5000Hz or higher).

One other important aspect to consider when applying diffusion is periodicity: for homogenous results any diffusive surface should at least be covered by a width of 4, or better still, 5 periods. Therefore it's sometimes better to design narrow periods (lower N-numbers such as QRD-7 or 11) in order to be able to have diffusive surfaces consisting of 4 or 5 periods next to each other.
Periodicity brings homogenous diffusion over the targeted working range. Applying one wide panel (period) is not a good approach, not even when it's a QRD-19 with a wide working range. In those cases it's better to apply 5 lower numbered periods of a QRD-7.

Defining the working range is, contrary to popular internetbelief not a simple matter of well depth, well width and quarter wavelength in a linear calculation. The function of N-number and choice of one of several depth-sequences to choose from are equally important in the calculation of the diffuser's working range.
By choosing one of various depth-sequences, one can lay more emphasis on extension of the bottom-end of the working range or choose for extension of the top-end.
Moreover, the choice of that sequence sometimes reduces the amount of wood needed to build the panels by 15% and may also simplify the construction of a period somewhat.

Finally the maximum well-depth of the diffuser plays a crucial role when applying them on the wall behind the listener or against the ceiling. The bottom-end frequency of a period will in turn determine the minimum distance this listener should be removed from the diffuser, in order to get homogenous diffusion over the desired working range.

Thanks for the followup. You have a lot more technical knowledge than I do, so I hope you don't mind a couple more questions.

I'm sure a 'period' has a technical definition, but in practical terms is a given RPG 24 inch wide diffusion panel '1 period' wide?

Also, what does the N-number specify? I take it that a lower-numbered diffuser either diffuses over a narrower frequency range, or specifically at a higher frequency range. Is a lower N-number one of these or something else entirely?

"Finally the maximum well-depth of the diffuser plays a crucial role when applying them on the wall behind the listener or against the ceiling. The bottom-end frequency of a period will in turn determine the minimum distance this listener should be removed from the diffuser, in order to get homogenous diffusion over the desired working range."

Can you give me some 'rules of thumb' for how to determine the appropriate distance from the diffuser?

thanks so much,

Brandt

An excellent and clear answer that removed several unclear points from my mind.

If you buy these difusors they are expensive. I found a places which sells skyline diffusors but they are not made of wood. I was considering them but your posts clarifies.

So based on all of the useful information provided by you in the first and subsequent posts, it seems like the groud is ready for me to build my own QRD-7 diffusors.

In my expereience, the wall behind the speaker is most critical and then the side walls. In my case, my back wall has natural diffusors in the form of CD shelves. I am not sure if the ceiling and floor are that critical for Apogees (because of their radiation pattern).

I will soon start off my project for building a whole bunch of QRD-7s for the wall behind the spakers first, and then the side walls.

Thank you very much.

quote:

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Originally posted by sea-speak:
I'm sure a 'period' has a technical definition, but in practical terms is a given RPG 24 inch wide diffusion panel '1 period' wide?

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quote:

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Originally posted by sea-speak:
Also, what does the N-number specify? I take it that a lower-numbered diffuser either diffuses over a narrower frequency range, or specifically at a higher frequency range. Is a lower N-number one of these or something else entirely?

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quote:

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Originally posted by sea-speak:
Can you give me some 'rules of thumb' for how to determine the appropriate distance from the diffuser?

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You are agreat asset to this forum, man.

A lot of knowledgeable members have left the forum and very few are still around. You are one of them.

Just one more question: Can the depth of the diffusor greater than 140 mm (or 5.52")
Let's say 6"? Can it be even more, say 8inches?

Thank you.

When you say there should be at least 4 or 5 periods(panels) for a given surface, do you mean they have to be next to one another and not separated? If I place 2 or 3 behind each speaker, with a large gap in-between,would I get the same effect?
One more thing. I have a large TV between and a little behind the speakers. Should I place the panels in front of the TV or behind it against the wall?

I started by examining the diagram of the QRD-7 diffusor posted by Toine and by calculating what kind and how much wood material I would need. I tried to follow Toines specs on thickness as much as I could. I am close but not always, so I made some approximations, which I think should be ok as Toine mentioned.

I then went to Home Depot and examined wooden sheets available there. They have 4' by 8' sheets of various widths. I decided to aim for 16 pieces of diffusors, each 8' tall (and about one feet wide as per Toine's specs). But since 8' is too long to handle, I decided to make two lengths, 5' and 3'. This would allow me to use either one or both to account for the slight slant that I have on side of my ceiling. It also makes it easier to assemble and place the diffusors. I can place one 3' on top of one 5' to make it 8' tall, if needed. Thus, I am making 16 pieces which are 5' tall and 16 pieces which are 3' tall.

I bought sheets with three different thicknesses: 5/8", 3/8" and 1/4" to cover the base, top, sides, fins, and well bases.

With some calculation, the total number of sheets turned out to be 16 to do all 32 diffusors, with some spare pieces for base, fins, and sides, just in case.

Home Depot have cut the base, back and sides pieces for me but need more time to cut the fins and well bases (which are 1-3/8" inches wide, and hence need more time to cut). They charge $0.25 for each cut. It turned out be a total 0f 220 cuts. The total amount for the material and cuting fee turned out be close to $410, including a can of glue and packs of nails. For 32 pieces, that is about $12 per diffusor. Not bad.

I will start assembling them slowly, by spending about 2-3 hours per weekend.

Will keep you posted with the progress.

http://www.pmerecords.com/Diffusor.cfm

Toine, if you have seen the picture of my room, note that I have a big carved painting on the wall behind my speakers. It is very dear to me but hey sound gets priority in the media room. I know it affects my sound negatively. Can I use QRD diffusors behind the speakers and skyline diffusors to replace the painting?

quote:

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Originally posted by gallant_diva:
Toine:
You are agreat asset to this forum, man.
A lot of knowledgeable members have left the forum and very few are still around. You are one of them.
Just one more question: Can the depth of the diffusor greater than 140 mm (or 5.52")
Let's say 6"? Can it be even more, say 8inches?
Thank you.

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Thanks GD...

quote:

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Originally posted by ccheng:
Toine,
When you say there should be at least 4 or 5 periods(panels) for a given surface, do you mean they have to be next to one another and not separated? If I place 2 or 3 behind each speaker, with a large gap in-between,would I get the same effect?
One more thing. I have a large TV between and a little behind the speakers. Should I place the panels in front of the TV or behind it against the wall?
Clarence

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quote:

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Originally posted by gallant_diva:
I found the following recipe for constructing a skyline diffusor, which I guess will be a future project for me as i have already started building my QRD diffusors.
http://www.pmerecords.com/Diffusor.cfm

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quote:

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Originally posted by gallant_diva:
Toine, if you have seen the picture of my room, note that I have a big carved painting on the wall behind my speakers. It is very dear to me but hey sound gets priority in the media room. I know it affects my sound negatively. Can I use QRD diffusors behind the speakers and skyline diffusors to replace the painting?
GD

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Interesting post! That machine you refer to is called a router in english, and I have some nice ones here that can be put to such uses once in a while! So - I wonder - would you have a close picture of the routed ends required in the other diffuser you mention?

I have a small shop in my garage and I will definitely build my own difusor out of wood.

Toine, it seems to me that with the woodworking skill to build it, the diffractal type is optimum. I'd appreciate if you could clear confirm my analysis of your quote which I've included below.

"The problem with building your own diffractal are the embedded diffusers, which is essentially a massive length of wood (about 3cm thick) with segments cut out and very thin fins left over."

First of all, do you mean that in each of the courses of the difusor has very thin fins cut into the surface of the wood? Are the fins aligned vertically or horizontally (or in both directions)?

Is there an equation that can be used to determine the number of fins and their thickness relative to the width of the courses?

Could you post a drawing of some sort that illustrates a diffractal?

if you google 'rpg diffractal' you'll find some material (in various places on the web), including a drawing of the basic cross section. no internal dimensions provided, though.

meanwhile - Toine - one more question for you. I will have a hard time in my existing apartment getting any traditional QRD or PRD diffusion on the walls. however I could probably use a DIY Argent Roomlens type of product. I don't need the helmholtz radiator part of the benefit (digital room correction takes care of that) but the diffusion could be quite useful. do you think someone in my shoes would be well served by some roomlens-like units?

curious,

Brandt

quote:

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Originally posted by Graz:
Hello Toine

Interesting post! That machine you refer to is called a router in english, and I have some nice ones here that can be put to such uses once in a while! So - I wonder - would you have a close picture of the routed ends required in the other diffuser you mention?

Curious - Graz

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Hi Graz,

Even though this page is in Dutch, I'm sure the images speak for themselves
http://www.soundscapes.nu/rb_9430.htm
Toine

quote:

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Originally posted by sea-speak:
meanwhile - Toine - one more question for you. I will have a hard time in my existing apartment getting any traditional QRD or PRD diffusion on the walls. however I could probably use a DIY Argent Roomlens type of product. I don't need the helmholtz radiator part of the benefit (digital room correction takes care of that) but the diffusion could be quite useful. do you think someone in my shoes would be well served by some roomlens-like units?
curious,
Brandt

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I have no idea Brandt! No experience with a room-lens, although I have a drawing of the device here. Always wanted to build one to experiment with.
I'm sure it will work, but once you've tasted the working of a diffuser, anything 'less' in this field will be a compromise though.
I have two customers using a so-called "holograph". Extremely expensive and magical... Not too effective though, when compared to true diffusers...
Toine.

I have no doubt that a roomlens wouldn't be as good as a diffractal, but the real question is whether it would be better than nothing.

I live in a 1-bedroom apartment in New York with my girlfriend, who has already been so gracious as to permit Divas and my 5-foot tall custom corner subs into the room. Furthermore, the front wall in my room is entirely glass, to which I will under no circumstances be allowed to apply diffractals! (The view is very fine, so even I wouldn't really want to put diffusion over the windows.) So... I'm thinking that under the circumstances, portable roomlenses might provide a significant benefit. And when they aren't in use I could hide them behind the Divas.

If you have other thoughts on the roomlenses or other ways to get diffusion into this kind of situation, I'm all ears.

cheers,

Brandt

quote:

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Originally posted by sea-speak:
hi Toine,

I have no doubt that a roomlens wouldn't be as good as a diffractal, but the real question is whether it would be better than nothing.

Brandt

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Brandt:

See the picture of my system with room lens that I made:

http://www.audioworld.com/sw/Forum1/HTML/005410-2.html

I made 5 such lens, 2 go behind the speakers, 2 on the sides and one between the speakers. I cannot listen to my system without these, and I guess that explains.

The ones behind the speakers and the one in the middle have a profound effect on midbass and imaging. I moved the ones from the side to behind also, which means double lens behind each speaker and that is even better. The ones on the side have not much effect, perhapd due to the diffusor on the wall.

My first "QRD-Toine" diffusor, 5 feet tall, is almost ready.

Also, congrats on getting some real diffusers built. I am fantasizing about a front wall covered in rpg diffractals. Ah, what strange dreams an audiophile can dream...

cheers,

While trying to adjust the saw precisely, he first cut one sample. He measured the sample and said "It is just tiny bit short".