Diffuser frequencies and listener distance

Nordenstam

New Member
Aug 19, 2010
37
0
0
Norway
nordenmaster.no
#1
Hello!

A poster sent me a PM with some of questions and I thought it would be better to do the reply in public. Not only so others can read and possibly learn, but also to open up for others to comment.

I seem to recall reading that Trevor Cox recommended a minimum seating distance from a diffuser as being 3X the minimum design frequency's wavelength. What I can't find is whether this advise is diffuser-type specific? In otherwords, does the 3X apply to QRD and semi-circular polyfusers that are both 1D, but doesn't apply to a Skyline which is 2D?
Math diffusers have a near field and a far field response. Sitting too close to them will, like multidrive speakers, not allow the various reflections(virtual sources) to properly mix before they hit the ears. A semi-circle does not have anything particular to mix as it's just one single reflection. So this caution does not apply to them. A Semicircle is only providing spatial diffusion, it have no temporal diffusion attributes.

1D and 2D math diffusers sends out a bunch of different reflections that needs to mix before they hit the ear. The 3x design frequency is a good thumb rule. Though, the important thing to realise is that it depends on complexity of the diffuser. A 2D diffuser generally have much more complex response so there'll be a huge number of reflections at any point given some distance. A 1D diffuser is generally much less complex, so they typically needs a bit more distance to the listener as the consequence of being in the near field is larger. I think the 3x distance rule may be shrunk to 2x with a complex 2D diffuser, but that's a very vague guesstimate. Somewhere between 2x and 3x should be safe.


A corallery question is how do you determine the minimum design frequency by diffuser type?
Math diffusers (with both spatial and temporal scattering) have a certain base frequency. This is the prime number used. Neither PRD's nor QRD's actually use the prime number. The physical depth is prime number minus one at most. Ie, a prime 7 PRD counts 1 to 6. So if 6 is X deep, the diffuser base frequency is (X/6)*7. A 6 inch deep PRD 7 diffuser will have a base frequency that corresponds to 7 inch depth. A prime 7 QRD sequence only use 4 as the max depth, so the base frequency is (X/4)*7. In other words, a 4 inch deep prime 7 QRD will work at the same base frequency as a 6 inch PRD. Such large "free low end" gains are only available for low prime numbers and QRD sequences. Keep in mind that the period width (each sequence) needs to be large enough to accomodate that frequency.

The poly is a different scenario. A general guideline is to keep the panel size large compared to the wavelength. As wavelength gets larger than the array, the effect of the lump of mass gets smaller. It's typically doing some good to about half the wavelength, one octave below the physical size. There are some more advanced formulas around if you want to include more details like distances from source and receiver, angle distance from edges and so forth.


Regards,

Andreas Nordestam
 

kevinzoe

New Member
Nov 7, 2010
4
0
0
#3
If diffusion can add listener envelopement and help foster a sense of spaciousness, then is there any merit in taking it to the extreme like George Massenburg's 'Studio C' at Blackbird Studios? I can't help but wonder how my sound room would sound if I covered it with Skylines . . . humm.
 

audioguy

WBF Founding Member
Apr 21, 2010
2,767
40
225
Near Atlanta, GA but not too near!
#4
If diffusion can add listener envelopement and help foster a sense of spaciousness, then is there any merit in taking it to the extreme like George Massenburg's 'Studio C' at Blackbird Studios? I can't help but wonder how my sound room would sound if I covered it with Skylines . . . humm.
Apparently, that is not an original idea. Not Skylines but LOTS of diffusion!!

 
Aug 19, 2010
37
0
0
Norway
nordenmaster.no
#6
updates on your prd builds??
Nope, still waiting for access to the facilities where I can do the woodwork. The price of an urban life!

If diffusion can add listener envelopement and help foster a sense of spaciousness, then is there any merit in taking it to the extreme like George Massenburg's 'Studio C' at Blackbird Studios? I can't help but wonder how my sound room would sound if I covered it with Skylines . . . humm.
The Ambechoic room at Blackbird is more elaborate than the results achieved by adding lots of normal diffusers. The diffusers there are much deeper and much more complex than the typical skylines. A room with normal diffuseres would not achieve the response seen in that room. See page 8 and 9 in this PDF: http://www.rpginc.com/news/seminars/Diffuse_Seminar_2007_iRoom.pdf


Diffusion? I don't need no stinkin diffusion!
Bring on the concave arcs! :)

By the way. This was sent to me by a well know acoustician who is working on trying to make this room sound listenable! Apparently you CAN have too much of a good thing!
It looks like there's too much plastic in that room.. That aside, diffusers have specific frequencies they work better at and other frequencies they're less effective at. I build mines using different formulas and sizes to spread the effect of the diffusion across different frequency bands. It may have helped to use different sorts of diffusers in that room instead of putting all the eggs in one basket.
 

terryj

New Member
Jul 5, 2010
512
0
0
bathurst NSW
#7
the diffusors in the pic above look like those (swedish?) ones, I remember we spoke of them many many years ago..or so it seems! They *look* complex, but in reality not so much so.

Can't remember where I read it, but the prime number used in the blackbird studio was amazingly high. Obviously it would be the highest prime yet. No doubt you could tell us what it is!

the ceiling, seven feet deep??!!

Man, how much would that lot weigh?
 
Aug 19, 2010
37
0
0
Norway
nordenmaster.no
#9
the diffusors in the pic above look like those (swedish?) ones, I remember we spoke of them many many years ago..or so it seems! They *look* complex, but in reality not so much so.
Yeah, they're not the most theoretically advanced around. Though they probably work fine in practical applications, probably better than many other offerings. Hope no one misunderstands my post above. The diffusor.com lineup of acoustical products is pretty impressive. It's the excess amount of one particular implementation of treatment (and plastic to boot!) that sets off my warning lights.

Can't remember where I read it, but the prime number used in the blackbird studio was amazingly high. Obviously it would be the highest prime yet. No doubt you could tell us what it is!

the ceiling, seven feet deep??!!

Man, how much would that lot weigh?
Can't remember all details now. Some info in the PDF linked above. I do recall reading that they started out with 40 metric tons of MDF and ended up using something like 27 tons of it. Serious stuff. :)


Andreas
 

Johnny Vinyl

Member Sponsor & WBF Founding Member
May 16, 2010
8,571
12
38
Calgary, AB
#10
Hi Andreas,

If the poster you are referring to is kareface, I have not seeen him around at my other haunt at all these last few weeks. This really surprises me as he is usually quite vocal about this subject matter.

John
 

kevinzoe

New Member
Nov 7, 2010
4
0
0
#11
Diffusion? I don't need no stinkin diffusion!



By the way. This was sent to me by a well know acoustician who is working on trying to make this room sound listenable! Apparently you CAN have too much of a good thing!

Hi audioguy,
Did the acoustician describe the sound in the room as it's pictured or what the problems were? Was it too much diffusion throwing the imaging and localization off or was the sound degraded because of the plastic used to make the diffusers? Enquiring minds are curious . . . :D
 

kevinzoe

New Member
Nov 7, 2010
4
0
0
#12
Hi Andreas - thanks for your reply and knowledge sharing.

By the way, what do people think of applying a QRD sequence to a row of polys so as to improve their diffusion effectiveness when several are put side-by-side? So for example if I were to use a QRD sequence of p=5 which is 0, 1, 4, 4, 1 and lets assume that we seed the sequence using a 6" radius, then the poly sequence would be 0, 6" radius, 24", 24" 6". A p=7 sequence using the same 6" radius as a seed would be 0, 6", 24", 12", 12", 24", 6". Does "borrowing" this sequence make sense in order to improve the poly's performance? Of course the other rational things to consider would be to space them apart with random distances being best, or to alter the radius or to have them perpendicular to one another.

Is this crazy latteral thinking on my part or is there merit in the idea . . .
 

terryj

New Member
Jul 5, 2010
512
0
0
bathurst NSW
#13
hi kevin

I'll take a stab at the diffusion question whilst we wait for others to answer:)

My guess is the overriding problem is too much of ONE type of diffusor, I'm guessing design frequencies and range of diffusion (rather than concentrating on 'plastic').

Most of the modules you can see are the same, they may be 'pushed back or pulled forward' a little, but essentially the same unit (the ones on the side, are they rotatable?)

I guess the analogy would be tons of absorption...but all at the same frequency..leads to imbalance.
 

kareface

New Member
Jul 30, 2010
91
0
0
Seattle, Wa
#14
Hi Andreas,

If the poster you are referring to is kareface, I have not seeen him around at my other haunt at all these last few weeks. This really surprises me as he is usually quite vocal about this subject matter.

John
Sorry. I've been working on a research paper the last few weeks that really enveloped my free time. Nothing like spending 30+ hours of work to not even put a dent into the paper. Most of the first few days was just collecting and sorting periodicals for citation. I finished the paper and I'm just catching up on the work I'm late on now. I have a little more free time to think about things that don't relate to school (for about a week, lol). However John, I'm not the poster that sent him the question. I know who he's referring to because I did receive the same questions in PM. I responded to him a couple weeks back but I'm only now getting around to reading his replies and this thread for that matter.

Hi Andreas - thanks for your reply and knowledge sharing.

By the way, what do people think of applying a QRD sequence to a row of polys so as to improve their diffusion effectiveness when several are put side-by-side? So for example if I were to use a QRD sequence of p=5 which is 0, 1, 4, 4, 1 and lets assume that we seed the sequence using a 6" radius, then the poly sequence would be 0, 6" radius, 24", 24" 6". A p=7 sequence using the same 6" radius as a seed would be 0, 6", 24", 12", 12", 24", 6". Does "borrowing" this sequence make sense in order to improve the poly's performance? Of course the other rational things to consider would be to space them apart with random distances being best, or to alter the radius or to have them perpendicular to one another.

Is this crazy latteral thinking on my part or is there merit in the idea . . .
If you are talking about what I think you are taking about, it wouldn't have any measurable benefits over a random pattern. The blocks need to be the same width for the differences in phase to shape the wavefront. If you have a ton of different sized polys each 'block' would be a different corresponding size based on the width of the poly. It wouldn't work. You could repeat the poly pattern the length of the cell but you wouldn't see some dramatic improvement over just placing them randomly.

If you are talking about several polys that are the same size recessed from each in a QRD pattern than you'd have some positive results. But at that point you'd simply be tacking on polys to the end of a larger QRD. If you were planing on modifying a poly using principles of a different diffuser designs you're much better off with something like a binary sequence, or perforated diffuser / Helmholtz absorber. They are more compatible with the design. If you are worried about repetition effects from having them side to side you can just use the barker code to correct that.
 

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