Minimum size control room for quadratic residue diffusor?

[Amun Ra]

Well, I'm moving my studio to a new building this summer (very excited!!!), and I'm trying to plan the accoustic treatments in advance.

The CR is 4,1m*3,3m (13ft*9ft). Is there any point in putting a QRD on the back wall of the CR, or should i stick to bass trapping?

[jayfrigo]

Originally Posted by Amun Ra

Well, I'm moving my studio to a new building this summer (very excited!!!), and I'm trying to plan the accoustic treatments in advance.

The CR is 4,1m*3,3m (13ft*9ft). Is there any point in putting a QRD on the back wall of the CR, or should i stick to bass trapping?

Rule of thumb is that you want to be three wavelengths away at the lower limit of the effective bandwidth of the diffusor.

[jwl]

In a room that small I'd probably stick with thick absorption on the rear wall. If you do use diffusion, you definitely still want bass trapping there too.

[Glenn Kuras]

Originally Posted by Amun Ra

Well, I'm moving my studio to a new building this summer (very excited!!!), and I'm trying to plan the accoustic treatments in advance.

The CR is 4,1m*3,3m (13ft*9ft). Is there any point in putting a QRD on the back wall of the CR, or should i stick to bass trapping?

Jay gave you the tech rule of thumb, but for the non tech guy use the rule, for every inch deep of diffusion you should be 1 foot back. So if the diffusion is 5" you want to be 5 feet from it. Keep in mind though that this is just a rule. For your room I would go with THICK (I like 6") absorption on the back wall with diffusion around that. Also there is no reason you can not use diffusion on the side walls and ceiling behind the mix position.
See the following articles about diffusion
Diffusion is a Wonderful Tool by Jeff Hedback
How Diffusion Really Works. Spotlight on MASSIVE Mastering.

[audiothings]

i recall reading somewhere that there must be minimum 11' between the speakers and the diffusers for the diffusers to work appropriately. take this with a sack of salt because this is completely hearsay/second hand information. (just food for research...)

regards,

[avare]

Originally Posted by audiothings

i recall reading somewhere that there must be minimum 11' between the speakers and the diffusers for the diffusers to work appropriately. take this with a sack of salt because this is completely hearsay/second hand information. (just food for research...)

Thak you for qualifying the statement. It is a myth.

[andrebrito]

I have done in the last studio I have designed which is now under construction.

Recording studio - albufeira, portugal

Lenght of room is 4.5 m , width 3 m

[jayfrigo]

Originally Posted by avare

Thak you for qualifying the statement. It is a myth.

It is true for a certain diffusor for which that length corresponds to 3 wavelengths at the lower limit of the design bandwidth... That is, of course, far from being able to say it's a hard and fast rule for all diffusors.

The typical old-school RPG-designed 1D QRD prime-7 that we see in many control rooms works out to be about 10 or 11 feet, and that's probably where this myth came from. You can certainly design a diffusor with a higher bandwidth and put it closer, but you still need to consider those lower frequencies if you choose to leave them out of the diffusor's effective bandwidth.

If you get too close to the diffusor, your ears won't fall off. You'll just get some unintended behavior. You may end up hearing the cancellation patterns, and reflected energy will behave more and more like specular reflections (non-diffuse) as you get closer, not to mention being out-of-phase due to the different distances involved with the well depths. Like a 3-way speaker, you need to get a little distance between it and you before you get an integrated wavefront.

The kind of diffusor also matters. A poly (like a semi-cylindrical) and a QRD will both diffuse spatially, though only the QRD diffuses temporally, but to sit close to a QRD will sound different from sitting close to a semi-cylinder. Generally speaking in practical terms, a 2D skyline sounds different from a 1D QRD as well, and one might expect it to be slightly more forgiving of distance. Some of this can be traced simply to the design parameters for the typical sizes available commercially and used commonly, and some to the measurable behavior that simply isn't the same for different designs.

[audiothings]

If you get too close to the diffusor, your ears won't fall off.

thank you jay.

your post is a bit much for me right now... just rushing out... some very stressful theater mixing work going on. But i will def give it some serious time in a couple of days. i'd have to spend a lot of time getting a grasp of all of your post, and it will be a pleasure.

[avare]

Originally Posted by audiothings

thank you jay.

your post is a bit much for me right now... just rushing out... some very stressful theater mixing work going on. But i will def give it some serious time in a couple of days. i'd have to spend a lot of time getting a grasp of all of your post, and it will be a pleasure.

Jay's post is wealth of information about diffuser design and application. It is excellent.

Andre

[Ethan Winer]

Originally Posted by jayfrigo

The kind of diffusor also matters. A poly (like a semi-cylindrical) and a QRD will both diffuse spatially, though only the QRD diffuses temporally

Temporally? Would you please elaborate on that?

--Ethan

[Trev@Circle]

Originally Posted by Ethan Winer

Temporally? Would you please elaborate on that?

--Ethan

in the time domain (in addition to the space domain)

[jayfrigo]

Originally Posted by rimskidog

in the time domain (in addition to the space domain)

Exactly. The reflections not only spread out in space, but also in time (temporally). The time differences actually are related to how the QRD diffuses (different well depths), but subjectively this temporal diffusion (i.e. in the time domain) helps to create a more even and dense-sounding decay.

The reflections from a semi-cylindrical poly, though spread out in space, and therefore reduced in magnitude any any one location, still resemble specular reflections at individual locations in that you have a delayed arrival at a discrete time of the reflected energy after the incident energy, as opposed to spread out arrival times. This reflection which is similar to a traditional specular reflection in nature can conceivably cause comb filtering, though the reduced magnitude of the reflection will make any interference less significant.

This doesn't make one or the other better or worse globally, only better-suited to a particular application. Both have their place, though in small critical-listening spaces like control rooms, the addition of a temporal component has been shown often to be beneficial.

[Magickman]

From "Acoustic Absorbers and Diffusers - Theory, Design and Application" by Trevor J. Cox and Peter D'Antonio, pages 27-28:

"Figure 2.4 shows the effect of adding surface roughness to disperse the reflection. In this case, the semicylinder is used. It can be seen that the reflected wavefront is no longer planar, but is now semicircular. The semicircle has generated a virtual point source at the center of itself. Consequently, a semicylinder will generate spatial dispersion. The wavefronts generated are still very ordered; however, so although semicylinders are good at spatial dispersion, they are not the best diffusers because temporal dispersion is not achieved...

...A Schroeder diffuser changes the phase of the reflected wavefronts. The waves radiated by the secondary sources are complicated, and it is difficult to pick out clear cut wavefronts. As well as generating spatial dispersion, reflection phase grating diffusers generate temporal dispersion."

For those who are listening and actually care...

[Ethan Winer]

Originally Posted by jayfrigo

The reflections not only spread out in space, but also in time (temporally).

Originally Posted by Magickman

...A Schroeder diffuser changes the phase of the reflected wavefronts.

Got it. It was the terminology that threw me. I've always felt that the staggered delay times coming back from a QRD diffusor is the main reason it does such a good job of reducing the sound of comb filtering. The reflections are no longer coherent as they are from a flat wall or curved surface.

Thanks guys.

--Ethan

[Trev@Circle]

No problem Ethan. My pleasure

[Ethan Winer]

^^^ Sorry, thanks to you too!