Some First Reflection / Diffusion Questions from a n00b

[kornowsd]

OK...

Finally, some breathing room... having had the overall bass trap issue relatively figured out... I have one remaining question on the side wall absorption. My room is 10 x 13 x 9 and I'm firing the speakers down the "long" side of the room.

The primary reflection points, on the L/R of the mixing position is what I have the most question about. I'm planning on the absorbers being either 3" or 4.5" (I have 1.5" rigid fiberglass so those are my options)...

Would it be "better" to have those absorbers spaced away from the wall to provide a bit more "broadband" absorption? I have about 6" to play with... so would I fare better with 3" of rigid fiberglass spaced 3" from the wall? Or would 4.5" of fiberglass spaced 1.5" from the wall be better...? or 1.5" of fiberglass 4.5" from the wall?

As with anything I already know that "better" is highly subjective... my ultimate goal is to make the room as neutral as possible so that I can record, mix, master and whatever else in there with minimal changes. So, to that end, I know that spacing a panel absorber away from the wall, a bit, makes it more "broadband."

Finally... as per above, the room is 10 x 13 x 9 (W x L x H). I'm firing the speakers down the 13 foot dimension. I've read pretty much everything I can find on diffusors including ALL of the technical papers by the guy who made them a mainstream event and a "must have" Dr. Diffusor, himself.

It would seem, at least from his whitepapers, that there is a "critical minimum distance" that one must sit away from a diffusor before you get out of what he calls the "nearfield..." In one of his papers he notes that if you're in the nearfield of the diffusor you will hear acoustic distortion (phase distortion) as the primary image NOT a clear, diffused image of the original wavefront hitting the diffusor.

To quote, directly from the paper:

It is suggested that a listener should be at least three wavelengths away from scattering surfaces. Since diffusors used in listening room applications have a lower frequency limit of roughly 300-500 Hz, this would mean a working distance of 10’ (3 m) or larger is recommended.

Just as one would not listen to a 3 way loudspeaker with their ear close to the midrange driver, one should expect sonic anomalies when seated too close to a diffusing surface. Many of the phasing anomalies reported by room designers are simply due to the fact that they are not far enough away from the diffusor and they are hearing near field comb filtering and lobing effects. Furthermore, getting too close to a diffusor means that the reflections are dominated by the surface close to the ear, which means the temporal dispersion generated by the diffusor is not heard. The direct and reflected sound are then rather similar and interference gets worse.

• "Technical Bulletin on the Application of Diffusion in Critical Listening Rooms"

Peter D'Antonio and Trevor Cox

Being that my recording console desk is 30" deep and my chair would seat me back, from that, to beyond 3 feet, I'm actually WORSE OFF by putting diffusion in the room - according to that paper written by the diffusion experts.

F. Alton Everest, in his book "Master Handbook of Acoustics..." notes in several places, that absorption properly placed, diffuses the space pretty well as a result of it's being there. This statement is also backed in several college texts on architectural acoustics as well as several other books on studio design.

So, the question comes down to it... to diffuse or not to diffuse? I can make RPG-like diffusors for next to nothing in wood and have one put together in roughly 3 hours time... I have a QRD here and they're, well, relatively easy to make and put together if you're half good w/ a saw and some clamps. The question really becomes will I benefit from them...

The "experts" are, seemingly, saying "NO WAY", not in a space that small. When I contacted a certain foam company that makes plastic diffusor products they, of course, "highly recommended" diffusion. Of course they stand to make some additional $$$ by selling them.

Thoughts from those who've installed? I do plan to record vocals, guitar amps (loud), acoustic guitars and other acoustic instruments in that space.

A "tack on" question would I be better off, in the end, by using something closer to the "scatter blocks" concept - providing a way to control the strong primary reflections off the back wall... and still leave a little "life" in the room? My thought was that I'd absorb a rather large space directly behind the listening position and then alternate w/ absorption and flat wall... heck, for that matter, to control any residual "flutter" I could quickly route the RPG equivalent to "flutter free" which is also a quick and easy to make product given its symmetry, and place it betwixt the absorbers.

Anyway... as per the side walls... would one space those rear wall absorbers out from the wall, or is controlling the specular reflections enough - just putting 3-4" of rigid fiberglass directly on the wall surface?

Diffusion seems a bit "over the top" as with this smaller space it seems to be more about the control of the primary specular reflections and bass modes more than anything...

Thoughts from those who've been there, before?

Dar

[PaulP]

Originally Posted by kornowsd

OK...

I've had similar questions to yours and, after reading countless opinions
on the matter, have come to the conclusion that I'm going to have to
try what I think is best (a sort of mix of everyone's opinion) and then see
what it sounds like. Then go forward from there making whatever changes
seem appropriate.

I find you've described well the issue of diffusion vs absorption but I don't
think you'll get an answer that'll satisfy you.

Paul P