Absorption is much better than diffusion at reflection points, especially in a narrow room. Bookshelves are not really diffusors, but even real diffusors are better used behind you when listening rather than at reflection points.

--Ethan

Excellent question and I would like to hear more on Ethans reasoning. I hate to lose any energy, thus absorbsion is a bad thing to me. Next, the frequency response of any panel is important also.

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I would like to hear more on Ethans reasoning. I hate to lose any energy, thus absorbsion is a bad thing to me. Next, the frequency response of any panel is important also.

There are many issues and I could write for hours on this subject.

First, good diffusion cost more than good absorption, whether you buy commercial diffusors or make your own. So that's probably the biggest factor for most people. But let's say cost is no object. I still would not put diffusors at first reflection points because it doesn't solve the problem of comb filtering as well as absorption does. Anywhere else is a good candidate for diffusion, especially the rear wall behind you.

As for "losing energy," I think that's not the right way to look at it. Energy is cheap, and you can always turn up the volume control a notch. The more relevant issue is the sound of the reflections in a room. The smaller the room, the worse the reflections sound - congested, boxy, etc. In small rooms reflections are also stronger because the boundaries are closer. Again, if cost is no object, the ideal room treatment is as much bass trapping as possible, broadband absorption at all reflection points, and deep diffusion (6 inches deep or more) everywhere else.

--Ethan

Centered vertically at ear height is best.

--Ethan

You got it!

I'll add that it's not a single "point" but rather an area. So I suggest extending the coverage at least one foot in all directions, and two feet is better still unless you're really close to the loudspeakers.

--Ethan

Rock wool and rigid fiberglass are more or less equivalent, and neither is carcinogenic. Just wear a face mask when working with them. The main difference is that rock wool costs less but is much more difficult to work with because it flops around. Versus 703 or 705 rigid fiberglass that are truly rigid and can stand on their own without sagging.

--Ethan

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Quote:
I would like to hear more on Ethans reasoning. I hate to lose any energy, thus absorbsion is a bad thing to me. Next, the frequency response of any panel is important also.

There are many issues and I could write for hours on this subject.

First, good diffusion cost more than good absorption, whether you buy commercial diffusors or make your own. So that's probably the biggest factor for most people. But let's say cost is no object. I still would not put diffusors at first reflection points because it doesn't solve the problem of comb filtering as well as absorption does. Anywhere else is a good candidate for diffusion, especially the rear wall behind you.

As for "losing energy," I think that's not the right way to look at it. Energy is cheap, and you can always turn up the volume control a notch. The more relevant issue is the sound of the reflections in a room. The smaller the room, the worse the reflections sound - congested, boxy, etc. In small rooms reflections are also stronger because the boundaries are closer. Again, if cost is no object, the ideal room treatment is as much bass trapping as possible, broadband absorption at all reflection points, and deep diffusion (6 inches deep or more) everywhere else.

--Ethan

After some thought about it, you are correct. Absorbtion is better than diffusion. Absorbtion reduces the amount of first reflection signals that arrive to the listeners ear.
These first reflections arriving closely to the main signal tend to confuse and smear the original signal. Diffusion would create multiple delayed signals.

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Diffusion would create multiple delayed signals.

Well, yes, but that's not necessarily bad because the delayed reflections are not as coherent as when reflected off a large flat surface. The larger issue is that many rooms have too much ambience, not too little.

--Ethan

I should also point out that if you have lots of absorbtion, the cues you will get regarding space, imaging, etc, will be the ones in the recording.

To the extent that any modern recording includes them, that is, unfortunately.

Exactly, and I argue that almost daily. I don't know why some audiophiles believe that a dead sounding room sucks all the life out of a recording. It's actually the other way around. When the small sound of the room is removed, the result is usually a much larger sound. Or at least as "large" as the recording engineers made the recording.

--Ethan

What would be considered an optimal room size?

Optimal is considered a room that is at least 2500 cubic feet. However, 1500 cubic feet is considered the minimum for acceptable reproduction. Either size requires bass traps and other acoustic treatment for best results.

--Ethan