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RE: Asorption versus diffusion behind speakers?

Posted by David Aiken (A) on January 20, 2010 at 22:28:36

In Reply to: Asorption versus diffusion behind speakers? posted by MGH on January 20, 2010 at 20:55:18:

It depends on what you're trying to do and what kind of presentation you like.

Floyd Toole who has conducted and published a lot of scientific research on acoustics recommends absorption in his recent book "Sound Reproduction", and his reasons for that relate to imaging precision and the effects of first reflections, especially if the speaker is close to the front wall so the first reflection point is close to the speaker and the direct and reflected signal paths are close in length and therefore close in level. Absorption significantly reduces the level of the first reflection.

If you place diffusion there, you certainly weaken the strength of the reflection on that same first reflection path because you spread the energy that would otherwise be reflected to you from the first reflection point over a much larger area. Unfortunately what you also do is ensure that some of the energy reflected from points that would otherwise not be first reflection points will also be reflected directly to the listener so you weaken the first reflection you would get if the wall were untreated but at the cost of generating lots more first reflection to the listening position from a much wider area of the wall. Admittedly they are weak reflections but they do add up and they create a different effect to either an untreated wall or a wall treated with absorption.

In my limited personal experience placing diffusion in the centre of the wall behind the speakers, ie directly in front of the listening position, what I noticed was a much more expansive soundstage but individual images were also considerably broadened so singers sounded as if they had mouths a foot or two wide. This was in a room with absorption at the first reflection points and given the research on the effects of strong first reflections I suspect that had those first reflection points been untreated, the individual images may have been even broader than what I heard.

I'm not necessarily saying that pinpoint imaging is the goal because I certainly don't think it is for everyone and I think a fair amount of personal taste is involved in that assessment. I know when I was listening with diffusion in the centre of the front wall in my setup I thought it sounded great with symphonic music and that it reminded me more of my concert hall experiences with symphonic music than my system normally does when the only treatment on that wall is absorption at the first reflection points. Unfortunately I listen to very little symphonic music, my main musical tastes being small group jazz and other small group music. I hated the effect with that kind of music as much as I liked it with the symphonic music so I went with what works best for me with the music I play most of the time, removed the diffusion and stuck with the absorption.

There are others who have posted here from time to time who are very strongly in favour of diffusion rather than absorption and who obviously prefer a different presentation to what I prefer.

There is one other point to mention in relation to diffusion and that is that it needs space in which to develop. If you're sitting too close to a diffuser you not only don't get the full benefit of the device but you can also hear problems caused by the fact that you're too close and the sound distribution hasn't fully evened out. How close is too close depends on the diffuser you use but one frequently mentioned rule of thumb suggests not sitting closer than 10' to a diffuser. Given normal listening room setups you're probably more likely to be sitting more than 10' from the front wall than you are from the side walls (the room would have to be at least 22' wide by the time you allowed for diffuser depth and a single listening position, wider if you have two or more than people sitting side by side) or the back wall since most setups I've seen have not had 10' between the listening position and the wall behind. My feeling is that you need a fairly large room in order to really benefit from diffusion and most listening rooms I've seen, including living rooms, haven't really been large enough to get the full benefit of diffusion, especially since the shorter reverberation times of small rooms mean that you're probably not going to achieve a genuinely diffuse reflected sound field in the room even if you do use diffusion.

But those who prefer the sound of diffusion in their rooms will argue with me on that point.

Bottom line: absorption and diffusion on the wall behind the speakers create quite different presentations of the soundstage and imaging you hear. Different people have very different preferences when it comes to soundstage and imaging and, judging from my own experience, preferred taste in music may well have a relationship to what kind of soundstage and imaging people prefer. I don't think that this is the kind of decision you should make on scientific research or on the opinions of others including me. I think you need to hear each and decide for yourself which result fits with your own personal preferences because that's the approach I think you should use. The reason for acoustically treating a listening room is at least in part to improve your listening experience so treating it in a way that produces an outcome you don't like simply doesn't make sense to me.

If you do opt for diffusion I'd choose diffusers that achieve diffusion within a distance that's less than the distance between you and the diffuser in order to avoid the issues that arise when you listen too close to a diffuser. You may need to contact manufacturers directly in order to get that information about their products.



David Aiken

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