[davejcb]

Hi all,

Been reading up about diffusion, and how absorption can be bad in a small room because it robs it of kinetic energy, whereas diffusion will correct soundstage and imaging problems while leaving the energy in the room, making it sound more "live". At least that's how I understand it right now.

My question is whether there are any drawbacks to diffusion? Assuming the corners were trapped with fiberglass, and bass response is not a problem, would using only diffusers work well, as in this image from Decware's site?



Is absorption really necessary for reflections, or is it just an easier solution than building diffusers? Or are broadband absorbers absolutely necessary to correct bass response in a room?

[Ethan Winer]

Dave,

I wish you had included specific dimensions because that's a big factor. What's appropriate for a room 35x25 feet is not necessarily appropriate for a room only 10x12 feet.

> Been reading up about diffusion, and how absorption can be bad in a small room because it robs it of kinetic energy <

I don't know about the kinetic energy angle. The main issue is having enough ambience that the room sounds natural, but not so much that you can hear repetitive echoes or speech clarity is compromised.

> whereas diffusion will correct soundstage and imaging problems <

I'd say it's the opposite, at least when considering treatment for the first reflection points. In a typical small room where the side walls are less than ten feet from your ears, absorption is probably a better choice than diffusion. In a very large room diffusion can be good, but only if you want to retain all the ambience. Usually this is not the case, especially in a listening room versus a room where recordings are made.

Also, diffusion is often used on the rear wall to make the wall seem farther away and make the room sound larger than it is. So the rear wall is a more likely candidate for diffusion than the RFZ places on the side walls and ceiling. It's mainly the RFZ places that affect imaging and "sound stage."

> My question is whether there are any drawbacks to diffusion? <

If it's "good" diffusion, like RPG's QRD models, then being too close can sometimes give a strange sound. Those diffusors have a series of chambers that resonate at different frequency ranges. So when you're too close you can hear each individual resonance, which is not unlike setting one band on an EQ all the way to full boost. When you get far enough away all the resonances combine, so that problem goes away. With cheap/lousy diffuser this is less of a problem, but those don't diffuse well anyway and so are a waste of money IMO.

> Assuming the corners were trapped with fiberglass, and bass response is not a problem <

That's a mighty big assumption! How big did you say the room is?

--Ethan

[davejcb]

Thanks for replying as always, Ethan. This is my room...

I was considering building a few of Steve Deckert's Decware diffusers, got the plans... I will have about 7-8 feet on each side when the room is ready, and about 10-11 feet ahead of me. The couch will be basically on that line in the drawing where the ceiling becomes higher. So you're suggesting fiberglass on the sides/ceiling for first reflection, then diffusion on the rear walls? What about front wall?

I know bass will be an issue, I was speaking hypothetically.  

Thanks!

[Ethan Winer]

Dave,

> I will have about 7-8 feet on each side when the room is ready, and about 10-11 feet ahead of me. <

Is this a home theater or a stereo listening room? If there's no screen, I'd say you're sitting way too far back in the room. In a small room like that the optimum seating position for best low frequency response is 38% of the way back. So if your speakers are firing down the 17.5 foot dimension, you should sit about 6.5 feet from the front wall. The speakers will then be placed in relation to that prime seat.

> So you're suggesting fiberglass on the sides/ceiling for first reflection, then diffusion on the rear walls? What about front wall? <

Diffusion on the rear wall is iffy with the size room, but you could try it. If you sit where you suggested, you're probably too close to the rear wall for diffusion. If you sit where I suggest it's right on the border of the ten foot recommended minimum distance.

As for the front wall, maybe some absorption will help, but only if the room is generally too live.

Also, I failed to answer this yesterday:

> are broadband absorbers absolutely necessary to correct bass response in a room? <

Yes.

--Ethan

[davejcb]

In a small room like that the optimum seating position for best low frequency response is 38% of the way back. So if your speakers are firing down the 17.5 foot dimension, you should sit about 6.5 feet from the front wall.

Is that realistic? The 3A speakers want room to breathe, so if I had them out let's say 2-3 feet, then I'd be sitting 3-4 feet away? Seems very nearfiled...

[Ethan Winer]

Dave,

> The 3A speakers want room to breathe <

People breathe, speakers simply emit sound.

> so if I had them out let's say 2-3 feet, then I'd be sitting 3-4 feet away? <

You can sit anywhere you'd like!

I'm currently reviewing the blueprints for a listening room designed by a pro who I respect very much. This room is 21 feet long and has the listener about 9 feet back from the front wall. The speakers are about 3-1/2 feet from the front wall. This is the same pro who determined that 38% is the optimum distance from the front wall. Though in this design it's more like 43% of the way back. What really matters is being in front of the center line.

--Ethan

[davejcb]

What really matters is being in front of the center line.

Is there a technical explanation for this?

[Ethan Winer]

Dave,

> Is there a technical explanation for this? <

Yes, though I may not be the best person to give it.

My friend, expert studio designer Wes Lachot (http://www.weslachot.com), came up with the 38% rule as the optimum listening position based on a careful modal analysis of many rooms. All locations suffer from peaks and nulls, but some are better/worse than others. In this case Wes determined that 38% of the way back from the front wall is the best compromise of all the various factors.

I'll also mention that I do have a written explanation from Wes, but I can't repeat it here because it's going to be in an article we just wrote for the September issue of EQ magazine. The article presents the design for a properly proportioned and acoutically treated listening/mixing room.

--Ethan