Diffusers or Deflectors around surrounds?

Matthew J Poes

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Anyone played around with the use of diffusers or deflectors around surrounds to enhance envelopment or improve the realism of the surround effects? The idea has been on my mind lately. Surrounding the surround speakers with diffusor surfaces or maybe deflectors (barrel diffusors) to reduce the effect of the sound coming off the wall.
 

mike w

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Hi Matt: I’ve just finished my first DIY lean diffusor designed by Tim Perry (Arqen). I was going to make a more finished version and take final room measurements before posting on his site and AV NIRVANA but since you asked.


There doesn’t appear to be a consensus as to whether the First Reflection Point (FRP) should be treated with absorption or diffusion. The “Master Handbook of Acoustics” recommends absorption but I agree with Nyal Mellor and others that absorption is not the automatic default option - especially in a passive listening environment.


The location of this diffusor is in a living room listening space and not a studio control room. Therefore, it has some constraints regarding decor and size. The use of a diffusor was in part to deal with asymmetric speaker placement and differing FRP surfaces - windows vs. dry wall. The window side could benefit more from a diffusor. I was considering a clear SMT winged version but the architectural committee nixed that.


Does it change the sound? Yes, a little. It shifts the sound stage to the center some and gives the vocals a little more clarity. In terms of REW measurements, I see a difference in the Distortion Graph and the Waterfall Graph. However, almost anything changes those graphs, like moving furniture around, so its not clear to me if those measurements are significant.


Tim Perry says its technically daunting to try to measure the coefficient performance of a diffusor but it seems like we should be able to measure the over all results. What is your view?

regards,

Mike W
 

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Matthew J Poes

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Hi Matt: I’ve just finished my first DIY lean diffusor designed by Tim Perry (Arqen). I was going to make a more finished version and take final room measurements before posting on his site and AV NIRVANA but since you asked.


There doesn’t appear to be a consensus as to whether the First Reflection Point (FRP) should be treated with absorption or diffusion. The “Master Handbook of Acoustics” recommends absorption but I agree with Nyal Mellor and others that absorption is not the automatic default option - especially in a passive listening environment.


The location of this diffusor is in a living room listening space and not a studio control room. Therefore, it has some constraints regarding decor and size. The use of a diffusor was in part to deal with asymmetric speaker placement and differing FRP surfaces - windows vs. dry wall. The window side could benefit more from a diffusor. I was considering a clear SMT winged version but the architectural committee nixed that.


Does it change the sound? Yes, a little. It shifts the sound stage to the center some and gives the vocals a little more clarity. In terms of REW measurements, I see a difference in the Distortion Graph and the Waterfall Graph. However, almost anything changes those graphs, like moving furniture around, so its not clear to me if those measurements are significant.


Tim Perry says its technically daunting to try to measure the coefficient performance of a diffusor but it seems like we should be able to measure the over all results. What is your view?

regards,

Mike W
Hey Mike,

Thanks for sharing, those look great. Out of curiosity, how long did it take you to build these? I have the plans for them downloaded and I've read the articles pretty thoroughly. Seems like a great design, very space efficient. My take has been that it works its magic when used in large groups by extending the LF diffusion down some, but still works quite effectively in singles as well. The ideal minimum group is 5 which is hard, I don't have room for that, and looks like you don't either.

As for measuring, no I don't think you can measure the results of a diffuser in a room. I've read the articles by most of the topic experts on how to measure diffusion and what it is you are actually measuring. From that, I gather that it simply would not be possible to measure the effect of diffusers in a normal room with any certainty. There is little reason for it to reduce distortion, so if it is, I would question what that is. If the distortion reduction is all near or at the noise floor of the room or measurement, all the more reason not to trust it. As for decay, I'm of two minds. On one hand a pure diffuser should not have any effect on RT60 which means it should have no effect on decay. However, diffusion means taking a large soundwave and reflecting it in smaller components in many random directions. The total energy doesn't change, in theory. In practice air and existing absorption will absorb the sound and the diffused waves would be more easily dissipated, so I think there is something to be said for a diffuser allowing existing absorption to more effectively dissipate the energy (by spreading it across more of the absorption in the room in any given moment) and so I do think that would improve decay.

However, as a whole, I don't expect it to be possible to measure diffusion in a room. The way in which diffusers are measured is to take a diffuser and place it in a controlled ultra low noise space. Preferably an anechoic space to avoid other reflections from corrupting the measurement (but a reflective room could be used if large enough). Then placing many mics in an arc around the diffuser at a given space (Say every 2.5 degrees). In line with the center mic (roughly) is a sound source. That sound is reflected off the test specimen and each mic picks up sound waves. This is done with and without the diffuser in place. A calculation is used that creates a kind of difference score which reflects the diffusion at a given frequency. This is then averaged for the overall specimen diffusion coefficient.

I know a lot of folks have taken the view that if it can't be measured in their room then it isn't audible. This is really not true, mic's don't measure like our ears and their noise and distortion, their frequency response, and the way they pick up sound is both different and worse than our own hearing. There are lots of things that average joe cannot measure that we can readily hear. Diffusion is one of them. Diffraction is another (in all fairness average joe can measure this if he knows how and has the right environment to do it).
 

mike w

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I'm an amateur furniture maker so I have a lot of woodworking tools although all you really need is a table saw and some clamps to make the lean diffusor. It took about 20 minutes to cut the pieces out of MDF stock and about two days of glue up (because I don't have enough clamps).

Initially, I made the model 4, a three panel version with a 70mm standoff to the center module. I didn't hear a difference with the 70mm module so I went with the higher WAF, lower profile version. I'm sure that the larger you can make these diffusors, the better.

Lots of discussion of options in the link below.

https://www.gearslutz.com/board/studio-building-acoustics/766977-diy-sound-diffusers-free-blueprints-slim-optimized-diy-diffuser-designs-fractals-11.html
 

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I currently have diffusers on the top half of my first reflection point panels..
They are approx. 450 x 450mm. and extend 100mm out from the centre point..
Even at that size there is a much improvement in the spread of sound across the soundstage..there is more of an open feeling to the sound...

I'm now planning on building 4 diffusers..They will be from floor to the underside of my ceiling tray ..These will be 450mm. wide and extend 150mm. out from the centre..
I will be using 3mm. thick MDF for the curved panels
They will be placed at first and second reflection points..
Underneath the diffusers will be 100mm. thick fibreglass insulation to give absorption as well..
 

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I covered the whole side speaker area with diffusors. Sounds great, more detailed effects, better envelope. While there is also absorption at the back of the diffusive surface the response is good too.
 

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Early reflections can either really mess with SS&I or really enhance it, depending on timing and direction relative to the direct signal.
 

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I've added one more diffusor since this discussion started. It's in the back upper corner of the room (sort of). It gets the wife's approval where as something like thick foam would not. And since every diffusor also absorbs, I get a small benefit there as well.

My latest effort in room treatment is to flatten the T30/ T20 curves. I've read that the inverted U shape of my curve is a function of too little treatment or furnishings in the room. So, I've been adding things slowly since it takes me awhile to build what I want.

Correction to my first post: The walnut diffusor is not exactly at the FRP but within 12" of it based on the mirror reflection test.
 

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I agree it’s hard to measure, has anyone measured to see if there is any improvement on the impulse response. I want to believe it would be a little better, but may not impact it at all, I’m totally reaching here.
 

mike w

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I see minor differences in the RT60 and ETC overlays between before and after but those are two different measurement set ups and I don't trust that I have the microphone in exactly the same position. Bottom line: I can't say that the measurement differences would repeat. And it would be marvelous fun to get that diffusor on and off the wall to A/B in a single set up.
 

Matthew J Poes

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In general it should make no difference in the RT60. Diffusion by definition isn’t removing any energy from the room. If you see differences it’s probably measurement error.

You really can’t measure the effect of a diffuser in room using an omnidirectional microphone and simple measurements like REW. Diffusers mostly change the amplitude of reflections by breaking them up into smaller energy reflections that go out in random directions. The same total energy is in the room.

To measure their effect you need to be able to specifically measure the reflections and isolate specific reflections from others. That’s a hard task.

It also won’t change the impulse response much if at all. The likelihood that you could see it’s effect over all the other garbage is small. Just listen and don’t worry about it.
 

Negatron

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I have the room for many diffusers, but I am wondering if they would be a good idea in a live room.
I am looking at absorption panels from our 13' high French cleat down to the bottom of our stand mount K402 horns, and then I have 5' more above that could be used too. I have thought of making the same diffusers or making absorption panels angled the same. Is it advisable to use diffusion in a concrete/clay room with large windows? My room is not dedicated, and is 19'wide 40' long with a curved 32' high ceiling. I am on a limited budget, and will be making my own.

Years ago I made 2 RPG 4x4 diffuser clones for my rear wall, and loved them. I really like the angled diffuser though. I also wonder what would happen if you made the angled diffusion part, and attached it to the front of an absorber.

For those that have made the angled diffusers... how heavy are they?
 

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Negatron: I'm not sure what you mean by angled diffusors. The lean diffusors that I have made are heavy.

I'm not an acoustical engineer so cannot comment on your situation beyond noticing that your problem has all the hallmarks of no possible solution (large multi-purpose space and no budget). I would have your wife pick out carpets, curtains and tapestries and call it good.
 

Negatron

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Negatron: I'm not sure what you mean by angled diffusors. The lean diffusors that I have made are heavy.

I'm not an acoustical engineer so cannot comment on your situation beyond noticing that your problem has all the hallmarks of no possible solution (large multi-purpose space and no budget). I would have your wife pick out carpets, curtains and tapestries and call it good.
Thanks, I have a bunch of Roxul for panels, and much more will be ordered. I will only be able to get some large throw wool rugs to put in front of the speakers for absorbing on the floor. Wall I can do up to 18' with absorption and diffusion. I was talking of the lean diffusers. If diffusers will help in a live room too, I could make some of them also...maybe make a combo lean/absorber? My budget is maybe $2k to finish the acoustic treatments (not including wool rugs, bass traps and some first reflection panrls).
 

mike w

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Well...it sounds like you have some budget and flexibility for decorating your house like a studio ;-).

Here is the link to Tim Perry's diffusor design on Gearslutz. https://www.gearslutz.com/board/studio-building-acoustics/766977-diy-sound-diffusers-free-blueprints-slim-optimized-diy-diffuser-designs-fractals-28.html?highlight=lean+diffusor#post13250925

It is a long thread that has diffusor construction details and comments about how it works. Not sure about large volume spaces. This site is all about pro audio and their working spaces are usually quite small.
 

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Hmmm. The Gearslutz article referenced above seems to suggest diffusors pretty much everywhere. Not sure how that could lead to any kind of image sharpness.
 

j.man1503

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Frank Toole, in his well-known book, suggests diffusers alternating with absorption for the best effect in a surround sound environment.

What I have been thinking lately though is that most diffusers need a good bit of space between it and the listener to be effective. Something around 3-5' minimum distance. My HT room is barely this size from the side walls to the MLP.
 

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Any of you guys have diffusion on ceiling in the region of 1st reflection point?
 

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Any of you guys have diffusion on ceiling in the region of 1st reflection point?
No, but I have a bit of reflective area up there, a mistake really as I did the treatment a long time ago. When I treat again, my priority will be as much area of as thick cloud as possible. I would not have any reflection in the early reflection zone, but if I were doing a whole ceiling, sure at the back. Laths probably rather than real diffusion.
One sees Diffusors overhead in some Studio designs, also at the back of the room, typically just behind a couch. My contrarian nature immediately thinks, really? Can we hear diffused HF from behind or above? Take a look at jim1961's My Listening Room thread on GS. He ended up with reflectors and artificial 'diffusion' in the form of a Lexicon Reverb.
DD
 

Negatron

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I ran absorption for my first reflection panels on the walls, and ceiling of my last HT...with 2 rows of first reflection panels on the ceiling too.
 
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