Diffusers in early reflection zone?

Bjorn

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I've considered to add some diffusers on my absorbent drop ceiling and some RPG Bad panels (or somthing similar) on sidewalls closest to the listening position. The reason is because I would like to get some more liveliness and it would also be good to cover the frame of the drop ceiling which are causing some reflections.

If I do this, it will be in the area of early arrival sound, within 10 ms. And many are saying that it's better to use absorbents for early reflections and rather add more dffusion for later reflections, typically after 20 ms.

Another thing I'm uncertain about is if using diffusers in a narrow place like this (on drop ceiling and sidewalls), will cause unwanted specular reflections.

I've attached some pictures and the lines on the drop ceiling are the places I've thought using diffusers. The pictures are somewhat outdated. I've removed the diffuser on the frontwall, using less absorption on sidewalls and started to make some changes on the slanting ceiling now, but they still give a good impression.

I use diffusion only on the backwall today. The absorbents you see behind the speakers and on the slanting ceiling behind speakers are covered with a reflective membrane of mids and highs. An exception is the superchunks in the cornes that absorp all the way up.

Would be great with some input concerning adding diffusion. Will using diffusion in those areas cause specular reflections? Am I better off adding diffusion on the frontwall in front of basstraps?
 

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Nicholas Bedworth

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Maui, where else?
Bjorn... how does this arrangement of panels sound? Do you have any impulse response tests that you could post? I'd be very interested in Art's take on this as well.
 

DonH50

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I am interested in Art's reply as well. My experience has been that diffusion can be as effective as absorption but results in quite different sound since more energy is "retained" in the room (versus being aborbed in the panel). Diffusors generally cost more, and obtaining broadband diffusion is often more difficult than broadband absorption. Whether you absorb the energy, or break it up so it is no longer correlated to the primary signal, it should improve the image. Again IME, diffusors provide a "bigger" sound while absorbers provide a "tighter" image. Which you prefer is always a matter of some debate... The latter (absorbers) should be more true to the source material, assuming a fairly dry mastering studio (not always a good assumption), but most seem to prefer the former (diffusors) for the sound's "presence".

All imho - Don
 

Nicholas Bedworth

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Maui, where else?
Don,

Fascinating response and thanks. Let me run this by the diffusor panel manufacturer, who in so many words has told me the same thing, and also the acoustical engineer who's working with us.

Nick
 

microstrip

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Disclaimer - I am not an acoustic expert, but I did a lot of experiments with absorbers and diffusers in my room. My experiments also point towards Don conclusions, but I would like to add there are many different types of absorbers, and most of the classical DIY recipes have severe dips in the absorption spectrum and produce unpleasant results due to their non uniform or irregular absorption.

My final preference went mainly to diffusion with some corner absorption. But I look for a spacious sound.
 

Nicholas Bedworth

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Maui, where else?
See, if you were an expert, you'd set up a room that measured perfectly, and sounded poorly! :)

The notion of diffusors with corner absorption sounds good to me, as I prefer a spacious sound as well.
 

naturephoto1

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When I discussed my room with Bryan of GIK Acoustics we discussed using a combination of absorption and diffusion. I started by installing absorbing Bass Traps and Acoustic Panels:



The above photo is a portion of the front wall of the room. Additional Bass traps were placed in the rear of the room with Bass traps placed above the subwoofers.

Bryan and I discussed what would be the next step after the first round of Acoustic Room Treatments. We decided to use diffusers on the side walls and for the ceiling for the first reflection points (we hope we caught them correctly). When the diffusers were installed I also installed additional Bass Traps (Tri-Traps) above the seating and added fiber fill to the diffusers that were installed on the ceiling. As a result, the diffusers on the ceiling act as both diffusers and absorbers at the same time. The third round of acoustic room treatments were additional Acoustic Panel absorbers that may be at/near the 2nd reflection point (we were still noticing an echo around the seating). The next 2 photos show the diffusers and acoustic panels on the side walls from round 2 and 3 of the installation:





The last photo shows the diffusers on the ceiling that also has fiber fill in wells of the diffusers to also act as absorbers:



I am pleased with the results thus far.

Rich
 
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DonH50

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I should note that my current room is pretty dead, for various reasons. One of them being its relatively small size. In a good-sized room, my preference is a mix, with absorption in the corners and first reflection points and diffusors further back along the side walls and ceiling, plus the middle of the front wall and back wall. I find diffusors at first reflection points tend to muck up (that's a technical term :) ) the image to me and so I prefer absorbers there, but I like having some diffusion around to provide a more "spacious" sound.

I believe that puts me in line with Rich's room, somewhat a comfort considering how well that looks and (I assume) sounds. - Don
 

John Calder

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May 16, 2011
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Hmm... interesting.

I would make two points:

People tend to use "diffusor" and mean only one type of diffusor - the quadratic-residue type. RPG has done a good job marketing QRDs as 'the' diffusor, based on their research, but I encourage everyone to also think about "coherent" (more accurately, "phase coherent") diffusors, which have been in use for hundreds of years.

I like phase-coherent diffusors because our species has a few million years' experience with naturally-occurring phase-coherent diffusion, and only a few years of never-before-heard-in-nature non-coherent diffusion. Quadratic-residue diffusion is non-coherent. Coherent diffusors, like polycylindrical and cylindrical types, are much easier to use and more affordable as well (disclaimer: I develop products for Acoustic Geometry, which makes phase-coherent cylindrical diffusors).

OK, three points: vertically-oriented phase-coherent cylindrical diffusion used at the first-reflection point, along with diffusors placed "next to" the listening position (on the walls, also vertically oriented, 90-degrees on either side) makes a listening room sound much more open and natural (a few horizontally-oriented cylindrical diffusors in between helps a lot, too). Try it. I really dislike absorbers in the first-reflection point - they deaden the whole room, and they are never truly broadband.

My $0.02. I expect others will differ.

Nice thread!
 
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naturephoto1

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I should note that my current room is pretty dead, for various reasons. One of them being its relatively small size. In a good-sized room, my preference is a mix, with absorption in the corners and first reflection points and diffusors further back along the side walls and ceiling, plus the middle of the front wall and back wall. I find diffusors at first reflection points tend to muck up (that's a technical term :) ) the image to me and so I prefer absorbers there, but I like having some diffusion around to provide a more "spacious" sound.

I believe that puts me in line with Rich's room, somewhat a comfort considering how well that looks and (I assume) sounds. - Don

Hi Don,

My room was too live with a number of kinds of echo, bass issues, and we knew that I was having problems in the 200 to 300 Hz range. Also, we were mixing horns with dipole speakers and a limited amount of space. We tried to put more weight to the 2 channel performance than the surround portion of the system. The room is 19' 4" W X 14' 7" L X 9'H. We had to deal with the bass issues that were masking much of the detail and information in the highs and midrange. We tried to deal with those issues. So Bryan of GIK Acoustics and I agreed to stack 2 Tri-Traps in the front corners but we couldn't do that with the rear corners because of the size and the position of the rear subwoofers. We placed Monster Bass Traps above the subwoofers. Additionally we placed 2 Tri-Traps at the interface with the ceiling and the rear wall above the seating. We also decided to place 2 Acoustic Panels behind each of the OMA New Yorker speakers and 2 above the TV all to deal with some of the reflection issues in the front of the room.

Bryan and I had more discussions and we felt that we wanted to increase the diffusion and make the sound and soundstage more spacious. Bryan asked me about the large OMA New Yorker horns which are only somewhat directional and he felt that because of my 9' ceiling we could use diffusion on the ceiling near the first reflection point (each diffuser was installed 180 degrees to the adjacent one and we also decided to fill the diffusion channels with Poly fill so the diffusers would do double duty as diffusers and absorbers at the same time). Bryan also felt with my circumstances that we should use their new and best wooden diffusers at the first reflection point on the door and the side wall. In my room and circumstance Bryan thought that diffusion was the better selection at the first reflection points.

At a previous visit Jonathan Weiss of OMA and I both agreed that I was getting some echo in the rear of the room and we agreed that I should place either diffusion or absorbing panels above the turntable and on the opposite wall. I opted for installing the absorbing panels though I could have gone either way but the absorbing panels were substantially less expensive and I had less concern about the panels above the turntable than the diffusers. When Jonathan returned to the house and we were fine tuning the setting up my Teres Illius Tonearm and Soundsmith Strain Gauge Cartridge during the listening session Jonathan indicated that system really sounded quite good, better than it ever had with the treatments and the source material that we were using sounded just the way it should.

More possibly to come possibly more diffusion.

Rich: what brand r your equipment racks? Very nice (as is your room)

Chuck,

Thanks. The racks in my system are Adona Signature. The double rack was a custom made rack initially designed to place my Center Channel on the 2nd shelf below the Plasma TV but I never installed the speaker that way.

http://www.adonacorporation.com/audiorack.html

http://www.adonacorporation.com/av45g3.html

Rich
 
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DonH50

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@Rich: Sounds like you got it right!

@John Calder: I have little experience with RPG's diffusers; mostly with the "older" kind in a variety of forms. I still prefer absorption at first reflection points, but admit a slight bias toward a deader, or at least "less live" sound due to hours in studio control rooms that were (intentionally) pretty dead. Even "phase coherent" diffusers have somewhat limited frequency range and I found with my dipoles they tended to emphasize comb filter effects.That said, a large control room at a certain southern studio had some massive diffusers and almost no absorption, and it sounded great... Beyond a certain minimal treatment I think a lot of it is personal preference, especially with treatment done right.
 

naturephoto1

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I see this thread is taking a little different approach then my intentions. My question is for using diffusers in the early time zone, within 10 ms. A general discussion when it comes to absorption vs. diffusion isn't my intent.

Hi Bjorn,

I am sorry about this. I was trying initially to substantiate the usage of the diffusion at the 1st reflection point to counter some of the questions by some of the forum members and it went on from there. :( Hopefully the thread will return to deal with your specific question.

Riich
 

Bjorn

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Bjorn... how does this arrangement of panels sound? Do you have any impulse response tests that you could post? I'd be very interested in Art's take on this as well.
It sounds good. I can honestly say it's one of the best setups I've ever heard. It's narrow here, but the acoustic treatment makes a lot up for that. The combination of reflective basstraps, absorption of 1reflections and diffusers on the rear wall makes the sound clear, precise, dynamic and with a pretty good combination of both pin-pointing/holography and developing sound. It's perhaps a little bit too much on the dry side, but not much. And I plan to add more diffusers which should bring both a bigger soundstage and more liveliness.

I do have some cancellations and I'm working on improving that. If I'm not able to deal with them with the treatment it's not a big problem. Because I'm getting a new speaker system with separate bass units in august with much more bass and it will have a processor similar to DEQX/HolmImpulse, but even more advanced.

This is how the ETC looks for eack speaker now. The reflections you see for both speakers before 5 ms I believe come from the chair and are probably not effecting when i sit in it. The other within 10 ms are coming from the frame of my drop ceiling and I know how to treat these now. And the ones starting to arrive at 17 ms are from reflective basstraps and diffusers on the rear wall, so they probably don't need to be addressed at all.
 

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Bjorn

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This is how the front of the setup looks now. The blue/black/white "pillows" on top of the basstraps are there temporarily. I removed a basstrap on the sloping ceiling and I'm considering adding a perforated panel resonator to deal with a standing wave.

I hope Art Noxon could comment on his opinion about using diffusers in the early time zone. If you ask people in the recording area, most seem to be negative towards it. They say that it can lead to specular reflections, that's better absorp to early reflections and rather use diffusion on later reflections. That's also my standpoint today, but I know others have a different opinion on how a listening environment should be treated. Hopefully Art can contribute.
 

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Nyal Mellor

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Hi Bjorn

My wife is Norwegian, in fact her brother is called Bjorn too!

On to the question:

I think it is ok to use any form of treatment in the early zone as long as the targets for reflection control are met. Namely that any reflections should be -15dB in level relative to the direct sound.

In a small room it might be difficult to get that amount of attenuation using a diffusor, since they are not absorbing any of the sound, only redirecting it in space and time. And the distance is not enough for the SPL to drop due to expansion of the sound wave over distance. Note poly diffusors do not redirect in time.

The only reflection point that one could argue about would be the side walls, where the treatment of the side walls can be used as an 'imaging vs spaciousness controller'. Using absorption on the side walls audibly narrows the soundstage and improves imaging, leaving them reflective makes the sound wide but loses out on pinpoint imaging. Using a diffusor on the side walls is a reasonable compromise between the two approaches. So what you do with sidewalls is IMO personal preference related and depends on how you like your sound and what music you mostly listen to.
 

Bjorn

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Hi Bjorn

My wife is Norwegian, in fact her brother is called Bjorn too!

On to the question:

I think it is ok to use any form of treatment in the early zone as long as the targets for reflection control are met. Namely that any reflections should be -15dB in level relative to the direct sound.

In a small room it might be difficult to get that amount of attenuation using a diffusor, since they are not absorbing any of the sound, only redirecting it in space and time. And the distance is not enough for the SPL to drop due to expansion of the sound wave over distance. Note poly diffusors do not redirect in time.

The only reflection point that one could argue about would be the side walls, where the treatment of the side walls can be used as an 'imaging vs spaciousness controller'. Using absorption on the side walls audibly narrows the soundstage and improves imaging, leaving them reflective makes the sound wide but loses out on pinpoint imaging. Using a diffusor on the side walls is a reasonable compromise between the two approaches. So what you do with sidewalls is IMO personal preference related and depends on how you like your sound and what music you mostly listen to.

That's a very good answer and summing up what I've heard from others.

Of course using diffusors on the sidewall would also give the problem with attenuation of early reflections, but you're thinking that still is beneficial and perhaps has Flooyd Toole's research in mind? What I don't know from Tool's reasearch is who much distance they had to the sidewalls. In my case getting a broader soundstage would be nice. Adding diffusers on the frontwall would give me some of that (considering the fact that I already have diffusers on the backwall), but probably not as much as diffusers on the sidewall.

If I added diffusers on the sidewall I assume they would have to be a nearfield diffuser and maybe only use them on the walls closest to the listening postion. A caveat is that I would loose some bass absorption that the absorbers are giving, but it stil might be a win situation. Difficult to know before one tries. Like you're saying you loose some of that pin-pointing and I would say greater insight to the recording.

What kind of diffuser would you recommend for sidewalls in mye situation?
 

Nyal Mellor

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Jul 14, 2010
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That's a very good answer and summing up what I've heard from others.

Of course using diffusors on the sidewall would also give the problem with attenuation of early reflections, but you're thinking that still is beneficial and perhaps has Flooyd Toole's research in mind? What I don't know from Tool's reasearch is who much distance they had to the sidewalls. In my case getting a broader soundstage would be nice. Adding diffusers on the frontwall would give me some of that (considering the fact that I already have diffusers on the backwall), but probably not as much as diffusers on the sidewall.

If I added diffusers on the sidewall I assume they would have to be a nearfield diffuser and maybe only use them on the walls closest to the listening postion. A caveat is that I would loose some bass absorption that the absorbers are giving, but it stil might be a win situation. Difficult to know before one tries. Like you're saying you loose some of that pin-pointing and I would say greater insight to the recording.

What kind of diffuser would you recommend for sidewalls in mye situation?

I am a fan of the RPG BAD Arcs at sidewall reflection points. Their reduced absorption at higher frequencies helps to rebalance the spectral content of the direct and reflected sound for quite a few speakers (since off axis the response droops towards higher frequencies due to increasing tweeter directivity). You could also try a polydiffusor (either diy from a piece of ply or bought) or a Primacoustic FlexiFuser (these are cool since they have experimentation factor built in!) or even try using 6" thick strips of absorption interspersed with 6" of bare wall (not really a diffusor but has an similar balancing effect between imaging and soundstaging.
 

Bjorn

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I am a fan of the RPG BAD Arcs at sidewall reflection points. Their reduced absorption at higher frequencies helps to rebalance the spectral content of the direct and reflected sound for quite a few speakers (since off axis the response droops towards higher frequencies due to increasing tweeter directivity). You could also try a polydiffusor (either diy from a piece of ply or bought) or a Primacoustic FlexiFuser (these are cool since they have experimentation factor built in!) or even try using 6" thick strips of absorption interspersed with 6" of bare wall (not really a diffusor but has an similar balancing effect between imaging and soundstaging.
I've looked at RPG Bad panel (not arc). I mentioned them to a person who works in the field of acoustics. He basically said that they wouldn't work that great in my case since they will be placed in front of me and not right on the sides.

When it comes to polydiffusors, the "word" has always been that they need more distance to the listener then a QRD to get a homegenous diffusion.
 

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