# Stone walls good for diffusion?



## Owen Bartley

I've been thinking about what I would ultimately like to have in our basement theatre (whenever that will happen), and my wife and I both love the idea of stone, or more likely faux stone cladding on the walls. I figure the randomness of the stone might serve as a decent diffuser for sound, even though it is a hard, reflective surface. I thought that stone for the lower half, and fabricmate panels with some acoustic material for the upper half would provide a good mix, and it would also allow me to hide in-wall speakers and make them invisible for the surrounds. 

I am planning on an acoustically transparent screen and front wall if necessary, with the sub hidden behind the front wall as well. I originally wanted to have a curved drywall ceiling with a soffit around the outside containing small pot lights for direct lighting, and rope lighting above for a hidden diffused light, but I don't think I'll have the height available to add the curve. Carpeted floor, standard front row and raised 2nd row on the back wall.

Thoughts on sound?

Here's a really rough Sketchup model:


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## bpape

Will be pretty hard sounding :rofl: Seriously, the thing is that down lower at ear level is where your reflections are going to be and where you want some absorption. 

Bryan


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## Owen Bartley

Thanks Bryan. What about if I switched it around and had the stone at the top, with the acoustic panels underneath? It will make it a little tougher to install the surrounds, but it should look nicer too, with the lights directly highlighting the stone.









_
Edit: To help keep things in check, I could do a full acoustic panel back wall if necessary. I find I usually like a more 'live' rear stage rather than easy to locate point source surrounds, but since this room will be small and well-speakered, excess reflection may be a bad thing._


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## bpape

I too prefer a more lively and diffuse surround environment. Stone doesn't really diffuse but it will help some at upper mid and high frequencies to at least scatter better than a flat surface.

Swapping them around could work fine pending the height of the main LCR speakers and where you transition from soft to stone.

Bryan


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## Owen Bartley

Thanks again Bryan. I'll play around with the design, I just wanted to get some early ideas to see if I was starting out on the wrong foot. I plan to have identical L/C/R behind the screen, but I'm still in the concept stage, and the actual room dimensions and proper measurements are down the line.


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## fitzwaddle

A couple that came to mind when you mentioned use of stone, maybe will help give you some different ideas of how to integrate the stone, but less overall coverage, so less impact to acoustics?










http://www.avsforum.com/avs-vb/showthread.php?t=1330596&highlight=stone

Or this one:


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## Owen Bartley

Thanks Brad, very much appreciated! We still have a lot of research to do on the design, but these two are right up our alley. The original idea came from an ad in a magazine years ago for Runco, in a castle-themed theatre, which I loved. Then I saw a more recent implementation in someone's demo (I've seen other forum members sucked in by the same picture). The final straw was visiting a boutique AV shop here in Toronto in an old downtown building that had a little viewing/listening room set up with old brick and stone walls, and just sitting in their comfy recliners in that dim room under a PJ and big screen felt like what home should be.


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## Ile

onder: How about resonator made of vertically bricked hole bricks and mineral wool behind it?










Would look bit strange in HT.


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## Owen Bartley

It might look a little strange. But at least it would be unique!


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## JIMKOVACS

I like this one, but have you ever worked with SLAT RESONATORS (Google that) ??
Great looking, and they do both reflection AND absorption, plus you can tune them.
In Studio design, we have a concepr called LEDE (Live End - Dead End) where the back wall is very absorptive (to avoid standing waves), and the front is more lively.
Have fun !


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## Owen Bartley

Hmmmm... thanks. I think I could add some of these, possibly even design them right into some of the fabric panels. I'll need to do some reading before I get into this project. I think I'll pick up a copy of the "Master Handbook of Acoustics" which I have seen referenced a few times in my reading. I also saw some really nice examples of slat resonators in studios, they can be made to look very cool.


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## Owen Bartley

I've been doing a lot of reading on acoustics lately, and the more I learn, the more there is to keep learning. I did order the MHA, which should be here next week and keep me busy for a little while. I've also been studying build threads here and at AVS, and I've picked up some good ideas. Probably the most important though, is that I am going to scale down and simplify my plans. For every little added feature or idea, it seems that there are a dozen complexities to go with it.

I will probably end up going with fairly standard broadband absorption treatments, bass traps at the front of the room, and if I'm feeling adventurous I might look more into building a QRD or skyline diffusor or two. 

Tweaked my sketchup model a little bit, and added a proper door, fixed the lower panels, and added some lighting effect for the rope lighting hidden by the soffit. I think I'll stick close to this concept and see how it plans out.


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## SAC

I'm a bit late to the party, but let me offer both a 'small' and a larger perspective.

First, in the room a primary concern will be to control modal behavior.
And in treating this you will most likely use porous bass traps (as opposed to tuned resonant traps - due to the relative cost and DIY complexity).

the caveat I would add this this - one that is often overlooked, is that the bass traps must be exactly that - BASS (low frequency) traps! As porous bass traps are by nature broadband traps, the detrimentally absorb far too much mid and high frequency energy! Thus the first challenge is to use sufficient quantities of LF bass trapping _that is specific and limited to just those frequencies!!!_

Beyond that, you want the room to be reflective. And saying that, you will use Only surgical treatment to control early arriving specular (focused) reflections arriving in what is referred to as as the Initial signal delay gap (ISD). I say surgical as you ONLY want to absorb the _specific anomalous reflections_ that cause listening anomalies in terms of imaging, localization, and intelligibility - no more!

Now beyond this LF and early time period response addressed by the above topics - you have the latter arriving soundfield. This is the part of the response that will benefit from diffusion. The optimal response of which is to return such energy in a diffuse manner oriented laterally - from the rear sides.

I would recommend 1D QRDs or PRDs (as opposed to the 'cool looking' but less optimal 2D 'Skyline' QRDs) as they constrain the available energy to the horizontal plane, and maximize the return of said diffuse energy to the listening position as they do not scatter it to the ceiling or floor, orientations that do not offer advantages psycho-acoustically in a small space.

(2D diffusion is best used in large spaces and not small rooms.)

So much for the larger perspective - now the smaller perspective.

As far as a stone surface, such a surface can help to retain the energy in the room for later diffusion. But the surface irregularities of such a wall re seldom sufficient to scatter any but the very high frequencies featuring small wavelengths - low energy content frequencies above the range that we are concerned.

Thus, while it can definitely help retain mid and high frequency energy in the room while minimizing the unnecessary absorption that is all too common in a small space... it will not provide the degree nor the control of diffusion that is optimal. For that you will want to use appropriately placed 1D QRDs or PRDs.


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## SAC

BTW, (at least to me) your intended use is not defined. Is it a home theater intended primarily for movies, or is is primarily a 2 channel listening room where you also watch movies?

This distinction is important as the acoustical response that one aims for is different.

In a 2 channel listening room, one tailors the early and later returned energy within the room. Your implemented design is responsible for the character and acoustics/psycho-acoustics of the space. In this environment you have a larger role to play in establishing a psycho-acoustically correct later arriving diffuse soundfield.

In the home theater things are much simpler (but not necessarily better...). In a home theater, the dominant later arriving soundfield are not necessarily a product of the room.

Instead, to a significantly greater degree, you are relying on the 5.1/7.1 encoding utilizing the surround speakers to provide the later arriving acoustic soundfields. It is no surprise that the fundamental surround speakers are oriented laterally - the optimal orientation for the later diffuse soundfield return in a 2 channel system. but rather than the room providing this return, in the surround configuration, the 'mix' provides this information - and you are in large measure at the mercy of whether the production folks did a good job or not, and to an even larger measure, on the strengths and limitations of the surround model itself!

thus in a home theater configuration, your challenge is to surgically moderate anomalous high gain specular reflections. An ETC measurement for each speaker at the listening position can be used to precisely identify and to locate the incident points on ten boundary surfaces, sufficient to allow for the surgical application of absorption to mitigate them. And no more!!! You ONLY treat the anomalous destructive reflections (and all are NOT bad! ...Unless your goal is a 'dead' zone).

In this scenario you do not use additional diffusion (except if one chooses to mitigate hot specular reflection in this manner _without _reintroducing high level returns back to the listening position competing with the surround signal....and this takes a bit of additional planning for it to work properly with the associated issues of gain and signal alignment relative to the surround signal, etc!).

Thus, unlike the goal for a 2 channel music critical listening environment, for a theater environment you seek a middle ground - not excessively dead, and not excessively 'live' - with the modal behavior addressed and the strong specular reflections mitigated. Instead you rely upon the encoding of the surround system to create the total soundfield without the excessive interference of the room.


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## Owen Bartley

Wow, thanks SAC. The primary use for this room is definitely (almost exclusively) HT. There might be some music listening, but not critical, it would just be to try out an SACD or two since I don't have proper multichannel music capability now.

I was planning to have a false wall about 2' from the front wall with the screen on it, speakers and sub behind, and bass traps in the two "L & R side" and the "top" (front wall & ceiling) corners. The front wall behind the screen would be covered fully with OC703 or similar.

When you mention the horizontal QRDs, do you mean like this:

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or like this:

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___________

Where should they be located in the room? I will have trouble putting anything much deeper than 1" on the side walls because it is a small theatre and will be narrow. the back wall should give me a little more depth to play with, but not a lot. 

Would I be better off waiting until after the room is built, saving the fabric panels for last, then running Audissey and measuring the results to see if there are specific issues present? then I could fill the panels as needed... OC703, diffusion, or leave them empty. 

Approaching this from the very beginning before the design really starts, what is the most practical way to plan for room treatments? I'll try to measure this week to see roughly what I have to work with pre-framing.


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## SAC

1D QRDs are arranged primarily with the wells oriented vertically , thus dispersing the energy on a horizontal , loft-right, plane.

As to placement, it is hard to say abstractly.

Such treatment is intended to treat specific specular behavior, breaking the focused energy into a diffused energy.

Generally speaking in a HT, one would want to surgically apply porous absorptive panels a MINIMUM of 4" thick spaced 4" from the wall. Using simple 1/4 wave theory (as opposed to a program such as Soundflow), and assuming an incident angle of 45 degrees, the effective thickness would be ~11.3" and the quarter wavelength and low frequency cut off would be ~ 45.25" and ~300 Hz respectively. At normal (90 degree) incidence, the quarter wavelength distance and cut off frequency would be 32" and ~422 Hz. Not brick wall calculations, but general functional estimates.

But you only want to use these surgically - where actual anomalous specular reflections exist, in order to avoid a dead response. Generally, these would be applies at point of incidence on the forward side walls, and on the ceiling approximately mid way between the speakers and the listening position. I would suggest using the measured ETC response for each speaker driven individually to determine both the specific reflections and their specific path and boundary incidence points.

If you choose to go beyond what the surround encoding supplies (and of which is not 'supported' in their standards), you might consider the application of the diffusors to the rear wall (potentially with deeper LF bass traps located behind them, as the diffusors are not massive and will not stop the LF energy.

But again, what is appropriate is driven by the response model you wish to achieve, not to any general one-size-fits-all 'its good to use diffusion' mantra. In a 2 channel setting, I am a big advocate of the proper use of diffusion.

However, in a home theater, if one is adhering to the precepts of the surround sound formats, one does _not_ want the room to add such additional energy that arrives significantly out of phase. In this case we are mixing acoustical response models, and simply creating a hodgepodge crazy quilt of varying responses without a comprehensive plan supported by accepted psycho-acoustic models is not generally a path to optimal performance.

And as far as measurements and planning, one would FIRST decide on an acoustical response desired! THEN, measurements are made and lans developed to address modal behavior and to then implement the LF plan for addressing modes that is designed to integrate with any additional treatment (its ALL part of a complementary system) before messing with mid and high frequency specular treatment. In order to measure specular reflections in order to both identify specific anomalous reflections and their vector paths, you need to have reflective walls in place. (Satisfactory ETC approximation measurements that will qualify the actual steps to take to implement the acoustical model can be made during construction by applying temporary salvaged panels to the wall surfaces for anticipating and determining the actual anomalous reflections).

For specific steps, PM me per the below. I would be glad to provide an effective method to the madness. (And no, I'm not selling anything!)
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If anyone wishes to pursue the various approaches in more depth, and or wishes to know more about various accepted acoustical response models, and/or how to specifically use measurements to determine specular reflections and their paths, and how such measurements are used to both determine which treatments are appropriate and where to place them, PM me with your Yahoo Messenger or Skype username, and I will be glad to discuss the topic by voice and clarify any concerns you might have.


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## Owen Bartley

SAC, more great stuff, thank you very much for your input. To be honest I still don't understand a lot of it, but I'm working on it.



SAC said:


> 1D QRDs are arranged primarily with the wells oriented vertically , thus dispersing the energy on a horizontal , loft-right, plane.


OK, got it. I haven't seen many panels with horizontal fins, except in a mixed vertical/horizontal array.



SAC said:


> Generally speaking in a HT, one would want to surgically apply porous absorptive panels a MINIMUM of 4" thick spaced 4" from the wall. Using simple 1/4 wave theory (as opposed to a program such as Soundflow), and assuming an incident angle of 45 degrees, the effective thickness would be ~11.3" and the quarter wavelength and low frequency cut off would be ~ 45.25" and ~300 Hz respectively. At normal (90 degree) incidence, the quarter wavelength distance and cut off frequency would be 32" and ~422 Hz. Not brick wall calculations, but general functional estimates.


I think I understand where you're going with this, and how the angle increases the thickness, unfortunately I just won't have room for panels that thick. I have considered a couple of custom printed panels done up to look like movie posters (using this method) at the primary reflection points, but I won't know how feasible this is until I get the room built and some furniture in. I don't want to compromise space for seating or viewing angles in this tight space.



SAC said:


> If you choose to go beyond what the surround encoding supplies (and of which is not 'supported' in their standards), you might consider the application of the diffusors to the rear wall (potentially with deeper LF bass traps located behind them, as the diffusors are not massive and will not stop the LF energy.


Could high frequency diffusors be used on the rear wall to help cope with rear surround speakers that will be quite close to the back row of seating? Almost as a sort of BSC for the in-wall speakers? Just a thought.



SAC said:


> But again, what is appropriate is driven by the response model you wish to achieve, not to any general one-size-fits-all 'its good to use diffusion' mantra. In a 2 channel setting, I am a big advocate of the proper use of diffusion.


I think what I'm looking for is a "typical" theatre, fairly flat response, without too much reflection to confuse the surround channels.



SAC said:


> However, in a home theater, if one is adhering to the precepts of the surround sound formats, one does _not_ want the room to add such additional energy that arrives significantly out of phase. In this case we are mixing acoustical response models, and simply creating a hodgepodge crazy quilt of varying responses without a comprehensive plan supported by accepted psycho-acoustic models is not generally a path to optimal performance.


I think this makes sense too, with a lot more speakers to handle the processed surround sound, you can't use the same ideas used for a 2-ch setup where the sound of the room, including the reflections, create the desired ambiance/sound for your listening position.



SAC said:


> And as far as measurements and planning, one would FIRST decide on an acoustical response desired! THEN, measurements are made and lans developed to address modal behavior and to then implement the LF plan for addressing modes that is designed to integrate with any additional treatment (its ALL part of a complementary system) before messing with mid and high frequency specular treatment. In order to measure specular reflections in order to both identify specific anomalous reflections and their vector paths, you need to have reflective walls in place. (Satisfactory ETC approximation measurements that will qualify the actual steps to take to implement the acoustical model can be made during construction by applying temporary salvaged panels to the wall surfaces for anticipating and determining the actual anomalous reflections).


So it sounds like I will have to try to plan for some flexibility for the ability to treat the room when finished, but ultimately will have to measure the room to determine what steps will need to be taken. If I had more leeway in the dimensions, I could try the temporary panel method, but I am pretty much stuck with what I have due to support poles and other basement features.

Thanks a ton for all of your input. Once I get a little closer to starting the project, I will likely bother you some more. Right now I am still in the pre-planning stage, and I just wanted to approach all aspects that I could before putting something on paper.


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## bpape

In general, in a home theater type environment, diffusion is usually used on the upper portion of the rear 1/2 of the side walls. Other places in the room are generally (not always) better done using absorption to derive the correct decay time curve and address specific issues like bass reflections, boundary reinforcement, etc. The rear side wall placement generally provides a more spacious feel and a more diffuse surround environment without removing additional energy from the room.

Bryan


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## Owen Bartley

Thanks Bryan. This aspect of the project is spiraling into deep complexity pretty quickly on me.  I now have a copy of the Master Handbook of Acoustics, and am suitably overwhelmed. It's a good thing I have some time before this gets underway, because I have a lot of technical reading ahead of me. Hopefully after I make my way through some of it I'll have a better understanding of the help you and SAC have provided.


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## Owen Bartley

I've had some more thoughts about the treatment of my still-hypothetical room, but I wanted to float them out there to see if I'm on the right track. This is concerning the front of the room, which would have a false screen wall. I'm planning to DIY an acoustically transparent screen (probably Seymour AV) and grille cloth panels for the rest of the wall. I will need about 2' of clearance for my sub, and I had some ideas how to treat the actual wall. Excuse my crooked sketch, I was doodling at work.

1) I thought I would build floor to ceiling superchunk corner traps in the front L/R corners (green)
2) The rest of the wall would be covered with 1" or 2" OC703 or similar (yellow)
3) I should have room for a proper 4" thick OC703 panel on the upper half, and be able to leave a gap to the wall (orange)

This should give me some bass control and absorption, and allow me to treat the seating area of the room itself as needed with spot treatments... I think. Am I on the right track here?


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## bpape

From a decay time perspective, the 4" on the front wall would help. From a purely HT perspective, 2" would be sufficient for the front wall coverage unless the speakers are pretty close where you'd have SBIR issues. The chunks will do an excellent job.

Bryan


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## Owen Bartley

Thanks Bryan. I'm trying to learn what I can from you and SAC, and my MHA reading. With the space constraints, I think the front wall is the only place I'll have some actual depth available. 

Would I only need to worry about SBIR if the speakers are close to the side walls? There should be some flexibility of placement behind the screen, at least from L to R, front to back there won't be a lot.


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## bpape

Boundary interactions occur from side wall and from the wall behind. What frequencies and the intensity is based on distance. 

Bryan


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## test4echo101

Hey Owen,

Not sure if you have seen RealTraps video demonstrations but if you haven't, they will certainly clear up some confusion. Actually seeing and hearing the before/After can really help gain a much better understanding.

Watch them all!

http://www.realtraps.com/videos.htm

Test,


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## Owen Bartley

Thanks Test, I liked the one on diffusion, the comparisons he shows were very helpful.I know every room and every situation will be different (and he is promoting his product), but it was nice to have an example of bare wall, absorber, and diffusers one after another.


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## basscleaner

Hi, Owen. You’re right, every room has it own individuality, but I would like you’ll pay attention to the early reflections role, especially you want to use stone like acoustic material… Let us to carry a simple experiment: take, please, two CD, oriented to your ears in both hands vertically. If you will direct it to reflect sound from main stereo speakers (I recommend to close your eyes, when doing it, during turning CDs “to catch” first reflections) respectively left CD to the left speaker and right CD - to right side, and with cross situated (i.e. left CD – right speaker and right CD to the left speaker) you’ll see great difference! It seems to me, after your conclusion you‘ll know significantly, where to place stone diffuser and absorb panel inside you room. I divide early reflections on "bad" and "good" types. Cross reflections are "bad" and must be eliminated by acoustical treatment.


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## Owen Bartley

Thanks basscleaner, and welcome to the Shack! I do plan to work in some kind of treatment for the first reflections. I never thought to try a test like yours, but that's a great idea, I tried it at my desk at work and could immediately hear the difference, even at low volumes with small cheap speakers.


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## basscleaner

OK Owen, thanks for kind words. This idea has come to my head, when I observed how old people take your hands to ears to hear better. Our hands are simply leather membrane, and this fact "ampliphiers" (if it so may be spoken) our sensibility to higher frequencies. Maybe it's interesting to construct armchear with such a surfaces on both sides, don't you? It seems to me, it's possible to make it for front and rear speakers too.


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