# Treatment Recessed or Protruding



## aardvarcus (Oct 6, 2009)

When treating a room is it better to install room treatments over walls such that they are sticking out, or to design them in and recess them into the walls where the fronts of the treatment are flush with the surrounding wall. Just from pondering the issue it would appear to me that recessing them in would make the low frequency room modes “cleaner” especially as the number of room treatments increased, as there wouldn’t be multiple objects protruding from the surface of the walls disrupting the standing waves. To me I think it would look cooler with them built in. Even if it isn’t any better, are there any issues with recessing them?

Some background information-I am still in the planning stages of my Home Theater which is in the Basement of my new house, which is currently under construction. I want to do it once and do it right when it comes to the construction of the room. I am planning on a “room in a room” setup for sound isolation. This is a 100% DIY job. 

Thanks for your help!


----------



## SAC (Dec 3, 2009)

It makes little difference aside from the aesthetics.

Modes are determined by the underlying massive structure, not porous surface materials.

The limitation of building treatment into the walls is that, unless you are able to do a preliminary mock-up prior to the actual installation, that you are speculating as to where treatment will be required. Thus treatment will not be placed strategically but in accordance to the structure of the room, the result being overdamping of non-critical specular behavior while and potentially missing critical energy points of incidence.

One method of determining the specular behavior is to source salvaged paneling and to brad it up temporarily and to then 'shoot' the room in order to obtain the ETCs (of each speaker individually) and to then resolve the anomalous high gain energy sources into their various paths and to determine the actual points of boundary incidence. Thus you will have a good pattern from which to work. Note, you will need follow up proof of performance measurements with the mic placed in literally the EXACT same position (so a bit of planning to achieve that is required!). And allowances for treatment modification must be made and completed prior to the final installation steps, as after that, modification will be difficult if not impossible.

Modes can likewise be reasonably estimated with waterfalls based upon the massive boundaries. But aside from modifying listening position placement and speaker placement, if using velocity based porous LF/bass traps, placement is already pretty much a foregone conclusion - in the corners. The only real variables thereafter are 'how much', and of a reflective facing that will render the bass traps from full range broadband traps into high pass low frequency specific traps.

So, yes, in wall treatment is _possible_, but it requires MUCH more preparation and planning. Otherwise it will accomplish little more than simply serving to deaden the room, possibly while leaving the destructive specular reflections in play if care is not taken to determine their actual pathways during planning... which is the worst of both worlds.


----------



## patchesj (Jun 17, 2009)

The other issue to consider is sound containment.. If you are trying to build the room to be isolated or at least minimize sound leakage to other areas, you'll want the walls to be as sealed up as possible. Building absobers into the walls will create a lot more work to acheive this.


----------



## SAC (Dec 3, 2009)

Note: Isolation (sound transmission) and internal modal and specular treatment are _two separate_ issues. Please be careful not to confuse or equate the two. They are neither interchangeable nor will one serve the function of the other.

Energy transmission is mitigated by mass, as well as the boundaries being literally air tight and the flanking vectors (paths) addressed.

If you concern is isolation, let me now and I will re-post the list of reference docs addressing this non-trivial issue.


----------



## aardvarcus (Oct 6, 2009)

Thanks everyone for the responses. Just to clarify, yes I am discussing room modes and specular treatment, not sound isolation. I have a plan worked out for sound isolation that I will post in the appropriate sub-forum at some time before construction to get ideas on that, but I really wanted to get feedback on this part first.

Back to the room treatment, I am planning on using both absorbers and diffusers. My plan was to first recess bass traps in all the corners, next determine first reflection points for my speakers and recess absorbers in those locations, and lastly recess diffusers or possibly other absorbers in other locations. My plan is to have the treatments “interchangeable” by installing them with screws such that I could remove an absorber and install a diffuser, or even a flat wall panel. The work would be in determining the locations of these recessed areas, as the bass traps and the locations available for putting treatment would be the only “fixed” items in the room, so I would be able to test and adjust other actual treatments as necessary. I don’t want to be stuck with a dead room and specular reflections if I mess something up.

The real reason I am wondering about the modes is not for an item like an absorber, but for a large wooden diffuser. For example, on my back wall I would like to install a few large 2D QRD/PRD diffusers, probably built to a foot deep or so and taking up a decent percentage of the rear wall area. If I build my room in the standard fashion of constructing my wall and then hanging my treatments in front of that and my diffusers are taking up 50% of my wall area and my corner bass traps are “hiding” 20% of the wall area, will the last 30% of the clear wall really set the modes of the room, or will the modes be partially set off of the “average” depth of the diffusers? Would it help if the diffusers were recessed into the wall by the “average” depth of the diffusers, so a 6” recess for a 12” diffuser? Should the absorbers be protruding and only the diffusers recessed?

I know I am probably making something out of nothing, but I just can’t wrap my mind around how a diffuser wouldn’t be at least partially acting as a massive boundary to the standing waves. Thanks again for taking the time to assist me, I really appreciate it.


----------



## SAC (Dec 3, 2009)

You might want to play with QRDude (have I given Collo sufficient accolades lately???:bigsmile and develop a workable LF design before you assume that it is viable allowing for the limited depth available for a diffusor - plus the fact that you may be looking at the need for a 'nested' diffractal design for both low and effective mid band bandpass effectiveness.

Not impossible, but not trivial either. Especially considering the limitations regarding the degree of freedom stated. 

Also, you might want to consider only using a 1D diffusor for many reasons, not the least of which is to retain the finite and limited diffused energy on the horizontal plane. 

Additionally, plan on significantly more energy being lost due to edge diffraction. While the diffusor may be (ideally) constructed of purely reflective material, there will be a significantly larger amount of energy loss than conventional expectations will assume.


----------



## aardvarcus (Oct 6, 2009)

Attached is an example of the treatment of my room I am considering. Please note that this isn’t finalized, I am just providing it as an example. You have made me realize that recessing the non-bass trap absorbers is a bad idea, but I am still wondering about the diffusers.

 

I am not expecting the diffusers to operate in the LF (sub 300hz) range, I am just wondering how they will affect the room modes determined by the room dimensions. As you can see from my drawing, the blue diffusers will change the “massive boundary” of the walls if they are recessed into the walls, but if I recess the diffusers by half of the build depth, shouldn’t that minimize the disruption to the “calculated’ room modes? 

I have calculated my room modes and am pleased with the spacing up through 300Hz. While my room dimensions are not one of the “ideal ratios” the spacing on the modes is good and it makes best use of the room I have available. On the diffusers with a 12” build depth, I should get diffusion to around 600Hz and scattering to around 300Hz.
Modes:
39.76
79.52
94.25
119.29
134.64
159.05
188.50
198.81
238.57
269.29
278.34
282.75

So, does the concept (not the specific placements) look workable? I really want to know if I am headed in the right direction before I put a lot of work into the specific placements.


Thanks again for all the help.


----------



## SAC (Dec 3, 2009)

Sorry, but your modal calculations will not be correct.

A few important points.

Modal calculations ASSUME a perfectly massive rectangular boundary - meaning its acoustical impedance is completely resistive and reflective. It also assumes a completely (and ideally) rectangular space. While this seems simple, it assumes NO variation in surface - meaning for windows, doors, any surface irregularities or variations in acoustical impedance - which typical of doorways, windows, etc. It is an ideal rectangular perfectly reflective bounded space. 

They don't exist - at least not in rooms you want to live in or can get into or out off, seeing as how it precludes a doorway.

Thus the modal calculators give you a very rough overly simplified estimate (at best!). They are NOT to be taken as authoritative. Thus you have an idea before building, but after which measurements are authoritative - in that they provide the actual response with all of its complexity and variations.

Also, variations in the massive boundary will effect modes and modal distribution. How? With FEM or BEM analysis, and an extremely detailed CAD drawing of the room and complete complex acoustical impedance figures regarding the rooms boundaries, you can calculate this - but not from the simplified calculators which only address the space based upon dimensions (assuming ideal conditions about which the remaining variables are effectively ignored). But yes, the irregularities and variances in the acoustical impedance in the boundary surface will effect the modes and their distribution within the room. The modes will not be diminished, while the irregularities simply make them harder to predict - and hence the necessities of measurement on order to determine the real nature and distribution.

So if you are planning a room, and if you do not have the luxury of having additional boundary waste space, you can use the modal calculator to avoid dimensional ratios that allow modes to become coincident (think avoid dimensions that have common denominators) and reinforced (and that's all) and then you measure the room.

And as to the larger concept. No it is not what I would consider workable. As all that has been done is to alternate treatments.

It appears that not even basic abstract ray tracing assumptions have been attempted, let along treatment placements that considers the spatial polar Q/dispersion of the speakers and how that will play out in terms of the energy incident upon the boundaries, nor minimum spacing requirements that put the listener in the far field relative to the diffusors in order to avoid nearfield specular reflections and lobing.

The problem is that treatments do not work by virtue that you have a bunch of hem scattered around the room.
They work by addressing real patterns of energy distribution which have real points of boundary incidence by which the energy is directed to the listening position. These result in high gain sparse reflections that superpose with the direct signals, resulting in destructive interference.

The spatial dispersion of the speakers are critical in this regard, as this determines the degree of interaction with the boundaries and he generation of indirect energy reflections. Thus that is one variable that is not only under your control by virtue of their selection, but it has a critical role to play in where the reflections will be incident and thus, where appropriate treatment needs to be placed.

In this plan, there is not rhyme nor reason to what is being absorbed or diffused. 

The preferable way? Build the room as a rectangular space. Then, after modes have been measured, seating has been adjusted such that the placement is forward or behind significant nulls or peaks, then apply LF bass traps. You may want to utilize porous corner traps or tuned resonate traps. Just be aware the resonance enclosures are interactive with the modal behavior and thus present an iterative process in that they modify the response of the modes by becoming a part of the behavior that they are affecting.

Once the modal behavior is addressed, THEN you make ETC measurements for each (and every) speaker. From this you determine the real characteristics of the direct and indirect signals. First you align the various source direct arrival in time. Then you analyze the indirect energy arrivals in terms of gain and arrival time. And from this determine the anomalous signals and determine their precise pathways and points of boundary incidence. Once this has been done, you select the appropriate treatment (absorption, redirection or diffusion) sufficient to affect the desired response, be it as a control for early arriving reflections within the ISD time region wherein the Haas effect is predominate (and where the surgical use of absorption is appropriate), or if it is part of the later arriving energy after the ISD region where you generally have a well behaved laterally arriving exponentially decaying diffuse soundfield (where the use of diffusion is appropriate). 

This is further modified in a HT environment as the premise of X.1 is that the surround speakers supply that later arriving soundfield that the room normally provides.This where you have a few choices dependent upon the intended use (2 channel, surround channel music, HT, hybrid music HT, etc.). In this, it is possible, depending upon the intended usage to select n appropriate balance of surround supplied ambiance and room supplied ambiance.

But in any case, the placement of treatment is specific to the distribution of the specular energy in the room based upon the REAL distribution - not assumed possible distribution based upon some generic speaker dispersion model. In this manner real issues are addressed and one does not either over-damp the room or inadvertently create excessive levels of early arriving energy.

Build the room as a normal rectangle. Then make the measurements and then strategically place the treatment based upon real objectively identified behaviors. You will save time, energy and money doing so. And you will end up with a much better performing room that addresses the real, versus the assumed, behaviors. And you will effectively accomplish what it is that you actually desire - with the ability to objectively verify the progress and the actual performance with each step through proof of performance measurements. Thus each treatment will have a specific verifiable beneficial impact upon the space, rather than a detrimental effect - as is too often the case of treatments are not surgically placed based upon actual behavior.

If you like, PM me and I would be glad to discuss any or all of the above in much more detail and to explain the method to the madness in more practical detail....

Oh,and DON'T be discouraged! I applaud your attempt! But we can help you to more quickly zero in on a productive process, explore the underlying psycho-acoustic principles as well as the actual physics, and save you time, energy and money, and achieve a more optimally treat the room.


----------



## aardvarcus (Oct 6, 2009)

Ok, I will put my crazy ideas away and build a rectange. Thanks for taking the time to explain all of this to me, your posts have been much more helpful than the dozens upon dozens of articles/guides/tips that I have been getting off of the internet.


----------



## patchesj (Jun 17, 2009)

How is your project going? Sometimes it's best to keep it simple. If you are concerned with the walls being "flush" after treatments are put up, you can always fill the space in between with a false wall.


----------

