Control Room Construction

Rod, As I pointed out in a few sentences before the link to the pdf. - the link is a seriously drafty draft, and I have changed an incredible amount since I posted that. In saying so, the references list was, and still is, not up to date. I have a physical list written down on paper, of ALL my references, wether from word of mouth, a book, internet, diagrams, anything... it's all on that list. At the time I had only listed books and websites in my references list, and have not completed the list yet... but I assure you that both your book, and your diagrams are referenced clearly in the appendix. Andy

andihopkins wrote:

Ok, here are the page/chapter details in MHoA: Page 442 - Chapter 21 Just above the title, 'Outer Shell of the Control Room' he says...

These frequency zones determine construction of the control room—a massive shell to contain and distribute the low-frequency modal energy, and an inner shell for reflection control.

He then goes on to talk about the two main outer shell shapes, rectangular and trapezoidal, etc.

OK - this is tough - we are discussing the same thing in 2 different threads...... so after this - let's try to consolidatethis into one location - this from the other thread:

when Everest speaks of the Outer shell - he is speaking of the ENTIRE outer shell - which consists of both the inner and outer walls. It is those 2 walls together that create the outer shell - and they can only be measured (in terms of isolation) together. The inner shell he refers to is the innermost surface of the wall system - which it is critical to the acoustic qualities of the room - and this is independent of the shell as a whole.

So the massive outer shell (the ENIRE wall assembly - that which is tested for isolation) is being used to "contain and distribute" (within the space) the low frequency modal energy. That is clearly defining an isolating assembly - and he is focused of it's design being effective at low frequencies (and not bothering to mention higher frequencies) because? - Because if you can handle low frequencies with an assembly the highs are guaranteed to be even better to begin with. No bother to concern yourself with them escaping. THEN he says: "and an inner shell for reflection control." - Again - we know we just locked all of this highly reflective energy in this room when we locked in the low frequency energy - and we now need to deal (and this is only with the inside most face of the room) with reflction control. There is nothing in this that contradicts my understanding of acoustics - and it doesn't conflict with what Stuart is saying either - although he does not have my smooth manner of presenting things....... lol

If we forget isolation for a second, and assume that we have treated the MSM with materials specific to that of acoustic treatment (and not isolation)... would it be correct to assume that the inner shell/outer shell system would work similar to that of a membrane absorber?

Actually - John sort of attcks things that way with his inside out wall design....... but the short answer is this: All design begins with an understanding of the isolation needs of the client - it does you no good to work out an entire design of a beautiful studio only to find out that it won't fit inside the space you have to build it when you add (outside of it) what you need to make it function in the real worl. So first the space and degree of isolation. Now you can begin with the artistic side of things...... and I suppose the answer is "yes" the amount of sound escaping the room will leave a residual - and it is that residual that I now have to deal with in order to have a room that is acoustically pleasing - so in that sense the shell does play a part - in sort of a round about manner of picturing things. But is does not translate directly to absorption - that is the difficulty - because the tests for isolation and the tests for absorption are 2 totally different animals - providing totally different data results - one in real world measurements I can reach out and touch (decibel loss........ ) - the other in a pink sort of fuzzy concept with an open window (1 sf) relating to the time of decay of a sound after the source has stopped transmitting (sabins). There is no way to directly translate one to the other - and there are no tests done on walls as absorbers or room treatments as isolators....... SO that is what you are up against with this line of reasoning......... and why producing emphirical data to support the position is one of impossibility. The fact that Everest says something (or I do - or Newell does) doesn't make it Gospel........ we'll all made mistakes - we're all human - we all learn more today than we knew yesaterday.......... (or at least we do while we're alive). There are known errors in MHoA that have survived multiple editions - there are known errors in my book (that hopefully will be cleared up in the 2nd edition) which is why we have errata sheets - That doesn't make Everest any less for having made errors - or Newell - me - you or Stuart...... aftr all - when it all comes down to it - we are all failable human beings.........

As the 1st mass is thinner, and in most cases, less dense, would the 1st mass really have any effect on frequencies bellow 100 Hz or so? It has been pointed out to me that the purpose of the MSM system (when relating to isolation) is not absorption, but reflection.

OK - forget the term reflection for a moment - this is something that is a by product of the assembly - but not it's goal....... The goal is to stop unwanted noise from either getting out - or getting in - period....... Are we together so far? So the question is this - how do we acheive that? In a lot of cases we begin with a very massive outside wall surface......... say 8" of concrete or block....... Let's look at the value of that wall in and of itself for a moment......... For some reason you keep looking at the wall components - and not the "wall" itself. If you begin with an 10" concrete block wall - standard weight block bare on both surfaces- you have an STC value of 49...The numbers in the middle are the TL values for this wall. On the right side are the TL values for a double 2x4 wood frame wall - 16" centers - 2 layers of 5/8" drywall on each face...... STC these are the TL Values in corresponding frequency: Freq HZ 125.....32.4....42.8 160.....31.2....49.6 200.....34.0....55.3 250.....36.2....61.1 315.....39.8....65.9 400.....40.7....69.1 500.....43.5....72.4 630.....46.6....75.1 800.....48.9....77.9 1000...51.6....80.7 1250...53.9....83.7 1600...56.4....83.9 2000...56.0....74.6 2500...58.6....75.5 3150...50.5....82.4 4000...61.0....88.4 The above taken from Report # BRN-217Sound Transmission Loss of Masonry Walls, Tests on 90, 140, 190, 240 and 290 mm Concrete Block Walls with Various Surface Finishes by Warnock & Monk. and the wood frame walls are from IRC-IR-761 Test ID, TL-93-269 (STC 67). Note the advantage over the masonsr wall with a less expensive 2x4 assembly. Now - take that same masonry wall as the exterior leaf of a MAM system and add a single 2x4 wall with 2 layers of gyp on the inside face (with fluffy fiberglass in the void) and it will outperform the 2x4 wall by far. Rod

andihopkins wrote:

Rod, As I pointed out in a few sentences before the link to the pdf. - the link is a seriously drafty draft, and I have changed an incredible amount since I posted that. In saying so, the references list was, and still is, not up to date. I have a physical list written down on paper, of ALL my references, wether from word of mouth, a book, internet, diagrams, anything... it's all on that list. At the time I had only listed books and websites in my references list, and have not completed the list yet... but I assure you that both your book, and your diagrams are referenced clearly in the appendix. Andy

No problems Andy........ as I said - I was just suprised to see my artwork in a parper from someone who neither owned nor read my book - had me thinking for a second there.......... I am not one to :horse: LOL Good night. Rod

There is nothing in this that contradicts my understanding of acoustics - and it doesn't conflict with what Stuart is saying either - although he does not have my smooth manner of presenting things....... lol

Aw geee! And I was trying to be diplomatic! :) - Stuart -

Thanks for the info Rod. Two quick things before I continue: 1. Thank you for taking the time to help. I really appreciate it. 2. I will not be embarrassed if I truly have missed to point of the books I have read, however, I want to be 99% certain that I really have missed the point.

To contain the low-frequency sound energy associated with con- trol-room activities, thick walls, possibly 12-inch-thick concrete, are required.

That is the last sentence of the section 'Outer Shell of the Control Room' on page 442 of MHoA. Here he depicts the outer shell to be one, singular, 12-inch wall, and does not allude to stating that the outer shell is a system of multiple walls and materials.

The purpose of the inner shell is, among other things, to provide the proper reflection pattern for the operator at the console. Consequently, its construction can be relatively light. For the inner shell, shape is everything.

If Everest really is trying to say that the outer shell is the MSM system, then wouldn't he call the inner section a 'space' - and not a 'shell'? A shell describes something that surrounds another, it does not describe what exactly the shell is surrounding (in this case, acoustic treatment). He speaks of construction, does that mean that another wall, is added to the 2-wall outer shell? If one couples together the information found in both sections of this particular part of the chapter, isn't it apparent that when he talks of the outer shell and inner shell, he is talking about two separate constructions, and therefore, two separate walls? If he is trying to convey that the outer shell is actually the MSM system, wouldn't he attempt to explain this at some point? The only time he describes an example of the outer shell, is when he speaks of the 12-inch, concrete wall. Also, the section on inner/outer walls relates purely to acoustic treatment, and not to isolation. Would it be correct to say this? If this is the case, in order to understand what his views are on isolation I'm having a read through that section of the book now. I understand that mistakes will always be present in books, however this section is under 2 bold headings, and leads from the section on frequency regions, thus, one would imagine that if this were a mistake, they would have picked it up, due to the headings, and importance of the subject. Likewise, I also understand that Everest's perspective is not the be all and end all of acoustic knowledge, and thus I have referenced my views to various authors, such as Philip Newell. I'm going to go back over Newell's book again, and try to understand exactly what he means, then I'll try to dissect it, much like the above for MHoA. For those wondering.. the above quotes come from F. Alton Everest's, Master Handbook of Acoustics, 4th Edition. You can find the text in the section on Control Room Acoustics, Chapter 21. Andy

To put my questions into a practical application, here is an example of my perspective, as to how isolation and bass trapping may be able to be treated using a 2-shell design. My conclusions are drawn by combining the 2-shell bass trapping ideas posed (in part) by Everest, Newell and Huber, with the isolation concepts of users on this forum, as well as information from various sources. Any opinions, from anyone, are welcome. However, please do just say that it is 'wrong' and not provide a detailed description as to why your opinion is so. Ok, so here's the question to get us started.... If one creates an effective MSM system, with air acting as the spring, would the application of (for example) acoustic hangers in the spring (similar to the one's in the image below) absorb low frequencies, as they are purposed to do? The air in the spring is still present, therefore isolation is still possible. Does that make sense? As many of you have since pointed out, isolation and absorption are two different animals, therefore, is it to say that the hangers will not effect isolation? Who here agrees that frequencies in region B, with wavelengths ranging from 16 m to 3.4 m, will not be reflected by a thin, inner shell ranging (for example) from anywhere between 5 - 15 cm? But, will reflect off a larger, and denser secondary wall? Correct me if I am wrong, but I have been studying the behavior of sound for quite some time now (both at SAE and personally), and one of the first things you learn is the difference between high and low frequency wavelengths, as well as the difficulties in trapping and containing lower frequencies, because of much longer wavelengths, and the ability of the 'wave' (air pressure) to bend and twist more easily through objects and materials. Therefore, if we agree that low frequency wavelengths will be reflected chiefly by the secondary wall, what effect does the MSM system have? Transmission loss is what I have gathered so far. Is this correct? Transmission is defined as being 'passed on from one place or person, to another' Transmission loss, then, can be defined as 'some form of loss or decreasing of quality, through the act of passing from one place to another.' If the above description is correct, then sound is always effected by transmission loss. As for sound to exist, there must be propagation (transmission), and as air has density, some loss will always occur. Again, correct me if I am wrong, but does the description of transmission loss also work to describe absorption? They seem insanely similar. Picture it this way, instead of building bas traps OUT... this theory builds them IN to the wall. But the same theories for bass trapping still apply, it's just that the panel isn't sticking out from the wall. I'll try to think of some more examples/ways of explaining myself. Cheers! Andy

andihopkins wrote:

That is the last sentence of the section 'Outer Shell of the Control Room' on page 442 of MHoA. Here he depicts the outer shell to be one, singular, 12-inch wall, and does not allude to stating that the outer shell is a system of multiple walls and materials.

Andy, Understand a couple of things here...... first off - Everest was not writing a primer for the uninitiated to learn the basics from - he was writing an engineering manual for experience acousticians and engineers to work from. And (especially for non acousticians) this is probably one of the more difficult manuals to understand. He didn't need to take this to the basic level of explaining what made a wall assembly - but rather strictly what that wall assembly had to achieve - and some directions that could be taken to make it from A to B. Anyone in the field knows that the amount of TL value in an assembly has to do with the entire assembly as a whole - from side A to side B - and not the individual parts and pieces of that assembly. This is why any assembly is tested by constructing the entire assembly in a lab - and then measuring to establish the value of that assembly. And the minute you introduce another assembly adjacent to that assembly - you just changed the TL values - sometimes in reverse of what you intended, which is why you need to determine exactly what you are going to construct from the begining before you have testing performed. I do not agree with your assumption that he is talking about to totally separate entities......... this because (first of all) it is physically impossible to separate the 2 when it comes to the isolation values. The value of the 2 walls as constructed will determine the total loss from one side to the other. It is that simple - and thus - it is the wall assembly as a whole that is the outermost wall he is speaking of. If that consists of masonry with an inner frame - or 2 walls of masonry - or 2 framed walls is irrelevant - the 2 combined create the whole. Those will determine the isolation. Then you have a thing which is separate from that - which is the inside shape of the innermost face - and this has nothing to do with the shape of the outer wall - this will determine modal activity as well as higher frequency reflections. Listen - I didn't invent any of this - I am not really that bright......... but I have constructed and designed numerous world class recording studios and movie studios along with national museums, schools, hotels (both high and low rise) convention centers, etc., etc., etc. and all of these have their own issues with sound isolation as well as sound quality..... and I have an extremely firm grasp of the concepts and physics at play here. I am telling you - you are missing the big picture when you try to view this the way you are. Sincerely, Rod

you are missing the big picture when you try to view this the way you are.

Rod, The thing that is most frustrating about all this, is that it seems no one has explained to me the big picture. (unless of course I have missed it) We seem to be running around in circles endlessly, yet no one has informed me as to why my perspective is not ideal, most have just stated that it is 'wrong' and have continued by stating they're perspective. STC ratings and the like mean nothing if you don't understand the big picture, and that is what I have been trying to establish. My apologies if I seem a little dull and confused, Andy

andihopkins wrote:

If one creates an effective MSM system, with air acting as the spring, would the application of (for example) acoustic hangers in the spring (similar to the one's in the image below) absorb low frequencies, as they are purposed to do? The air in the spring is still present, therefore isolation is still possible. Does that make sense?

Not preserved: Hangers 1.gif

Ah - but the hangers are not inside the spring of the system........ they are inside of the control room. Installing a false ceiling does not necessarily create the inner leaf of a MAM system. I create false ceiling and false walls all the time to develope trapping behind so I don't have my rooms all cluttered up with "traps" - but that just means that the room boundaries are not what they appear. Behind those false wall ceiling are the isolation assemblies consisting of MAM construction.

As many of you have since pointed out, isolation and absorption are two different animals, therefore, is it to say that the hangers will not effect isolation?

Although they might have some small effect on this - it is so small as to not make any seriously measurable difference betweent them being there and the room being empty.

Who here agrees that frequencies in region B, with wavelengths ranging from 16 m to 3.4 m, will not be reflected by a thin, inner shell ranging (for example) from anywhere between 5 - 15 cm? But, will reflect off a larger, and denser secondary wall?

As I pointed out - false walls and ceilings do not define the room boundaries........ you are talking about apples when looking at oranges.

Correct me if I am wrong, but I have been studying the behavior of sound for quite some time now (both at SAE and personally), and one of the first things you learn is the difference between high and low frequency wavelengths, as well as the difficulties in trapping and containing lower frequencies, because of much longer wavelengths, and the ability of the 'wave' (air pressure) to bend and twist more easily through objects and materials. Therefore, if we agree that low frequency wavelengths will be reflected chiefly by the secondary wall,

Well - there begins your problem......... as I have repeatedly tried to point out to you - the TL value is for an entire assembly - not for one side of the assembly. So you beginat a point whre we cannot agree.....

what effect does the MSM system have? Transmission loss is what I have gathered so far. Is this correct?

That is correct.

Transmission is defined as being 'passed on from one place or person, to another' Transmission loss, then, can be defined as 'some form of loss or decreasing of quality, through the act of passing from one place to another.'

Nope - it has nothing to do with quality..... TL is defined as The decrease in power that occurs during transmission from one point to another. Thus - if you have a signal of frequency "A" in a space - and the space has an envelope with a TL value of "B" at that frequency - and B = 30dB...... and you transmit frequency "A" constantly at an amplitude of 80dB - then outside of the wall (36" from it's face) you will receive 50dB of frequency "A" assuming that on the other side of that space you had no residual sound to deal with - you would have a very clear reading of that 50db. If at frequency "A" you had a TL value of 85 - then outside you would not hear any of that sound. Now - that leaves the space itself with a reflection from the envelope that keeps almost all of that remaining 30dB (in the first case) and 80 dB in the 2nd case - still within the room. I say almost because a small amount of what is passing from side 1 to side 2 will be converted into energy.

Again, correct me if I am wrong, but does the description of transmission loss also work to describe absorption? They seem insanely similar.

Consider yourself corrected...... although they might seem similar - they are not. TL is always measured with a constant state signal - whereas Absorption Coefficencies are always measured after the initial signal stops - and are a function of how quickly reverberation drops.

Picture it this way, instead of building bas traps OUT... this theory builds them IN to the wall. But the same theories for bass trapping still apply, it's just that the panel isn't sticking out from the wall.

I said earlier (yesterday I believe) that John Sayers uses an inside out wall design so that the inner wall is sound attenuation within the room - and the 2 leaf of he isolation wall is installed on the inside face of the inner most wall.... Nice trick - works - hides the panels you refer to - BUT - it only changes the location of the room surface - it does not make the panels treating the room inside the wall cavity the way you picture it.

I'll try to think of some more examples/ways of explaining myself.

I'm waiting Rod

andihopkins wrote:

you are missing the big picture when you try to view this the way you are.

Rod, The thing that is most frustrating about all this, is that it seems no one has explained to me the big picture. (unless of course I have missed it) We seem to be running around in circles endlessly, yet no one has informed me as to why my perspective is not ideal, most have just stated that it is 'wrong' and have continued by stating they're perspective. STC ratings and the like mean nothing if you don't understand the big picture, and that is what I have been trying to establish.

Andy, Go to the IRC and begin studying the tests that have been conducted on varying types of wall systems - you'll get it. Rod

Thanks for explaining things Rod. As I said before, really apprrciate it. Ill have a good read over the next few days and then check back to confirm im on the right path :) Will let you know how I go, Cheers! Andy

Behind those false wall ceiling are the isolation assemblies consisting of MAM construction.

Rod, I am confused. Wouldn't this create a 3-leaf system? An additional false wall, in front of the MSM system, would create another leaf? Is that correct? Therefore, I have been told by members of this forum, that a 3rd leaf, or 3-leaf system, is actually worse than a 2-leaf. Or does the false wall not have any effect on the MSM system? If so, what materials would be used for the false wall? Thanks, Andy

andihopkins wrote:

Behind those false wall ceiling are the isolation assemblies consisting of MAM construction.

Rod, I am confused. Wouldn't this create a 3-leaf system? An additional false wall, in front of the MSM system, would create another leaf? Is that correct? Therefore, I have been told by members of this forum, that a 3rd leaf, or 3-leaf system, is actually worse than a 2-leaf. Or does the false wall not have any effect on the MSM system? If so, what materials would be used for the false wall? Thanks, Andy

Andy, typically I use a full frame assembly with my treatments inside the space plus rigid fiberglass at the face - and then cover the entire assembly with fabric. more often than not I will use wood stops to create a finished frame for the wall surface. In the case of the ceiling - I make frames = then use rigid fiberglass panels with fabric wrap so they are essentially just breathable ceiling tiles - except they are also doing what I need acoustically. I have had hidden treatment areas inside of rooms that are as much as 6' deep by almost the entire surface of the wall (in the rear of the rooms). No - breathable fabric faced walls and ceilings are NOT leafs...... in the same way that helmholtz trap are not. Soimetimes I do everything using a hybrid helmholtz trap for etire wall and ceiling assemblies in rooms...... The point is that thetreatments are in the room - and the double wall system is outside of the treatments...... Rod

Ok, that makes sense. I think I'm starting to understand the differences. One thing that I am still a little confused about however, is what dimension is now determining the rooms resonances? My unprofessional guess would still be the outer wall of the MSM. Or perhaps the MSM as a whole? Is the common construction of an MSM deep, and dense, enough to reflect the majority of low frequency, longer wavelengths? So, to reiterate... One could possibly place acoustic hangers between the MSM system, and the fabric faced walls? Cheers! Andy

Looks like I'm going to have to go further than referencing you Rod. Perhaps a dedicated 'thank you' section will suffice! Andy

andihopkins wrote:

Ok, that makes sense. I think I'm starting to understand the differences. One thing that I am still a little confused about however, is what dimension is now determining the rooms resonances? My unprofessional guess would still be the outer wall of the MSM. Or perhaps the MSM as a whole? Is the common construction of an MSM deep, and dense, enough to reflect the majority of low frequency, longer wavelengths?

Andy, Please visit the IRC site I placed the link to and examine the document for wood and steel framed walls......... Even the most simple wall - 2x4's @ 16" centers with 1/2" drywalleach face with celluose fiber insulation in the cavity will introduce enough isolation at low frequencies to cause issues with modal activity....... From IRC-IR 761 for that wall assembly: 50 Hz 21.5 dB 63 Hz 19.5 dB 80 Hz 21.8 dB 100 Hz 16.3 dB 125 Hz 12.5 dB 160 Hz 11.4 dB 200 Hz 22.6 dB 250 Hz 32.7 dB 315 Hz 36.7 dB 400 Hz 42.0 dB 500 Hz 44.2 dB 630 Hz 48.4 dB 800 Hz 53.4 dB 1000 Hz 56.1 dB 1250 Hz 57.3 dB 1600 Hz 57.4 dB 2000 Hz 55.4 dB 2500 Hz 45.5 dB 3150 Hz 43.0 dB 4000 Hz 47.4 dB 5000 Hz 53.0 dB 6300 Hz 57.4 dB

So, to reiterate... One could possibly place acoustic hangers between the MSM system, and the fabric faced walls?

Most certainly. Rod

Andy, In all fairness - when I find someone as insistent as you have been - I go hunting...... I did locate a study by the BBC where they experimented with room treatments INSIDE of the wall cavities of MAM wall systems. As a whole (primarily at lower frequencies) there was a boost in the TL values of the walls..... BUT - the the Tl value was still for the wall as a whole - and the innnermost face of the wall assembly would still define the modal activity within the space. Here is a link to that study if your wish to review the data: www.bbc.co.uk/rd/pubs/reports/1995-07.pdf BUT - this is NOT what Everest is discussing in MHoA. And, (after reading the report) it is not anything I would use for studio design purposes, Rod

typically I use a full frame assembly with my treatments inside the space plus rigid fiberglass at the face - and then cover the entire assembly with fabric.

Rod, I'm wondering... what if you wanted to employ a Controlled Image Design? Whereby the false walls act to redirect early reflections away from mix position, creating an RFZ? Is there a material that is reflective enough to redirect initial reflections, but still desist from becoming a 3rd leaf? Hmm... I wonder.. Cheers, Andy

andihopkins wrote:

typically I use a full frame assembly with my treatments inside the space plus rigid fiberglass at the face - and then cover the entire assembly with fabric.

Rod, I'm wondering... what if you wanted to employ a Controlled Image Design? Whereby the false walls act to redirect early reflections away from mix position, creating an RFZ? Is there a material that is reflective enough to redirect initial reflections, but still desist from becoming a 3rd leaf?

Andy, if that's the design you wish to employ - then you make use of the inner leaf to make it work. You have to understand that when I do as I speak of - it's by creating pockets in certain locations to hide the room treatments behind - I do not create an entire false room inside of the room. That would end up eating up too much real estate and also force one to begin adding back reverberance that the original room already had.........

OK, i see.. The image below... is that still an MSM system? Even with such variation in distance between the 1st and 2nd mass throughout the room. Wouldn't that result in varying TL throughout the room? If you wanted to incorporate both designs... would thin wood slats be an option?? That way, some of the high frequency early reflections are redirected, and the rest goes through and is slightly absorbed by the false wall, before hitting the MSM system? Or would it not be possible to incorporate both designs? Cheers, Andy

andihopkins wrote:

The image below... is that still an MSM system? Even with such variation in distance between the 1st and 2nd mass throughout the room.

Yes it is

Wouldn't that result in varying TL throughout the room?

Nope - the room as a whole will have a single TL value - and it will more often than not be controlled by flanking - not the MAM wall systems.......

If you wanted to incorporate both designs... would thin wood slats be an option?? That way, some of the high frequency early reflections are redirected, and the rest goes through and is slightly absorbed by the false wall, before hitting the MSM system?

I do not understand the question - if you want to treat the inside of the wall assembly with wood slatds - then do so - what does that have to do with the isolating assembly? Rod

Rod, I think Andy was asking if he can replace the inner leaf walls with slats. In other words, forget the sheetrock and just build the inner leaf with slats. I guess he means as a resonator of some sort. Andy, the answer is no. You can build a resonator next to the MAM wall, inside the room, but you cannot make holes in your MAM wall without drastically reducing the TL. - Stuart -

replace the inner leaf walls with slats

No, no Stuart. I have gotten past the understanding that the leafs must be air tight... a slotted leaf wouldn't work. My idea was, if you used your design for a false wall, but placed slats on the inside of the false wall, to create the CID, whilst also being able to install acoustic hangers behind the false wall. I was wondering wether a false wall similar to your design, with slats on the front to redirect high frequencies (RFZ) would create another leaf. But my guess, (judging from Stuart's reply) is that because the wall is slotted, it would not act as another leaf. Could this be an effective idea?? Andy

OK, Andy, it looks like I misunderstood you there! My bad. To me, it looks like you are trying to combine a slot resonator with a bass trap? The basic concept of "slots in front of a wall" is something that you will see on many designs on this forum, so yes, you can do that. It does not create another leaf. I've seen a discussion here from a few years back about whether or not a panel trap directly on the MAM wall creates a third leaf, but I don't recall anyone questioning slots. In any case, as Rod said: "... breathable fabric faced walls and ceilings are NOT leafs...... in the same way that helmholtz trap are not." And of course, slot resonators ARE helmholtz. So yes, slots are fine, and won't harm your MAM. I'm not certain about putting bass traps in there, though, for a combined resonator/trap. I guess it might work, to an extent, but I think you might be compromising something on both sides of the equation. AFAIK, in order to get predictable performance in the resonator you want uniform absorption, and putting hangers in there would not give you that. In addition, each section of the cavity in a resonator acts as if it were tuned to a specific frequency, but in your design you'd also have another "thing" in there that is tuned to a totally different frequency. So my guess is that you would broaden the Q of the resonator, flattening it to a very broad-band and lowering the peak, and also your hangers would not be running at maximum efficiency, since they would be inside a system that is tuned to a different frequency from what they are tuned. So the "loudest" sound where the hangers are would be sounds at a frequency other than the one they are tuned to. But the above is more hunch than calculation. I'm sure Rod, John, Andre, Ro, Ethan and others can give you the exact theory behind it, and explain what will really happen in there, but my off-the-top-of-my-head guess would be that it would work, but neither system would work well: both would be very broad band, reduced effectiveness. - Stuart -

Hi Stuart, Thanks the reply. And my apologies for our heated argument in the other post... I haven't had much sleep lately!

I'm not certain about putting bass traps in there, though, for a combined resonator/trap.

Doesn't this combination create a Membrane Absorber? However, in saying so, this is not the intention... Rod was saying in a previous post that he does this sometimes... that is, constructs a false wall to place acoustic treatment behind.

AFAIK

Haha... huh? Rod:

typically I use a full frame assembly with my treatments inside the space plus rigid fiberglass at the face - and then cover the entire assembly with fabric.

My guess is that placing slats on the front of a breathable false wall, such as above, will only reflect higher frequencies.

So my guess is that you would broaden the Q of the resonator, flattening it to a very broad-band and lowering the peak

Many of the materials I have read conclude that broadband bass traps are more efficient in the long run... as peak and nulls occur at all frequencies... not just at room resonances. Ethan Winer has a video where he discusses this: http://www.realtraps.com/video_wave.htm However, some of the materials also say that tuned traps are better. So I'm not 100% sure on that one... Might have to see what Rod says to get a better idea. Cheers, Andy