Wall Isolation Plan : regarding isolation of speaker baffles

Greetings all. I have a question regarding construction of speaker baffles and any isolation requirements. The space is in a detached garage and looks like this: Full details can be found in the Studio Design thread: viewtopic.php?f=1&t=11758&view=next The actual framing is a little different as I decided to double all the interior walls. Here is the floor plan (exterior shell not shown): The interior hard walls (i.e. not acoustically treated) will be isolated from the exterior walls using decoupling devices. I'm happy with this design as I feel it represents a good compromise between isolation and floor space. I started off making the entire flush-mount assembly, which includes the baffle and bass trap below, as rigid as possible, attaching it to the interior walls for further rigidity. But then I started wondering if it would be beneficial to decouple the assembly from the walls or would the benefit be minimal and hence not worth the expense, since the speakers will be isolated from the assembly itself and any vibration would be from sound within the room hitting the walls. In that case the assembly would be subjest to the same conditions as the rest of the walls in the mix room. Now for the question regarding the flush-mount speaker baffle: Should the entire assembly be decoupled from the interior hard walls or is it enough to isolate the speakers from the baffle and flush-mount framing assembly? Here is a view of the assembly:

I can't find any specific recommendations in any of my reference books but common sense and deductive reasoning suggests decoupling wouldn't be detrimental. Any thoughts? Perhaps I should ask the question in one of the other threads: Acoustics or Speakers.

but common sense and deductive reasoning suggests decoupling wouldn't be detrimental. Any thoughts?

I agree! Not that I found any more basis for it then you did, but it makes sense that you don't want your soffits coupled to your walls. I once asked John if soffits should be sealed airtight, and he answered that they should not, if that's any help. so mine will be free-standing, perhaps with some wool-and-cloth around the edges for aesthetics. Take that for what its worth, as I don't have any solid basis for saying that, though! - Stuart -

They can't be airtight if you want to cool inbuilt speaker amps. But they should be rigid! Glue and screw as they say :) I wouldn't decouple from the main wall. Just make it as strong as you can. BTW Sandersd - what is the font you are using in your pics - very nice :)

Unfortunately, I can't offer an informed opinion about whether or not to decouple from the hard wall , but I am curious about your baffle design. It isn't clear in the drawings, so I'd like to know how you plan to decouple your speakers from the baffle itself. From information I have found here and in other reading, I understand this a critical design concern with soffit mounted speakers. I like the idea of making the bottom cavity a bass trap, and it seems you have separated the two cavities, so to speak. But I don't see how you plan to mount the speakers for the purpose of decoupling them from the baffle. I like your design, so I'm hoping that you might share that information with the forum. Also, I see layers of what looks like drywall in that bottom cavity - what's that material? Are you intending to put that many layers in there? If so what are the benefits that you anticipate from this kind of layering. Thanks for posting.

John Sayers wrote:

BTW Sandersd - what is the font you are using in your pics - very nice :)

It's called PGTEXT and came with a program called The Blues Guitarist. It's by the folks that make Band In A Box. It has that hand-lettered look, doesn't it. They also have several other fonts with notation symbols, chord symbols, etc., that I occasionally find use for. You're right about the vents for the powered monitors. I only asked about the decoupling because I read somewhere that an extended baffle can introduce sympathetic resonances that would distort the sound, and I wanted to avoid that, although at some point such concerns over the minutiae of acoustics and the physics of sound become silly for me. I realize I'm not building a world-class studio in my 400 sq ft detached garage, just a comfortable area to practice and experiment with different sounds and techniques. Thanks for the reply.

11coco wrote:

Unfortunately, I can't offer an informed opinion about whether or not to decouple from the hard wall , but I am curious about your baffle design. It isn't clear in the drawings, so I'd like to know how you plan to decouple your speakers from the baffle itself. From information I have found here and in other reading, I understand this a critical design concern with soffit mounted speakers. I like the idea of making the bottom cavity a bass trap, and it seems you have separated the two cavities, so to speak. But I don't see how you plan to mount the speakers for the purpose of decoupling them from the baffle. I like your design, so I'm hoping that you might share that information with the forum. Also, I see layers of what looks like drywall in that bottom cavity - what's that material? Are you intending to put that many layers in there? If so what are the benefits that you anticipate from this kind of layering. Thanks for posting.

My intention is to build a shelf to attach to the bottom of the speakers using the mounting holes provided in the speaker cabinet. Then I will either suspend from the joists or float the speaker on its shelf with spring/neoprene isolation hangers or spring supports. The spring supports have the best isolation. I still have to model a detail that keeps the speaker in position while maintaining isolation from the baffle. I have a good idea how to do it, I just didn't want to spend time modeling it until I resolved the baffle-to-wall isolation issue. The stuff in the cavities that looks like drywall is drywall. I can't build a wall from concrete, so I thought I'd increase the mass by layering drywall between the studs to make, in effect, a solid wall. My understanding is that heavy and rigid is recommended for a baffle wall. When I finish working out the details and modeling the baffle structure I'll post it - probably this weekend. I want to build the entire structure and work out all the interconnecting details in the model before I start the interior construction. It really is the most efficient way to go, modeling the entire structure, addressing all the problems, working out the sequencing in the virtual world - and much cheaper. At my job, we model entire petroleum refineries before any construction begins. We actually build the entire plant: from the biggest piece of equipment to the smallest pipe fitting in 3d models. Construction goes a lot smoother and all those ideas you wish you'd had before you built such-and-such, you do!, and can incorporate them into the finished product. :yahoo:

At my job, we model entire petroleum refineries before any construction begins. We actually build the entire plant: from the biggest piece of equipment to the smallest pipe fitting in 3d models. Construction goes a lot smoother and all those ideas you wish you'd had before you built such-and-such, you do!, and can incorporate them into the finished product.

That explains the excellent, well thought out drawings that you have posted. All the detail is helpful and allows for precise interaction and detailed discussion here. I am also (with much less prowess) modeling my final ideas for a build that will begin momentarily :D . The speaker mounts are the final elements that I am working on so, I am locked in here. I appreciate that you are willing to take the time to answer questions while you look for answers. Before seeing your design, I had planned to build a tubular concrete support for each speaker that went through the soffit structure to the concrete floor decoupled entirely from the frame and baffle. What are your thoughts on this vs. spring/neoprene decoupling in your design? Also, with regard to your design, Will the increased mass that you are adding to the bottom cavity, or bass trap actually reduce the resonant qualities of the enclosure? My understanding is that the resonating structure is part of the mechanism which creates heat that is then dispersed/absorbed by absorptive material like rigid fiberglass insulation or other. So then, does this level of increased mass, in your application, actually reduce the effectiveness of the structure to do what you intend? i.e., absorb/trap bass frequencies. Similarly does your baffle's framing structure have the same effect of damping the resonance of the actual baffle in your design? Is it attached at all points on that frame or not? Most of the designs I've seen have a larger frame for the baffle allowing it to resonate, with the caveat that the thicker the resonating baffle, is the lower the frequency that can potentially be absorbed. :blah: Ok, I'll cut it here - I may be getting too eager... but It's because I am truly curious. Thanks

Similarly does your baffle's framing structure have the same effect of damping the resonance of the actual baffle in your design? Is it attached at all points on that frame or not? Most of the designs I've seen have a larger frame for the baffle allowing it to resonate, with the caveat that the thicker the resonating baffle, is the lower the frequency that can potentially be absorbed.

I don't think that you want your soffit baffle to resonate at all, if you can help it! The concept is based on the way speakers used to be tested and their response measured: they were literally buried in the ground, with the entire speaker below the surface, all except for front face, which ended up flush with the surface of the ground, pointing straight up at the sky. That creates about as perfect an infinite baffle as you can get, forcing the speaker to radiate perfectly into half-space, for accurate measurements. Done like that, the "soffit" has effectively infinite surface area (the entire planet!), effectively infinite mass (the entire planet!) and effectively infinite damping (one again, the entire planet). It's hard to get a better baffle than that! And I can guarantee you that such a baffle does NOT resonate... So that's the concept of the flush mount soffit baffle: It needs to be as massive as possible in order to emulate a theoretically perfect infinite baffle as closely as possible. It needs to be as large as possible too, within reason, (remember, "infinite" is part of the title!), so your speaker should be positioned as close to the center of the soffit as you can get it. But the key is a large, flat, smooth and very massive baffle. Mine will be concrete, since that covers all of the above, and is also cost effective and relatively thin. Others here have used MDF, HDF, plywood, OSB, etc. As long as you make it massive, any rigid and dense building material should work. John Sayer's soffit design makes use of the lower part of the baffle for bass trapping, and my guess is that that part will also not be resonant. He uses hangers inside the cavity as his bass trapping elements, not so much the structure itself, from what I can figure out of the design with my limited understanding. It's his own proprietary design, as far as I know, so he can explain the theory behind it much better than I can, but then again he does design and build world-class studios around the entire planet for a living, which means that his proprietary designs are his livelihood, so it would be entirely understandable if he chooses to keep a few secretes up his sleeve, and not reveal absolutely everything that he knows on this forum! - Stuart -

Now i`m very confused :? In another thread Ro told me that it is better to have a heavy but NOT rigid wall to avoid resonating of the wall. he says a rigid wall resonates more. You can read his points here: viewtopic.php?f=1&t=11791 So what is the truth????????? :roll: Rigid or not???? cheers Mika

The thing is; the most dense materials are rigid. Exceptions are Lead for example. The stifness/mass factor depends on the resonance frequency of the material. Below that frq is a stiffness-controlled region while above that frq, the mass law steps in. You can imagen a very dense material being very stiff. Brick for example. The resonance frq of a heavy brick is much lower than a gypsum board. You want the resonance as low as posible. Like Stu's doing with his soffit wall; make it concrete. There's heavy mass in concrete, but it's also very rigid. Comes with the material. The resonance of concrete is already very low, so the stiffness doesn't play a big role (any more). If the resonance of the material is high enough to be annoying (aka: you can actually hear it buzz) you want it less stiff. Hard material resonates better than soft. confusing, yeah. I sometimes don't understand neither.

11coco wrote:

At my job, we model entire petroleum refineries before any construction begins. We actually build the entire plant: from the biggest piece of equipment to the smallest pipe fitting in 3d models. Construction goes a lot smoother and all those ideas you wish you'd had before you built such-and-such, you do!, and can incorporate them into the finished product.

That explains the excellent, well thought out drawings that you have posted. All the detail is helpful and allows for precise interaction and detailed discussion here. I am also (with much less prowess) modeling my final ideas for a build that will begin momentarily :D . The speaker mounts are the final elements that I am working on so, I am locked in here. I appreciate that you are willing to take the time to answer questions while you look for answers. Before seeing your design, I had planned to build a tubular concrete support for each speaker that went through the soffit structure to the concrete floor decoupled entirely from the frame and baffle. What are your thoughts on this vs. spring/neoprene decoupling in your design? Also, with regard to your design, Will the increased mass that you are adding to the bottom cavity, or bass trap actually reduce the resonant qualities of the enclosure? My understanding is that the resonating structure is part of the mechanism which creates heat that is then dispersed/absorbed by absorptive material like rigid fiberglass insulation or other. So then, does this level of increased mass, in your application, actually reduce the effectiveness of the structure to do what you intend? i.e., absorb/trap bass frequencies. Similarly does your baffle's framing structure have the same effect of damping the resonance of the actual baffle in your design? Is it attached at all points on that frame or not? Most of the designs I've seen have a larger frame for the baffle allowing it to resonate, with the caveat that the thicker the resonating baffle, is the lower the frequency that can potentially be absorbed. :blah: Ok, I'll cut it here - I may be getting too eager... but It's because I am truly curious. Thanks

Excellent questions all. These are exactly the kind of responses to posts I find are the most helpful and productive, because I have to think about what is being done and why and hopefully find support for the methods used. I intend to answer all your concerns, but I'm at work so give me a day or so - plus I need to research my supporting documentation for my response so you can evaluate it for yourself. Thanks again for your input.

In another thread Ro told me that it is better to have a heavy but NOT rigid wall to avoid resonating of the wall.

Exactly! The key point being "to avoid resonating". My reasoning goes like this: Limp objects (non-rigid) don't resonate very well: take the rubber floor mat out of your car, hold it up by one corner, and try to make it resonate by tapping it with a hammer: Compare that with tapping the same size piece of steel plate with a hammer.... So non-rigid is good, if you can get it. But mass is also important in "resisting" resonance. The resonant frequency of most objects is inversely proportional to the density (AFAIK!). So the more dense it is, the lower the resonant frequency is. You want that as low as you can possibly get it, so denser materials are better for soffit panels than lighter ones. It's also harder to make a very massive object vibrate, then it is to make a less massive object vibrate. So the more mass you have in your soffit panel, the more energy is need to make it move. The problem, as Ro pointed out, is that it's hard to find materials that are both non-rigid and also dense. Lead is a good one, but it's not cheap at all. (I guess thick gold foil would also be good, but it's just slightly outside of my budget range, so I think I'll skip on that one... :) ). Then there is MLV, but the opinion of most expert here on MLV is widely known... :) Apart from that, I can't really think of anything else that is both very dense and not rigid at the same time. So, since mass is more important than non-rigidity for soffit panels, heavy materials are better than limp ones. Just go with the highest density you can afford, and it should do the trick. - Stuart -

So, since mass is more important than non-rigidity for soffit panels, heavy materials are better than limp ones. Just go with the highest density you can afford, and it should do the trick.

I can't disagree with you Stuart, but My curiosity is activated by some prior reading that speak of bass trapping in such a way as here on Ethan Winer's Website. http://www.ethanwiner.com/basstrap.html Where he says:

you'll get much better results using resonating boxes designed specifically to absorb low frequency energy. These boxes are called bass traps, and they absorb the lowest frequencies where fiberglass and foam stop working. The bass traps I have found most effective are built from plywood panels, and designed to vibrate over a broad range of bass frequencies. Fiberglass is mounted behind the panels to damp the vibration, thus absorbing the bass energy from the room.

It makes sense to me that when you allow for some resonance which you then damp with fiberglass or rockwool or something else, for in order to "trap" bass. I have a keen interest in seeing where this thread leads. I like the design, so I am interested in hearing out the reasoning behind it, and the discussion that might follow.

but My curiosity is activated by some prior reading that speak of bass trapping in such a way as here on Ethan Winer's Website

Well, yes, what Ethan said is, of course, absolutely correct: a panel trap (or "membrane trap") is also a very good way of trapping bass. But that's not a soffit. That's not what John is doing. A panel trap uses a different design principle from what John is doing with his "bass trap in a soffit", and it must have a sealed cavity in order to work. John's design does not use a sealed cavity. I reckon that Ethan would agree that combing a speaker soffit with a panel trap is not a good idea, and wouldn't work either way: It would make a lousy panel trap (since it isn't sealed) and also a lousy soffit (since it would resonate). Panel traps are one thing, based on one principle of acoustics, and speaker soffits are a totally different thing. Soffit panels should not be part of a panel trap. You don't want the soffit panel to absorb any frequencies, nor to resonate at any frequency, as that would certainly mess with the speaker's radiation. You want it to be neutral, neither absorbing nor resonating. What John does with his design, is to mix bass hangers with a soffit, and that makes sense. The lower part of the soffit panel (which john uses as a bass trap) is not a panel trap. It can't be, since it is not airtight! It is just a convenient place to put hangers. In John's design, the hangers absorb the bass, not the soffit panel. The the soffit is just a useful piece of real estate that John makes use of, but it isn't a panel trap, and should not resonate. John's desing might also act as a Helmholtz resonator (cavity with a "tube"), but my bet is that John designs it so that the resonant frequency is way down below the audible range. At least, that's the way I see. I may be wrong: it wouldn't be the first time... I hope John notices this discussion, and chips in with a confirmation (or if not, then a flame-thrower, to blow me out of the sky!). - Stuart -

What John does with his design, is to mix bass hangers with a soffit, and that makes sense.

Yes, it's clear that there are different methods to absorb the bass frequencies, and I think I read here that it is the fact that they resonate which makes hangers work as effective bass absorption. It is, in fact, John's expertise that brought me to this site and his soffit design is the one that I've been trying to model for my own build. I do, however, want to see where Sandersd is headed because his process has captured my attention. And I hope he'll share what he comes up with.

Soundman2020 wrote:

But mass is also important in "resisting" resonance. The resonant frequency of most objects is inversely proportional to the density (AFAIK!). So the more dense it is, the lower the resonant frequency is. You want that as low as you can possibly get it, so denser materials are better for soffit panels than lighter ones.

Yes, it depends on DENSITY AND VOLUME (thickness) of the material. As for absorption, TL is frequency dependent. Typical partitions have TL values that increase with increasing frequency with the exception of a dip in TL around a bending wave resonance frequency, known as the critical frequency. That effect is known as the coincidence dip. (as Rod also explained in his book). Critical frequencies generally decrease at the same rate as the material thickness increases. So, a doubling in thickness translates to cutting the critical frequency in half for the same material. Coincidence effects can be minimized by using multilayered partitions with different materials and different material thicknesses (having different critical frqs) stacked into a single partition. (assuming they are all connected as one leaf ofcourse) As I said earlier, heavy (dense) material have lower cr.frq. For example: The CF (critical frq) for 8cm Concrete is 100Hz (so doubling the concrete to 16cm will result in 50Hz!) 12mm GB has a CF of 3000Hz. Therefore, a 3x12mm (3,6cm) GB partition will have a CF of about 1000Hz. Making it as thick as the 8cm concrete example will result in a CF of 465Hz. for a 6mm Glass piece the CF will be 2450Hz. Note that the density of glass is about the same as concrete. So a soffit wall made out of glass would be a good idea, besides looking cool :) (remember that particular studio that has glass soffits? It's been discussed here on the forum before) The wall resonaters (bass traps) as mention on Ethan's website, take advantage of the CF. You tune them to absorb that frequency.

Soundman2020 wrote:

What John does with his design, is to mix bass hangers with a soffit, and that makes sense. The lower part of the soffit panel (which john uses as a bass trap) is not a panel trap. It can't be, since it is not airtight! It is just a convenient place to put hangers. In John's design, the hangers absorb the bass, not the soffit panel. The the soffit is just a useful piece of real estate that John makes use of, but it isn't a panel trap, and should not resonate. John's desing might also act as a Helmholtz resonator (cavity with a "tube"), but my bet is that John designs it so that the resonant frequency is way down below the audible range. - Stuart -

Actually it is my intention to make the lower trap sealed airtight, similar to a slat resonator, the only opening being at the bottom. Kind of a combination bass trap and resonator. I did some research on the speaker isolation from the structure and found a device that is interesting: IsoRock http://store.fastcommerce.com/prod_Brig ... b3f1a.html Apparently it is a speaker platform composed of multiple layers of materials separated with a polymer similar to Green Glue. Perhaps I can build a shelf of multiple layers of MDF with GG in between and support it on spring isolators. Another idea I'm also considering is hanging a box filled with sand and a 2'x2' concrete pad on which the speaker would sit. I didn't have a chance to work on the model this weekend. I'm finishing up a project in the house that I started long, long, long ago and it took all my time.

Actually it is my intention to make the lower trap sealed airtight, similar to a slat resonator, the only opening being at the bottom. Kind of a combination bass trap and resonator.

I'm not sure that that's a good idea at all. If it is sealed and there is only one opening, then it will basically be a Helmholtz resonator, tuned to a single frequency. What frequency do you plan to tune it to? But the biggest risk I see is that it will probably make your entire soffit assembly resonate at that frequency, including the speaker soffit panel. I can't see that as being a good idea.

Soundman2020 wrote:

I'm not sure that that's a good idea at all. If it is sealed and there is only one opening, then it will basically be a Helmholtz resonator, tuned to a single frequency. What frequency do you plan to tune it to? But the biggest risk I see is that it will probably make your entire soffit assembly resonate at that frequency, including the speaker soffit panel. I can't see that as being a good idea.

Excellent observation Stuart. As you pointed out in the design thread, predicting the response of an odd shaped room is difficult if not impossible. I have to provide myself with options. I will not know what frequency to tune to until I test the room. Then I will have the option of utilizing it as a simple bass trap w/ hangers or tune it as required. As far as the resonation is concerned, there are many options for eliminating any transmission to other parts of the structure, so I'm not too concerned about that, but it is something I have to make allowances for. As you pointed out, resonating the baffle would be bad, very bad. Thanks for your thoughtful input. Having someone point out problem areas is what the design phase is all about. Keep 'em coming.