Ceramic tile stiffening cabinet walls

[joecarrow]

Hi Guys,

I've been reading about different cabinet construction techniques, and I came up with something that might be of interest around here. I'm not the first person to come up with this, but I would like to bring up the idea of ceramic floor tile as a stiffening liner in cabinets.

I did some back of the envelope calculations (consulting my old mechanics of materials textbook), and I came up with the following:

If you attach a 3/4 inch piece of plywood or MDF with Young's modulus E=10 GPa to a 1/4 inch thick ceramic tile with E=60 GPa using an adhesive (whose influence we will ignore for now), the neutral axis for pure bending is approximately 0.292 inches from the outermost face of the ceramic tile. This leaves about 0.7 inches of wood on the other side of the neutral axis, telling us that the stiffness of this composite is equivalent to about 1.40 inches of solid wood material. If you keep in mind that the ceramic is stiffer than cement, and held at a distance from the neutral axis, it makes sense that this is an effective stiffener.

It may be an oversimplification to say that gluing a floor tile to a 3/4 inch sheet of wood gives the equivalent of nearly 1.5" of wood- but I think that's mostly because of the influence of the adhesive layer. The layer between the wood and tile will be mostly in shear, bringing about some viscoelastic damping if you use an adhesive that stays flexible.

Credit where credit is due, I have a few links to places I've been reading about this stuff:

Bob Brines uses a technique with a material called "hardi-backer". I read about this a long time ago, and I think it operates on this principle. One point, however- most resources I saw report E for ceramic to be in the 50-80 GPa range, and cement around 30 GPa.

This article on Audioholics talks about damping in panels and constrained layer damping. Reading it suggests to me that this method of cabinet stiffening could benefit from a glue that sets kind of rubbery for some viscoelastic damping. It also confirms my suspicion that the ACH-01 accelerometer is a good candidate for investigating cabinet vibrations.

Also, can't forget this thread: http://htguide.com/forum/showthread.php?t=16564&highlight=cabinet+vibratio n about cabinet vibration measurement. I hope to validate my final results of all this in a similar way.

Does anybody have any other interesting links, articles, or opinions? Did I make some bad assumptions? Let's hear it, I think it will be cool if we can substantially stiffen cabinets with floor tiles! 8)

PS- GrahamT, doesn't your Extreme MT cabinet use cement? How'd that turn out?

[posix]

in that case why not lay up some carbon fibre epoxy on the wooden panels and get the ultimate in stiffnes?

but I am not sure whether it's the stiffness or inertness and tautness that's the goal. if it's simply stiffness there are FAR FAR stiffer construction techniques out there than just a simple box. how about a proper space frame for some extra stiff bracing. but then again, not sure whether that's what's required for a good sound or whether only an inert structure that dampens vibrations in which case your tiles will ring like church bells.

[Dennis H]

Joe, I think your calcs will only be valid if you can cover the whole panel with a single tile. Small tiles with grout joints should make it more flexible.

[joecarrow]

Posix

Posix- great point! A quick check shows that you can get carbon fiber with E in the neighborhood of 500+ (compared to tile's 50 to 80)- though only in tension. I didn't consider this because floor tile seems to be cheap and readily available. If you wanted to make your own laminate, McMaster has a carbon fiber cloth for a good price.... though its physical properties aren't listed. Structural carbon fiber is MUCH more expensive- like $100/sq ft 8O

Space frames- that sounds to me like B&W's matrix construction technique. You raise some very good points with these, and I share your concern that the overly stiff cabinet could ring. My hope is that if it is stiffened enough, the main resonances will be raised to a frequency that is not excited by the passband of the driver, or at least is a frequency well damped by foam and fiber

My goal here is not to reach the "ultimate" in cabinet construction (not yet ), just to get another cheap trick with a good bang/buck ratio. Thanks for the comment.

And Dennis- you're absolutely right. I picture this as working best with foot square tiles filling the space between braces in a large cabinet, or one large tile covering the inside of something like a Modula MT.

[posix]

Another point about CF is that I once spoke to this turntable designer and commented on a CF arm that someone is producing. He wasn't too keen on that and explained about unwanted resonances there because basically it was too stiff and said that he prefers to stick to brass. From that point on, like Pavlovs dog I think "stiff" and I think "not necesarilly good for audio".

[Gargoyle]

IMO you made 2 bad assumptions, Joe C. You cannot presume infinitely stiff adhesive joint (for neutral axe) and elastic joint for damping in the same time. Furthermore, the mdf is not transversely homogenous - it has weaker core and harder surfaces. For second, if take into account Hook's law and the vibrations like a forces which are bending "sandwich" in all possible directions you can easily see that internal damping characteristic of the "sandwich" will approach the tile. How much close is beyond my knowledge for the moment.
Problem with effective connection of two "very" different materials, especially with only surface connection is hard to solve.

[joecarrow]

Hi Gargoyle- would you say that your main point is that my assumptions oversimplify the idea, reducing the usefulness of any analysis?

Also, I thought I implied that if the assumption of a perfect adhesive bond doesn't work out, then the viscoelastic joint area becomes more likely to have an effect. Maybe I wasn't very clear with that. I'm not sure if I understand the rest of your message.

I think this thread has probably had about enough conjecture about what *might* happen if you do something- what do you say we go cut some wood?

Here's what I propose for my experiment:

1) Cut several sheets of Plywood into the right size to act as a side panel for a Modula MT, about 17" x 12.5".
2) If Home Depot will cut some MDF to this size for me cheaply, I'll try to grab some to have some application for the rest of you.
3) Build a frame to hang one of these panels loosely from some rubber bands.
4) Devise a striker to deliver a repeatable "knuckle rap" to set the panel vibrating. This might be an object falling from a fixed height.
5) Record the sound each panel makes with my ECM8000 measurement microphone and computer sound card.

The panels will include at least the following:
a) Just 3/4" plywood
b) Two layers of 3/4" plywood glued together with yellow wood glue
c) One layer of 3/4" plywood with ceramic floor tile attached with some two part epoxy from my toolbox.
d) Just the tile- just for the heck of it.

Alternately, there may be some variation of MDF and/or the use of different adhesives- depending on my whim and people's suggestions.

Measurement method:

i) set them all up, ready to go
ii) record with ECM8000 mic, close miked, never touching the volume knob
iii) analyze recording for relative SPL and spectral content

That's all for now. I'll try to stop BSing you all, and post some photos and graphs.

[capslock]

Exactly that kind of experiment has been performed by Pico, but I can't find his paper at www.picosound.de.

However, he gives a good summary of what to do:

Construction of enclosures:

There are 2 principal strategies: stiff or damped.
In general chipboard or MDF-board (if painting is desired) can be recommended as building material. Depending on enclosure volume and application the material thickness should vary between 13 and 25mm. An estimation is:

Plate thickness [mm] = SQUARE ROOT (longest plate dimension [mm])

For a sub woofer or a 3 way system with a cross-over frequency to midrange of less than 500 Hz the behavior may be stiffened by (possible several complementary) ring shaped braces (massive plates with at least 50% opening share and about half material thickness). This ensures that the plate resonances are shifted clearly above the operating range of the bass speaker and therefore are barely excited.
Unfortunately this does not work for in general for 2 way systems with cross-over frequencies of 2000 to 2500 Hz (exception: B&W matrix enclosures). In these cases the following construction is recommended:

outer plate 16 to 19mm MDF,
intermediate layer 5 to 10mm bitumous material (e.g. roof sealing)
inner plate 4mm plywood
All 3 layers must be glued/welded together on the whole contact area so that the unavoidable resonance of the plates can be damped out effectively. Ring braces between the outer panels are not desired here as they would cancel out the damping effect!

[capslock]

Basically, you want to do stiff/rigid for sub enclosures in order to move resonances above the useful range. For mid, you are more interested in damping (cf. Siegfried Linkwitz' paper from the 70s). For midbass, pick your poison...

[Gargoyle]

Sorry for unclear words, english is not my mother tongue. Hook's law tells that tension is proportional to deformation time Young modulus. In our case two materials are used one very stiff and one weak. From this point of view, the majority of bending forces will be carried out throught the stiffer structure because the weaker will be less deformed. The more elastic glue is used the less the MDF is in game.

Few years ago (2000-02?) there were a long and exhaustive article in Axpress written by Tim Moriatsu(?) about various kind of enclosures and their dampenings. I remember few thing from it: In real world it is only possible limitedly move with box resonances up and down, limitedly change the Q of resonances and limitedly lower the magnitude of resonances, say -6dB. All measurements were made with some bass loudspeaker. All the hard-soft-hard materials falls short - the resonances only moved and changed their magnitude between. Overall was the same - only another "colouring". Not only for me it is nearly impossible acuratelly predict behaviour of layered structures. Must be measured.

So, the particular enclosures and strategies as capslock wrote seems to me as a meaningful. As T.M. wrote, heavily braced and strong structures together with driver decoupling partially help, for the bass at least.
New problem arise - how to efficiently decouple each other?

[joecarrow]

Gargoyle,

It's OK, I've worked with many people who spoke english as a second or third language. I understand better now that you have rephrased yourself. Thanks for the effort.

I think what I got out of all of this is that the constrained layer damping technique was shown to be more effective than the technique of using hard and soft materials together to create stiff composite layers.

I finally found a better link to the topic, on Art Ludwig's website:

http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damping_experiments

I think that my final decision is to use a thin layer of stiff material, attached to the insides of the enclosure with a flexible silicone caulk. Aside from this constrained layer, I will also use foam to line the walls, and a few handfulls of fibrous stuffing behind the midwoofer but far from the port.

Thanks again, all, for the comments.

[Martyn]

Here's a vintage research report published by the BBC in 1973! It seems that then, much as now, plywood with roofing felt glued to the insides is hard to beat.

Enjoy! 1977-03.pdf

[joecarrow]

Thanks a ton! This sort of report would normally be really hard for me to come by, and is exactly what I'm interested in. It should make for a good read.

[capslock]

Found the page with various sandwiches, waterfall and WAV files to listen to:

http://www.picosound.de/D_gehmat.htm

[capslock]

Just scroll to the table pretty far down and listen (or open the ppt presentation).

Spanplatte = particle board
Arbeitsplatte = kitchen top board
Kleber = glue (this was a damping glue)
Bitumen = self-adhesive damping boards (but he says roofing felt works just as well)
Fliese = ceramic tile
Weichfaserplatte = low density fiber board

[Dennis H]

Interesting, Eric! It looks like a sandwich of bitumen (roofing felt) between layers of wood is the winner, confirming Art Ludwig's testing.

[JonW]

Yeah, excellent link. Umm, at least I think so (Ich glaube yah?), my German is really rusty.

I'm impressed at how much of a difference you can notice just from listening to the .wav files- it's amazingly different.

What's Sperrholz? Maybe it looks like the best curve is the blue one which, I think, is 16 mm of MDF, then some roofing felt, then 4 mm of Sperrholz? Or is it 16 mm of MDF, then some roofing felt, then 4 mm of Spanplatte (particle board)? Heck, what is "particle board" in the US- the same as masonite/hard board? And how was this sandwhich made? Was glue used between the layers? And how many layers of roofing felt?

I'm hoping to start building my first speakers soon, so this is excellent info. :T

[posix]

sperrholz - plywood

[Martyn]

Particle board is like a low grade MDF - actually it's more like the stuff that cheap kitchen cabinets are made of. MDF is much better, and plywood (especially baltic birch) is marginally better than MDF but is more difficult to cut and finish.

I don't think that roofing felt has reached the level of an exact science yet, but half a dozen layers glued or even stapled to the interior seems to be adequate from what I have read.

It's easy to get carried away with the concepts of constrained layers or sandwiching roofing felt between layers of MDF, but it's probably not worth the effort unless you're an engineer who understands the mechanics and dynamics of vibration, absorption, damping, transmission loss, and so on. Furthermore, knocking up a flat test panel is one thing; reproducing its performance in a three dimensional enclosure is something else.

I'm no expert in enclosure design, but I am a mechanical engineer (even though it's quite a while since I've been in practice) and a firm believer in the KISS principle. Thus I offer the following rules of thumb:

* don't worry about the tweeter enclosure because it's probably already enclosed!
* do everything to the mid enclosure that you do to the woofer enclosure, only less so.
* make the woofer enclosure out of the thickest MDF you can find, to that the extent that you can still lift the finished product.
* brace all interior surfaces to stiffen them. The smaller the sizes of the panels between braces, the higher their resonant frequencies. The higher the resonant frequency, the easier it is to damp the vibration.
* avoid parallel internal surfaces if you can, especially along the longest dimension (usually top to bottom in a floor-standing speaker), to reduce the occurence of standing waves. You can do this by including a sloping "false bottom", i.e. a solid diagonal brace, but remember to compensate for the lost volume when calculating the total volume of the enclosure.
* contrive the panel and internal dimensions to be different from each other to minimise the probability of resonances and standing waves occuring at the same frequencies.
* line the internal surfaces with your choice of roofing felt, proprietary foam sandwich product, or whatever. The viscoelastic damping sheet designed for use inside car body panels should work quite well. Add convoluted foam for absorption. Pay particular attention to the sides and back of the enclosure behind the woofer.

Simplest of all, if this is your first forray into building speaker enclosures, find a good kit or proven design that meets your needs and follow it faithfully. Once you've got it working you can design imaginative new boxes of similar volume, re-use your drivers and crossovers, and then compare the result. You can, and quite possibly will, do this an infinite number of times!

[capslock]

I don't think that roofing felt has reached the level of an exact science yet, but half a dozen layers glued or even stapled to the interior seems to be adequate from what I have read.

Here, roofing felt comes in at least two varieties, heavily sanded and glass felt reinforced for top layers and lightly sanded and reinforced for the waterproof layer below (and it is certified for interior use so outgassing / odor is not a problem). I have used this indoor variety. One side is unsanded and protected by a thin foil. A gas burner will simply melt the foil, so you can stick it to MDF, or you can peel off the foil (not easy, depending on grade) and then use a hot air gun. Sanded side to MDF will stick well with white PVA glue, silicone or semi-flexible tile cement.

I have also done it the other way around, heating tiles in an oven and then pressing them onto the peeled side of the felt. These bitumen coated tiles can then be glued to the insides of a speaker enclosure.

Either way around, it is not too much trouble.

[joecarrow]

The roofing felt I saw at Home Depot does not have any backing, and there is not a sanded side. Both sides are the same- it's what I've always called "tar paper". It's the stuff you put between the plywood and the shingles on a roof. Is this what we're all talking about? It's not very sticky at room temperature.

[Martyn]

That's the stuff. Make sure it's the heavy grade (I think it's 30 lb).

[capslock]

Grades may be different in NA, but to me tar paper is a lot of fiber/paper with a little bit of impregnation. The roofing felt I'm talking about is lots of bitumen with a little fiber - it is meant to be "welded" onto the plywood and one sheet overlapping the next. You can cover it with shingles or with heavily sanded felt.

[capslock]

http://www.hille-dachbaustoffe.de/index.php?id=10

http://de.wikipedia.org/wiki/Bitumen

http://de.wikipedia.org/wiki/Bitumen-Schwei%C3%9Fbahn

[Dennis H]

Grades may be different in NA, but to me tar paper is a lot of fiber/paper with a little bit of impregnation.

That sounds right although there's really quite a bit of tar in it -- nasty on saw blades if you're remodelling. It's dirt cheap and it apparently works quite well as a constrained layer. Art Ludwig found it outperformed an expensive vinyl product that was marketed as an acoustic damping material. He used two layers of 30 pound -- just guessing, maybe 1/8" or 3mm total.

http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damp ing_experiments

[JonW]

Great discussion, fellas. I'm going to make some of the Modula MT speakers (if I can ever find the time...). They're small speakers and probably won't benefit from dampening as much as floorstanders, but it should be fun to experiment with.

If I were to try gluing layers of roofing felt and masonite to the inside of the cabinets, what do you folks think would be the best way to attach everything? The outside will be 3/4" baltic birch ply, then a few roofing felt layers, then the masonite. Screws? Some sort of glue? I don't want it coming apart a couple years down the road.

Dickason's nice book says just staple the roofing felt onto the sides, I think, with just 5 staples- one in each corner and one in the middle. No masonite layer was mentioned, though.

[capslock]

Glue by all means! Everything else will either be ineffective or even begin to rattle.

[JonW]

Glue by all means! Everything else will either be ineffective or even begin to rattle.

OK, thanks. So what's a good type of glue to use? One that will last years. And cover all sides with the glue or just put in certain places (e.g., around the perimeter)?

[joecarrow]

From everything I know about how this is supposed to work, I say cover the whole surface with glue.

What I do not know is what kind of glue will stick to bitumen! Capslock sounds like he's saying it's really sticky on its own, and others seem to agree with the tar-paper stuff I saw at Home Depot.

Right now I'm thinking that a spray-on adhesive is the way to go. Maybe 3M 90-strength; I've seen it recommended before.

[Hdale85]

That would probably work, tar paper is rather sticky although somewhat heavy. But still i think it would work with just some good spray adhesive just let it tack up a bit before you stick the paper on and make sure you get a good even coat of the adhesive.

[JonW]

Hmmm... spray adhesive gets all over the place. When you're working inside of a speaker cabinet, it would be tough to keep it confined. I've worked with it once before, for sticking poly batting to the inside of my sub. But I have no idea if it lasts well. And would it hold a piece of masonatie to tar paper and then to wood? Not sure. I'd think a viscous gel or liquid (e.g., caulk, white or carpenter's glue, rubber cement, hot glue, etc.) would maybe be a better way to go?

[Hdale85]

Well i was just talking about holding the tar paper to the inside of the cabinet. Its a lot stronger then youd think i used it for building my screen for my projector with the felt edge and its very strong. But then again thats cloth so i dont know?

[Dennis H]

Art didn't use glue, just screws around the perimeter. Screws in the center decreased performance.

[Martyn]

I wouldn't line the inside with Masonite - this is just going to make the interior reverberant again. I don't know which would be better - glueing the felt or using mechanical fasteners. I would expect glueing to have a vibration damping effect on the plywood (as long as the glue isn't too elasatic), but keeping them loosely attached might have a decoupling effect. Maybe glue a few layers and then mechanically fasten a few? (a few wood screws through fender washers should do it...

[Dennis H]

I wouldn't line the inside with Masonite - this is just going to make the interior reverberant again.

The Masonite, plywood or whatever is an essential part of the constrained layer construction. After it's in place, you need to add absorbing material to the inside -- foam, fiberglass or whatever to damp the midrange and keep it from exciting the walls as well as radiating through the cone. The whole thing, constrained layer plus stuffing, works as a system and the performance isn't as good if you leave any part of it out. Art covers all that in his summary and the results show in his measurement graphs.

Edit: Of course you can always cheat and buy Black Hole or Whispermat which combines both functions.

[joecarrow]

Martyn,

It is key to remember that there are two types of resonance-

First, there is the resonance of the air inside the cabinet. This is what they call "standing waves", and it would exist in a perfectly dead, perfectly stiff cabinet. Likewise, sound at frequencies higher than what would be considered a standing wave can bounce off the inside of the enclosure and re-radiate through the cone. I think that there is a consensus that the best way to treat this resonance is by paying attention to cabinet dimensions, lining with a sound absorber such as foam, and stuffing with fibrous material such as dacron or fiberglass.

The effect of the first type of resonance is present in the sound coming from the speaker cone itself.

Second, there is the effect of sympathetic vibration from the cabinet walls, where sound is radiated by movement of the cabinet walls. This resonance is excited by internal air pressure fluctuation (sound pressure), or the reaction force of the vibrating driver.

I'm sure you understand this, but your comment that Masonite would make the interior reverberant again caused me some concern that the conversation could veer off topic. Obviously, whatever treatment we do to prevent cabinet wall resonance will need to be paired with a treatment for internal air resonance.

On a final note, I wanted to mention something that's been on my mind lately.

Even with constrained layer damping, there is still a need for stiffness. Without resonance, the internal air pressure will still cause a deflection and you would hear an external sound from it. This is the 'forced response', and even though the amplitude would be lower than a resonance, it would be objectionable because it will be out of phase with the driver output. I don't think that's any news to anyone- it just says that the old rule of thumb for sub building "build it thick, brace like crazy" is well justified without talking about resonance.

[capslock]

From everything I know about how this is supposed to work, I say cover the whole surface with glue.

What I do not know is what kind of glue will stick to bitumen! Capslock sounds like he's saying it's really sticky on its own, and others seem to agree with the tar-paper stuff I saw at Home Depot.

Right now I'm thinking that a spray-on adhesive is the way to go. Maybe 3M 90-strength; I've seen it recommended before.

The stuff with lots of bitumen and little fiber will be sticky on its own -- if heated properly!

Tar paper, i.e. lots of paper and little tar (nasty stuff, by the way) or bitumen enhanced with some other polymer will probably not stick if heated.



Anyway, if you don't want to heat the bitumen, solvents or solvent based paints may work. Other than that, organic solvent or water based bitumen paste, acrylic paste, silicone paste and epoxy will work. If you talk about glueing tar paper or sanded bitumen mats to MDF or unglazed tile and you have chance of pressing the stuff, PVA (wood glue) will also work.

[Martyn]

Dennis, what Art has done in his tests isn't constrained layer damping. According to his description, he uses a couple of loose layers of felt sandwiched between his two skins. If you were to use this as a model for a complete enclosure, you would have something approximating a thin-walled box inside a thicker-walled box with the two joined at the corners. He would produce a much stiffer enclosure by constructing a single-walled box with a wall thickness equal to the two skin thicknesses combined (and stiffness is one of the things we want). If he were to stick his felt on the inside surface, he would have also done something about vibration damping too.

Constrained layer damping requires that the two skins be connected surface-to-surface by a suitable visco-elastic medium that is bonded to both surfaces by a high-shear strength adhesive. In cross-section this is analagous to an I-beam where the top and bottom flanges carry the tensile and compressive stresses, and the web carries the shear stress. In Art's example, even if the skins were bonded I suspect that he would achieve better performance if the skins were of more-or-less equal thicknesses. Unless Masonite has a much higher tensile strength than plywood, his adhesive would be subjected to a much higher shear stress than is really necessary, and would therefore be more likely to fail. Keeping it on the neutral axis would reduce the stress. The point I'm making is that things are rarely as simple as they seem.

I agree that the Masonite would need additional surface treatment on its inner face, but my point was that the Masonite isn't really doing very much anyway (other than making the construction a lot more complex). By keeping the construction simple, we can use the felt to help suppress the resonances of both the cabinet walls and the internal air that Joe discusses.

I've given this subject a lot of thought over the six months or so and have come to the conclusion that the next speakers I build will have a flat baffle but curved sides of changing radii. The curvature will add inherent stiffness and also help to reduce standing waves. Since I expect these boxes to be quite time-consuming to build, I will start with just a two-way design. I will also build a quick-and-dirty conventional box for comparison. I've thought about laminating different materials and including a constrained layer, but I really don't believe that it's necessary, and a decent analysis would be more appropriate for a master's thesis than a weekend woodworker's project :P

Of course, a woofer enclosure will be much more challenging...

[Martyn]

Joe, I'm sure you'll enjoy the elegance of the attached design. This could be taken to the next level of complexity by increasing the thickness of the walls, routing voids into the walls, and then pouring a visco-elastic cold-curing damping compound into the voids. Lots of fun, but would it really make any significant difference!

BTW, I like your present system.

[joecarrow]

Indeed! Those curves add a lot of stiffness, and those walls don't look very thin. That would probably let you stop worrying about cabinet wall resonance.

Also, I'd like to acknowledge your response on the CLD. I don't want to jump down your throat or anything. It's clear that you "get it". However, I think you're making too big of a deal about fully analyzing the behavior of the vibrating panel. I think that any panel flexing that is going to cause sound is going to be in the direction perpendicular to the face, and if there exists a method to damp motion in that direction it will have a positive effect.

I don't know about standing waves in that box. If you get something too close to a cylinder, you can get a really sharp resonance at a frequency corresponding to a wavelength related to the diameter. Of course, you might be able to address that with fiber.

My present system? Thanks I built something new, with better clarity:



Good learning experience there, mostly about newtonian reaction forces and the forced response of a box that isn't stiff enough. Doesn't resonate much, though. My next attempt will be the Modula MT, to be augmented with bass bins when I finally get out of apartment-country.

[Martyn]

I think we're agreeing with each other here. An infinitely stiff enclosure might sound good but would be rather heavy and expensive, and while it might be academically satisfying do do a rigorous analysis of an enclosure, we know that all we really need to do is to stiffen the panels sufficiently well that we move the resonances up to frequencies that are relatively easy to damp. That still leaves us to deal with all the (much smaller) non-resonant modes, so the question becomes: "at what level of damping do the effects of these vibrations become inaudible, and how do we achieve that level?". It might be interesting to calculate the amplitude of vibration from the theoretical deflection of the panels and arrive at an SPL, but for me the limiting factor is whether I can handle the box without a fork-lift truck!

[Dennis H]

Dennis, what Art has done in his tests isn't constrained layer damping.

Whatever you choose to call it, it measures better with the masonite than without it.

[jdybnis]

Unless Masonite has a much higher tensile strength than plywood, his adhesive would be subjected to a much higher shear stress than is really necessary, and would therefore be more likely to fail. Keeping it on the neutral axis would reduce the stress.

Isn't the goal to creat shear stress in the viscoelastic layer. That layer converts the shear force into heat, thereby damping the resonance.

I've never heard of a case where failure of the adhesive (i.e. seperation of the outer and inner layers) was as problem. There just isn't close to enough force.