Materials to reduce sidewall flex & vibration ?

I am wishing to reduce sidewall vibration without adding reinforcement 
within the cabinet. I'm curious about fiberglass and other products that can attach/bond 
permanently to the sides yielding the highest stiffness to weight ratio.
No Rez, designed by Danny Richie of GR Research. It is expressly for absorbing speaker enclosure wall resonance, being installed on the interior of the enclosure's panels. Very effective.
Thanks bdp24; however, in this case I'm only interested with
material that can be bonded to the exterior of the speaker. I'm hoping 
some have experience in materials such as fiberglass and carbon fiber
as well as with granite,etc. 
You could certainly go with granite, finding a really good installer to cut and glue the sides and top. Corian could certainly be tastefully done as well.a bit easier to do a cutout in the front for the speakers as well. If it works really well you could start a business.
Hard floor tiles! The thicker, heavier type without self-stick bottoms. They add mass, which lowers the cabinets resonant frequencies, and damp that resonance.
Texan Sonic dampers for speaker cabinets and subwoofers are, alas, no longer available. The problem with trying to use materials to damp the cabinets resonance is that almost all if not all materials trap as much as absorb. That's why Tekna Sonic dampers were so effective, they were a mechanical diode. They were also excellent for use on room walls as it tuns out. I used to use Tekna Sonics for my isolation stands way back when was big customer. IIRC I bought the last Tekna Sonics available from Music Direct eons ago.
You know I've thought about this off and on in the past and wanted to come up with a cheap DIY that would test if it would be worth pursuing.  I came up with one idea.  If you can imagine a bag of sand (or better lead shot) say 12" long 3" wide and about 1" thick - now sew as many of these together (kinda like a bullet belt) as required to drape over your speaker's top and sides.  Now the back wouldn't be covered unless you made a belt for it, but this method was only just a test I could easily fabricate and try out and would only be applied during listening sessions.  It's not a very aesthetic thing but it could work I think.
If appearance is not a concern, a C clamp attached to each pair of opposing walls. One top-to-bottom, one left-to-right on the upper half and another on the bottom half, and one front-to-back, between drivers. Kind of drastic, but it would certainly reinforce the enclosure and prevent flexing of the enclosure’s panels, It would also simultaneously raise the resonant frequency of each panel and the enclosure as a whole.
I got few area rugs from flea market near-new and same and hung around speakers on three sides and paid only $50 for all three.
Heavy paving stones have long been placed on top of subwoofer cabinets by owners, as have bags of sand or lead shot.
Please help me understand this. Is this a work around for a poor cabinet design-build? I can't imagine having to do something like this to my speakers.
Thanks guys.
Dentdog,Granite sounds ideal. For a business all I need is a warehouse and laser cutter for the granite; but, I'm retired so that won't work.
bdp,hard floor tiles,perhaps black, are interesting.
I mentioned fiberglass/carbon fiber as they are more rigid that rock by weight and I'm hoping someone here has experience with them, perhaps automotive or art related.
donjr, that's correct; and I would say at least 95% of cabinet sidewalls flex.
I don't really know if it will improve sound any; but it's idea I want to investigate.

donjr, that's correct; and I would say at least 95% of cabinet sidewalls flex.

Cabinet flex was/is more of a concern with old monkey coffins. Its pretty much non existent in todays narrower speakers with more emphasis placed on cabinet designs, thicker baffle and walls constructed from superior material, and internal bracing.    

Eliminating resonance by minimizing cabinet flex will improve sound, more or less. But I think your approach is all wrong. I go about it on the inside with hardwood dowel. Or by thickening wall density by laminating similar material to existing. Or some of both.    


meerzistar is correct. Doubled wall panels and internal bracing is the best, most effective way to combat enclosure resonance. The sound produced inside the enclosure makes it expand, like a balloon being inflated. Bracing prohibits the enclosure walls from flexing outward in response to the internal pressure created by the drivers moving inward. And doubled panels stiffens the enclosure walls, reducing their ability to flex and thereby create sound.
But, panels 'inside' the enclosure change the internal volume-
an area carefully calculated to work with the drivers used - to provide the right 'damping' for the drivers. This is why I am focused on the exterior.
Thats why I suggested dowels, they take up less internal vol.. If I were to add sheet it would be on the outer sides. Dowel braces may not decrease internal volume whereas you can hear any difference. I found this to be true, although results can and often do vary. You could also think of it as the lesser of two evils.

Whatever measures are taken if the scales happen to tip far enough I might even consider making new boxes with more volume to compensate for planned bracing, thicker walls, mdf or hdf.

You may wind up doing that sooner or later anyway. Until then no experience is bad experience here.

Yeah, the extra panels attached to the outside of the enclosure. For the interior, an alternative to wooden dowels is threaded steel rods, inserted into the enclosure through holes drilled into opposing enclosure walls, each rod screwed into opposite ends of a threaded coupler. The rods can be adjusted until they are tight enough to prevent outward panel movement. A locking nut can be threaded onto each rod, tightened right up against the wall interiors, thereby preventing inward panel movement. The rods can span the height, width, and depth of the enclosure, one set for each pair of opposing panels.

When I built my 4 cu.ft. sub enclosures, I built them double-walled, an enclosure-within-an-enclosure. I left 1/2" space between the two, maintaining the space with 1/2" "ribs". I then filled the space with #60 Silica sand. VERY effective wall damping! There were very large loudspeaker enclosures so designed and built in the 1950/60's, before Acoustic Research's introduction of the acoustic suspension principle killed them off.

Threaded bar with poly locks is a great idea.  

Baffle diffraction would be another concern if the widths or edges is altered. Or by using stone or composite tile, that sort of thing. Even slight changes can negatively effect how well the speakers image. 
Yeah, the edges of the added outside panels should be chamfered away from the baffle.
FYI. "Epoxy granite material has an internal damping factor up to ten times better than cast iron, up to three times better than natural granite, and up to thirty times better than steel fabricated structure." Answer found in other forums.  
Thanks to bdp24 and meerzistar for contributing to reducing side wall flex and vibration.
I'm still researching;here's new info..
"  Hardinge as well as many of these; others are quick to point out that the damping properties of epoxy granite are about 8x those of cast iron."  I'll be looking to confirm this as it appears very relevant. 
Well the gold standard is carbon fiber. Expensive and hard to find it what it does is absorb the back wave  and convert that energy into heat. Saving that I would look at bitumen, or modified bitumen. They are often sold as a water proofing membrane that is played under shingles. Soft and sticky it can be cut to size and attached to the inside of your cabinets. 
Maybe ceramic tiles?  Rigid, kind of light, and pretty inexpensive to try.
And the speakers would look right at home in the bathroom!
Seriously I would take out the woofer, which is the drive that usually causes most of the large back waves, and just stick some modified bitumen in the side and back panels. Self sticks, easy to cut to sizes. 
You will be crating a composite panel speaker. These bitumens I am referring to are often found in speakers and in electronics. Stuck to the inside. 

creating a composite panel speaker

The answer is obvious. Smart metal. 

cascade spray
Thanks johnk but the damping spray you mentioned,although it does have its application benefits, will not contribute to my goal of maximizing rigidity..  "I'm curious about fiberglass and other products that can attach/bond permanently to the sides yielding the highest stiffness to weight ratio." So far my research seems to indicate that epoxy filled with fiberglass or carbon fiber,or carbon nanotubes, combined with high mass material is the best option. I continue to research and appreciate contributions.   
Thanks Geoffkait. Yes smart metals have fascinating potential,along with gels that instantly harden upon attempt at deformation, but I feel the momentary time delay renders both non ideal. 
I think constrained damping can be more immediate. Of course I'm just thinking; I'm not a materials scientist---aha--you've given me the idea to search for the 'chat rooms' of materials scientists discussing cutting edge damping materials and techniques. Appreciated.
Thanks soundsrealaudio for your direction. I agree with you and am wondering what materials would do this best.
Just to clarify soundsrealaudio, I am seeking a composite of the 'most rigid' and highest damping' materials to use. So far the filled epoxy appears the most efficient. 
Well sometimes the most rigit is not the best dampening material. Sometimes they just store the energy and release it slowly Not good. If your speakers are vibrating, and most do, then glueing solid materials to the outside may not work out so well. The vibes might possibly loosen the rigid material right off the panels. Imagine glueing a rigid panel to the outside of a clothes dryer. 
Once they begin to loosen you really have a problem. I much prefer my recommendation. 

Soundsrealaudio, I agree that a composite material is ideal.
I'm searching for the 'ideal' practical solid,high mass deadening
sheet and the ideal adhesive. I want to create maximum stiffness
with highest possible damping. It seems carbon filled lead epoxy
is a contender. I'm hoping someone with industrial experience may
have something to contribute. Shipbuilding, aeronautical, train building
or massive motor vibration reducing techniques perhaps. 

You should note that low mass materials resonate at higher frequencies. Ping a wine glass and you can see how that works. Now take a hammer to a heavy piece of metal. Much lower frequency. High frequencies are easier to dampen. To stop the wine glass from resonating simply touch it with your finger. This illustrates another principle important to controlling resonance and that is composite technology. Your finger resonates at a different frequency then the glass and together they work to absorb the resonance. 
You might reset your goal to making the speakers sound better by reducing the cabinet resonance. There are not a lot of speaker companies out there that do more then stuff foam in the box. A few but not many.

Ptss, I suggest you read up on vibration theory, natural frequency and resonance before you attempt anything.  A engineering resource to be more specific.  I don't think you will gain much by just sticking other materials to mdf panels without an idea of what it would do to the resonance frequencies of your panels, might make your speakers sound worse.  
Hello rotarius. I appreciate your suggestions are very relevant so likely you
have some knowledge. Do you have some more specific suggestions? Thanks.

My final weight in. 

Buy new speakers and give up the science experiment. 
Spoken like a dealer.
Then don't buy new speakers, buy used ones on Audiogon. 

Hello rotarius. I appreciate your suggestions are very relevant so likely you
have some knowledge. Do you have some more specific suggestions? Thanks.

It's hard to be specific because induced vibration and resonances are so dependent on the masses coupled together and the natural frequencies of all the materials of a given system (speaker).  At work, I have access to vibration analyzers, so if I had to attempt an experiment like this, I would use some equipment to quantify the results.  Like someone suggested earlier, you could use materials that dissipate vibrations in the form of heat on the inside walls without much thought.  Just because something you stick on the panel is very rigid, it does not guarantee anything.  I mean it will still transmit vibration and it will resonate at some higher frequency. Is that better for your sound? 
Thanks rotarius. I very much respect the benefits of measurement with equipment; but it's not available to me.. You mention " you could use materials that dissipate vibrations in the form of heat on the inside walls without much thought ". If so, why could the same materials not be used on the outside? I'm simply looking for the best material for this. So far it seems like an epoxy with fillers of fiberglass,carbon fiber and lead slivers to both provide rigidity and dissipation of vibrations by turning them into heat. Your thoughts?
Well, for one thing, stuff like Dynamat, norez, etc. would look ugly on the outside! Your aim should be to minimize the excitation of vibration of the speaker walls so the material you use would ideally be placed between the speaker wall and the source of the excitation. I suppose there may be something to gain by sticking them on the outside.  I am just not sure if the effort is worth it all but hey, if you like tinkering like me, that's half the fun.
Thanks rotarius. As a fellow tinkerer I'll let you know that I do consider the final
look to be important and have decided to use good quality leather,in black (speaker color),as necessary to refine the look. I've already got the leather. I've found 3M Super 77 spray glue to be ideal for leather;just the right tack time then a nice permanent bond.