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Constrained layer damping

Arnandsway

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#1
Hi all,

For a while I've been busy trying to design a speaker for the first time. So I have learned a lot these last few months and I know it is only the surface of knowledge.
Now, my focus has shifted towards the enclosure itself and in this proces I came across this video which involved constrained layer damping, from someone probably some of you already know. (See the vid below.) It is a very interesting video for sure, so I recommend to watch it. I've linked it with a timestamp from where he shows what the effect is of dampening etc.

He explained how it can help to dampen the speaker enclosure very well.
And he also showed two products that could be used, Sorbothane (rubbery sheets very expensive though!)and Pyrotek Decidamp DC30 (two component paste as a adhesive and constrained layer). Unfortunately this product is not available in Europe, so I can't get my hands on it.

So my question to the experienced engineers/designers: do you know of alternative ways or products that could reach the same goal?

 

617

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#2
There are some good discussions from Earl Geddes on this subject on diyAudio. His recommendation is two braces from opposite sides of the enclosure overlapping, with a flexible adhesive holding them together. The idea is that you transfer the motion of the cabinet walls (in/out) into a shearing motion over a large surface area of flexible adhesive. This is essentially what CLD does - it puts a lossy, gummy material between two layers of structural material which are moving relative to one another. The differential motion between the outer layers is damped by the inner layer.

I am pretty skeptical of the conventional damping assessments looking at decay of impacts on the cabinet. The KEF LS50 whitepaper has some really good information, but essentially what they did was make a really small well braced cabinet, but undersize the braces and install them with a layer of flexible adhesive. KEF of course didn't do a 'knuckle wrap' test, or look at impact decay or use a stethoscope, they made a computer model of the cabinet rigidity vibrating at different frequencies.

Also I'm not sure what your aim is but I would steer you away from voigt pipes and full range drivers. Too many compromises.
 

Shadrach

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#3
I built laminate panel for my speakers. I used half inch marine ply and made what is best described as a tray. I filled the tray with the product below. You do in fact need to slightly over fill. Then I used another panel of quarter inch marine ply to so the Dum Dum was trapped between the panels.
Total panel thickness one and one quarter inches.
There is a theoretical problem though. Such a construction will act as a capacitor.
For the baffle I used a one and a half inch thick kitchen worktop made from composite blocks.
These are 60 liter enclosures with the crossover mounted outside the cabinet.
There are two horizontal braces and one vertical inside the enclosure.
Just an idea for you to think about.


https://www.morrisminorspares.com/b...-5kg-tin-no-longer-available-use-t18n-p828706
 
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Arnandsway

Arnandsway

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Thread Starter #4
There are some good discussions from Earl Geddes on this subject on diyAudio. His recommendation is two braces from opposite sides of the enclosure overlapping, with a flexible adhesive holding them together. The idea is that you transfer the motion of the cabinet walls (in/out) into a shearing motion over a large surface area of flexible adhesive. This is essentially what CLD does - it puts a lossy, gummy material between two layers of structural material which are moving relative to one another. The differential motion between the outer layers is damped by the inner layer.

I am pretty skeptical of the conventional damping assessments looking at decay of impacts on the cabinet. The KEF LS50 whitepaper has some really good information, but essentially what they did was make a really small well braced cabinet, but undersize the braces and install them with a layer of flexible adhesive. KEF of course didn't do a 'knuckle wrap' test, or look at impact decay or use a stethoscope, they made a computer model of the cabinet rigidity vibrating at different frequencies.

Also I'm not sure what your aim is but I would steer you away from voigt pipes and full range drivers. Too many compromises.
Thanks for the info. I'm reading through the thread right now. This one.
Next is to find which glue to use as a constraining layer. Titebond Melamine or Sikaflex 292 is what I found in this thread, mentioned bij Gedlee.

I wonder now though it would be worthwhile to CLD the whole enclosure? I agree that the methodology used in the video seems uncrompehensive but the panels did sound much well dampened where the resonance frequency lowered.

PS. Not planning on using full range drivers. ;)
 
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Arnandsway

Arnandsway

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Thread Starter #5
I built laminate panel for my speakers. I used half inch marine ply and made what is best described as a tray. I filled the tray with the product below. You do in fact need to slightly over fill. Then I used another panel of quarter inch marine ply to so the Dum Dum was trapped between the panels.
Total panel thickness one and one quarter inches.
There is a theoretical problem though. Such a construction will act as a capacitor.
For the baffle I used a one and a half inch thick kitchen worktop made from composite blocks.
These are 60 liter enclosures with the crossover mounted outside the cabinet.
There are two horizontal braces and one vertical inside the enclosure.
Just an idea for you to think about.


https://www.morrisminorspares.com/b...-5kg-tin-no-longer-available-use-t18n-p828706
Are your panels 1,25" (~3,1 cm)? That seems very thick. My speakers will be monitors of about ~14L, for which I don't think this isn't necessary. I plan to run them to 100 hz anyways and cross them over to subs.

The problem with this Dum-dum (however funny the name is) is that we don't know if the acoustic dampening properties actually help reduce the ones relevant. Per say they could dampen 30-60 hz but if the dominant resonance is around 150 hz, the CLD in ths case would be irrelevant.
 

Shadrach

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#6
Are your panels 1,25" (~3,1 cm)? That seems very thick. My speakers will be monitors of about ~14L, for which I don't think this isn't necessary. I plan to run them to 100 hz anyways and cross them over to subs.

The problem with this Dum-dum (however funny the name is) is that we don't know if the acoustic dampening properties actually help reduce the ones relevant. Per say they could dampen 30-60 hz but if the dominant resonance is around 150 hz, the CLD in ths case would be irrelevant.
Yes they are one and a quarter inches thick. The front baffle is one and a half.
For an enclosure of 14 liters none of what I've done is relevant.;)
 

gene_stl

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#7
I have watched many of that guys videos on jet engines and drones. They are interesting and he obviously is a very intelligent person.

But note that he call a single driver set up the "2nd best speaker in the world" His "Best speaker in the World" is(was) a bass shaker motor on a panel or at least was for a while. In my not particularly humble opinion, he ought to stick to drone and jets. (Very good videos and ideas!) The notion of a bass or whatever it was , loose attachable motor being the best there is and the above are both absurdities on the face of it.

I have a lot of audiophile friends who like single driver speakers and disrespect crossovers. If I am in a frisky mood I tell them ," yeah, they are great if you don't mind having no top octave , no bottom two octave , lots of IM distortion and modulation, and not terribly much dynamic range."

The laws of physic aren't suspended because you buy a Lowther, a Voxativ nor any of the newer versions. They do work GREAT with tube amps though.;)



I am not sure that constrained layer damping is even worthwhile if you build your enclosures solidly enough. I think it is more intended for rooms and less for speaker enclosures. If you apply horizontal grade Formica (laminate , horizontal grade is thicker) with the appropriate adhesive you get a composite material that performs pretty well. This material is stronger and more isotropic than wood veneers because it does not have grain nor fiber structure. The vertical grade is thinner. If you were to laminate the inside too it would be really stiff , even if the core was just mdf.

It will probably be difficult to find an adhesive that will remain flexible for a long time. You could mimic Sorbothane with polyurethane or silicone caulk. Silicone has less peel strength the poly urethane. Both are pricey but should even be available in Europe. Silicone is more resistant to attack and polymerization crosslinking (ie age related stiffening) than polyurethane. There are PUs that have Silicone too.

Might be overkill. But sometimes overkill is fun.:):rolleyes:
 
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Wombat

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#8
I used this on the inside of the braced MDF panels of my 14 cu.ft. enclosures. The result was a very dull, low level, 'thunk' from the knuckle-test. A big improvement.
 
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#9
Did research into this as well.
I came to the conclusion that a well built 1" (2.5cm) thick good quality MDF (/Valchromat) closed stuffed cabinet with enough bracing will not contribute audibly to the sound. You can go farther but I don't see any evidence there's anything left to gain.
Here a small test: http://www.hsi-luidsprekers.nl/index.php/projecten/12-kast-materialen-onderzoek
I think one of the important things is to keep your cabinets closed, not ported or with passive radiator. Proper stuffing with the right density fibreglass reduces internal resonances so much they don't matter anymore and it's especially those resonances that will become audible.
Also close bracing will help a great deal to stop the wood itself vibrating, closer than 15cm bracing (and proper stuffing) and 1" thick MDF will sound completely solid and dead.
I think this is better than constrained layer damping because the audible endresult is the same yet pure MDF with bracing is much easier to build and things like very round edges matter a great deal for sound (no edge diffraction) and may be much harder to build with contrained layer damping. (and also much more expensive)

Btw, no port and full stuffing this will matter much less for a sub. For a sub stiffness / good close bracing is most important.
 
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Arnandsway

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Thread Starter #10
Did research into this as well.
I came to the conclusion that a well built 1" (2.5cm) thick good quality MDF (/Valchromat) closed stuffed cabinet with enough bracing will not contribute audibly to the sound. You can go farther but I don't see any evidence there's anything left to gain.
Here a small test: http://www.hsi-luidsprekers.nl/index.php/projecten/12-kast-materialen-onderzoek
I think one of the important things is to keep your cabinets closed, not ported or with passive radiator. Proper stuffing with the right density fibreglass reduces internal resonances so much they don't matter anymore and it's especially those resonances that will become audible.
Also close bracing will help a great deal to stop the wood itself vibrating, closer than 15cm bracing (and proper stuffing) and 1" thick MDF will sound completely solid and dead.
I think this is better than constrained layer damping because the audible endresult is the same yet pure MDF with bracing is much easier to build and things like very round edges matter a great deal for sound (no edge diffraction) and may be much harder to build with contrained layer damping. (and also much more expensive)

Btw, no port and full stuffing this will matter much less for a sub. For a sub stiffness / good close bracing is most important.
Thanks for your reply, I remember you sending that experiment earlier! :)
How is your project going?
 
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#11
Thanks for your reply, I remember you sending that experiment earlier! :)
How is your project going?
Fiiinaly getting to the building stage :D Took a while haha :facepalm:
Will post on this forum when finished.
Good luck with yours!

Btw, the video you linked to isn't that great I think. First of all as others have mentioned full range drivers are not a good thing at all.
Secondly, ports, passive radiators and transmission lines all have compromises. For sound quality it's truly best to "throw away" the back wave. (with absorption). Btw all the ports / radiators etc make only limited use of the backwave around their tuning frequency. Besides all the additional problems they give, they all basically start acting like the isn't an enclosure at all below their tuning frequency which means that below the tuning frequency they are much worse than a closed enclosure and you will get very high excursion (with all the distortions that go along with that) of your woofer with very little SPL output. And increased output around tuning frequency will only be about 4dB in real life. Also since there is such a rapid dropoff below tuning frequency this means that there's basically an "open" filter in your bass. This is the "ringing" people talk about and it's certainly audible in the bass. And you cannot correct for this since below tuning frequency you have nothing to work with to boost as it's so inefficient with the open backwave cancelling the frontwave. Some people even add a HP filter below tuning freq to limit excursion but this making the port / radiator / transmission line "ring" even more.
Big drivers and amp power is fairly cheap, much better solution is to keep it closed in my opinion. At least for 3-ways or 2-ways with subs. Though if you must build a 2-way without sub then a port/PR/TL may be required to get enough SPL at 40-50Hz or so. It will be a compromise in soundquality though..

Edit: oh and btw his damping test is wrong too. Damping of MDF is better, you must test damping with the same frequency, his MDF impact test gives quite a lower freq than the plywood test yet the plywood still rings slightly longer. As for the contrained damping tests, he doesn't test it without damping layer which would show about the same effect due to different resonance frequencies of the ply and MDF and the extra thickness, and for damping one can simply add leadbitumen (just stick it on without another layer) it will make it more dead than either that damping paint or the damping sheet. (though it won't do much in the bass, for bass the close bracing is most effective). I beleive constrained layer damping is most effective when isolating against sound passing through, for instance a drywall add thick greenglue and another drywall and vibrations from the first drywall will be more isolated from passing on to the second drywall. This is good for sound isolation but less usefull for building a speaker enclosure.
 
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chebum

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#12
I think this is better than constrained layer damping because the audible endresult is the same yet pure MDF with bracing is much easier to build and things like very round edges matter a great deal for sound (no edge diffraction) and may be much harder to build with contrained layer damping. (and also much more expensive)
Lots of MDF has one important shortcoming - it's a lot of weight. In that experiment one driver enclosure weighted up to 105 kg / 230 lbs. Moving around such heavy speakers isn't convenient.
 

Francis Vaughan

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#13
Something I have used in a number of speakers is a rubberised acrylic "bitumen" sealer. It doesn't actually have bitumen in it (thankfully) but it is marketed into areas where bitumen is used (such as waterproofing.) I have used it with overlapping cross braces, and have also made up full constrained layer panels. I tend to use it like thick glue. You don't want or need a very thick layer, but you want to be sure everything has a good coat, without any voids. It can take a long time to set, and you cannot use it on sealed surfaces, as it will basically never set inside.

Remember, a constrained layer panel must not be fixed at the edges. I have usually added an interior panel to a partly finished box, one that is slightly smaller than the space it is to fit into, ensuring is edges are not touching anything. The panel can be fixed in place with short screws whilst the sealer sets, but the screws must be removed afterwards. Also the additional panel must have the same modulus as the panel it is fixed to. Using the same material for both layers is the easy way to ensure this.

I did some experiments many years ago with a contact microphone and various layups. Results were pretty good, with quite significant damping achieved. What I have not done is made two otherwise identical cabinets and measured, which I really should do sometime.

The stuff I used is this (Australian) but I'm sure it is pretty generic. It can get rather messy in use. But does clean up with water.
https://www.bunnings.com.au/gripset-betta-1l-waterproofing-membrane-bitumen-rubber_p0960165
 

chebum

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#14
Remember, a constrained layer panel must not be fixed at the edges. I have usually added an interior panel to a partly finished box, one that is slightly smaller than the space it is to fit into, ensuring is edges are not touching anything.
So you covered interior of the enclosure wall with that glue and then added another sheet of MDF on top of the glue, right? Where did you mount the braces then?
 

Francis Vaughan

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#15
So you covered interior of the enclosure wall with that glue and then added another sheet of MDF on top of the glue, right? Where did you mount the braces then?
Sorry to reply so late.
Cross braces can be mounted whereever you like. I make the cross braces as a laminate that is a form of constrained layer. (ie three pieces, in a laminate, where one end has the middle piece sticking out, and the other end has the middle layer recessed slightly, so lengthwise strain on the member places the damping layer in shear.) These are added after the inner constrained layer, and I simply glue them to the inner layer.
Using shelf braces and constrained layer damping is using techniques that are somewhat in opposition to one another. Damping requires that things move in order to dissipate energy. Solid bracing is designed to stop things moving. It breaks the panels up into smaller units. But at the corners of the brace it will stop the inner constrained layer moving with respect to the outer, and thus prevent the damping layer shearing, and so no energy will be dissipated. There is a lot of freedom in building up damped layers, bracing and the like. What matters is that the constrained layer is placed in shear by movement. If it isn't it won't damp anything.
 

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