Foamcore (posterboard), used by the OP of this thread, is basically that. Gatorboard is similar but with more substantial skins. Though both use XPS core I suspect.
https://www.gator-board.com/gator-board.htm
Yes, that should work well as long as the clamping is good. I may even try that just for fun. One challenge with my free-free beam method is that I have to tap it gently to avoid that the beam swings into the mic. Clamping one end would reduce that problem.
Eric
Sorry to be so late to reply... just catching up.
For sure, most 3-ply plywoods are very anisotropic, with the modulus parallel to the face grain being much higher (more than 10x sometimes) than the cross direction. The 5 ply plywoods are closer to balanced, and can actually go either way. Usually they are a little stiffer in the direction of the face grain, but also can actually be stiffer in the perpendicular direction if the face veneer layers are very thin (Revply is an example).
Eucy summarized my approach pretty well. I know (from finite element analysis, etc) that for a simply supported panel with balanced properties, a high aspect ratio panel (say 4:1) has a closer and more evenly spaced set of natural frequencies (at the low end), than any lower aspect ratio will provide. Hence, I favor a high aspect ratio panel.
But on top of that, I know that for a panel with unbalanced properties, if the modulus is stiffer in the long direction, that aspect ratio will have to be even higher than 4:1 (say 5:1 or 6:1) to get the same spacing of natural frequencies as the balanced panel at 4:1. And on the other hand, if the modulus in the short direction is higher, then the panel can achieve the same good distribution of natural frequencies with a lower aspect ratio, like perhaps 3:1.
Aesthetically, anything more than 4:1 looks funny to me, hence, I prefer the stiff direction to be the short direction, as that allows for a lower aspect ratio while maintaining a favorable (close and even) spread of natural frequencies. And at the same time, it looks odd for a wood panel if the face grain runs perpendicular to the length of the panel. Therefore, that best compromise I have found is to use a 5 ply plywood that is nearly balanced, or preferably slightly stiffer perpendicular to the face grain. For such a panel, I can use an aspect ratio under 4:1 and have the face grain running in the long direction.
Eric
Yes I noticed. Surprised it was as thick as it appeared to be. The PC speaker panels appear to be a lot thinner and probably need to be due to their small size..
A theory that I am definitely going to look into. Aspect ratio may figure as well.
You know you are right. I considered it some more and the stiffness would be comparable to 5/8" plywood (but much lighter of course) and that is pretty stiff.
I was looking for locally available thin wood veneers this morning but did not see anything. I agree though, something with a very thin wood face would be better.
From a performance viewpoint, I believe you'd be better off trying surfacing tissue/veil mat or ultra thin fibreglass cloth (0.03mm thick) with either polyester or epoxy resin coating to an XPS or EPS core of 3 to 5mm thick.
Why? ..I think even thin timber veneer plus glue to each side will deaden the panel too much. If you do try it, I'd use epoxy or polyester resin scraped onto the veneer and then placed in a press of some type to apply even pressure as per double top guitar building.
I'm considering making a flat bed type of hot wire cutter to try different thicknesses of XPS. I routed the last one... What a mess it makes.
That's the thing with this pursuit, it takes an enormous amount of experimentation to get the result you want, and then there's often little consensus between experimenters...Ahh the fun of it 😵💫🤭😃
Eucy
Why? ..I think even thin timber veneer plus glue to each side will deaden the panel too much. If you do try it, I'd use epoxy or polyester resin scraped onto the veneer and then placed in a press of some type to apply even pressure as per double top guitar building.
I'm considering making a flat bed type of hot wire cutter to try different thicknesses of XPS. I routed the last one... What a mess it makes.
That's the thing with this pursuit, it takes an enormous amount of experimentation to get the result you want, and then there's often little consensus between experimenters...Ahh the fun of it 😵💫🤭😃
Eucy
Hello Eric,
Thank you. Very clear.
I have in mind 2 other considerations not covered for now around the topic :
- One can read in papers (sorry I don't have at the moment in which one) that some orthotropy is beneficial to spread the effect at the coincidence frequency. A quick approach by the coincidence formula to spread it let say on what half an octave (?) might give an indication to the limit of orthotropy. Is the a standard 3 plies plywood to weak in one direction?
- Second is front and rear wave recombination : DML are said self baffled meaning the membrane is the separation between front and rear. Increasing the ratio, might increase the separation.
I made some tests by gluing together with PVA thin layers of wood (1mm balsa or pine veneer). The results are disappointing. Damping seems to high. The glue increases also very quickly the weight.
Is the PVA mechanically too "weak" ?
Plywood is made of glued layers. Does somebody know how plywood layers are glued (in the way it explains the difference for DML)?
Christian
I was more thinking of smaller panels and paper coated foam board that outfits such as Amazon sell. Those may use EPS or EVA.
Don't ask me where I saw mention. Too much web crawling to remember.My initial views on XPS were coloured by model makers. The blue these people use is high density. That left me wondering about pink but more looking around suggests that colour relates to the maker and both blue and pink are more or less the same thing. The type used for insulation is made to a din standard that spec's density. It looks like I can buy that fairly easily. Go to an outfit that sells various densities - I suspect I would have problems, order size etc.
I have that on my contemplation whiteboard too , The adhesive type cant add to the stiffness and whatever is used must be consistent thickness .
Whatever adhesive is used the only way to reliably get uniform glue thickness is vacuum bagging which can be done on the cheap , Youtube has a brazillion videos on that topic.
Caravan / RV manufacturers typically use 2 pack epoxy when they glue their fibreglass skins to the XPS core , they all use vacuum bagging , sometimes in runs 2.4 metres wide x 20 - 40 metres long in one sitting for efficiency.
"Plywood Manufacture
Four types of glue bonds are used for plywood manufacture, in decreasing order of durability.- Type A bond uses a phenol-formaldehyde resin which is specified for marine and exterior plywood and will not weaken under wet conditions, heat, or cold.
- Type B bond uses a melamine fortified urea formaldehyde resin and is suitable for exterior applications including concrete formwork.
- Type C and D bonds use urea-formaldehyde resin and are both interior bonds and are not recommended for structural applications.
I'll try...Oops... Earbourne beat me to it 😃👍
Plywood glues are typically amine resins. Urea Formaldehyde and Melamine Urea Formaldehyde being the most common for the plywood we use.
They are thermosetting glues which when cured are hard and rigid. Industrial usage would allow very thin layers between ply laminations.
Intuitively, the harder and thinner the glue, the less damping effect it will have.
UF powder is available at retail level but the application still needs a high quality approach.
An alternative worth trying is traditional hide glue which also sets hard. The application requirements however remain the same .
Eucy
It seems they use Urea Formaldehyde glue which is thermosetting. Laminating epoxy might give a similar effect or try some marine ply to see if ithat might work out.
When it comes to making composite with EPS and paper or veneer, keep in mind that really small changes to the surface can make a surprisingly big difference.
When I coat mine with hide glue and shellac, a fairly small change in the dilution will make a big difference, even if I think the difference in thickness of the resulting skin would be measured in 100ths of a mm. A 0.5 mm veneer or paper layer would change an EPS plate drastically. Only the glue needed to attach it would make a massive difference.
Something that is a good material as a plate on its own is unlikely to also be a good material for core, and I think for composites to work out well you need a less dense core that still have good compressive strength, and stiffer skin than really thin veneer or paper.
Otherwise you will get an overly heavy plate with low efficiency, or have to use a low density foam that lacks compressive strength, which also will kill performance. Honeycomb core does make a lot of sense, and if you had very high density EPS cut in a fine honeycomb pattern that could work.
If you find EPS is ringing too much, ensure dimensions are not unnecessarily large and that you have proper suspension first of all. That will tighten up the sound without loosing a lot of efficiency. You can also use a less diluted coating, but that will kill efficiency. PVA is a little bit more forgiving, but with hide glue and especially shellac it is really easy to overdampen the plate. I would guess that for example going from 1:4 (shellac to spirits) to 1:5 I gain at least 3dB, so need half the amp power.
But with the right dilution I find hide glue and shellac gives both better sound and efficiency than PVA.
When I coat mine with hide glue and shellac, a fairly small change in the dilution will make a big difference, even if I think the difference in thickness of the resulting skin would be measured in 100ths of a mm. A 0.5 mm veneer or paper layer would change an EPS plate drastically. Only the glue needed to attach it would make a massive difference.
Something that is a good material as a plate on its own is unlikely to also be a good material for core, and I think for composites to work out well you need a less dense core that still have good compressive strength, and stiffer skin than really thin veneer or paper.
Otherwise you will get an overly heavy plate with low efficiency, or have to use a low density foam that lacks compressive strength, which also will kill performance. Honeycomb core does make a lot of sense, and if you had very high density EPS cut in a fine honeycomb pattern that could work.
If you find EPS is ringing too much, ensure dimensions are not unnecessarily large and that you have proper suspension first of all. That will tighten up the sound without loosing a lot of efficiency. You can also use a less diluted coating, but that will kill efficiency. PVA is a little bit more forgiving, but with hide glue and especially shellac it is really easy to overdampen the plate. I would guess that for example going from 1:4 (shellac to spirits) to 1:5 I gain at least 3dB, so need half the amp power.
But with the right dilution I find hide glue and shellac gives both better sound and efficiency than PVA.
My father used to veneer things at times. I also did a bit but memory fades. Hot glue pot. When it comes down to actually applying the veneer use a hot domestic iron to smooth it down, This also causes excesses to ooze out but the coating needs to be thin anyway so that aspect shouldn't matter much. Afraid I can't remember the temperature of the iron or glue pot but info should be around for the pot. He also did some marquetry the same way.
I distinctly remember the mention of fish glue. Pass - that is now squirty stuff that is allowed to set under pressure.
If some one is into musical instruments I suppose they could also try french polishing it. That involves repeated very thin coats with a cloth. Each one effectively smoothing up the previous one. Same cloth each time. One of my grandfathers used to keep his cloth arrangement in a glass screwtop jar for the dining room table. Violin makers may use a different approach but I do know grandads method works, Maybe a day between coats.
@AjohnL
I find hide glue easier to work with than PVA thanks to the quick drying times, and it is cheaper too.
You should heat the hide glue to around 60c. Use a much higher dilution than typical used for gluing, with 3-4 parts of water. Use a roller to apply a single thin coating.
Then I use shellac (as used in french polish) on top of that, but strongly diluted with around 5 parts of alcohol, and brush a very thin layer on top of the hide glue.
The hide glue dries very quickly, and the shellac almost instantly, in fact you have to make sure to be quick to coat both sides, or even a 2 cm thick EPS plate will bend from the tension created when the shellac dries.
This should be the same combination used for soundboards in pianos, and I think it works wonders on the GPS plates I used it on at least.
I find hide glue easier to work with than PVA thanks to the quick drying times, and it is cheaper too.
You should heat the hide glue to around 60c. Use a much higher dilution than typical used for gluing, with 3-4 parts of water. Use a roller to apply a single thin coating.
Then I use shellac (as used in french polish) on top of that, but strongly diluted with around 5 parts of alcohol, and brush a very thin layer on top of the hide glue.
The hide glue dries very quickly, and the shellac almost instantly, in fact you have to make sure to be quick to coat both sides, or even a 2 cm thick EPS plate will bend from the tension created when the shellac dries.
This should be the same combination used for soundboards in pianos, and I think it works wonders on the GPS plates I used it on at least.
A little different to the approach I mentioned. Initially the coating is just grain filling really. Once the coat builds the same amount of french polish covers a larger and larger area, Very little on the cloth each time. As the same cloth is used this softens what is left from the previous coat, A small trial in my case using a piece of folder kitchen paper towel as I had used that on gun stocks with what are effectively friction polishes. People try all sorts but a sort of french polish approach works in much the same way. Very very little polish is needed once the grain is filled.
Furniture - also tried a particular brand of yacht varnish. Quick easier and and far more robust than french polish. Predictable too but the brand has gone.
I'm aware what a French polish is, but don't think it's relevant for DML since shellac will damp the plate a lot. Like I explained, even with much higher dilution than used when doing a French polish, a single layer can overdampen the plate way too much. So I suspect a French polish with many layers of shellac will not give great results.
But used very sparingly and carefully shellac seems very useful.
But used very sparingly and carefully shellac seems very useful.
For the shellac on my 3mm Poplar ply, I weighted it down with a compensating curve before applying the shellac. On drying and removing the weights, a straight panel resulted.
Same on the second side although there was less tendency for this to curl because of the shellac on the reverse side.
Eucy
Thanks for the trick Eucy. How do you apply the weight? I mean you probably need some attach points do to that... How letting the full face to varnish free?
Your technique remind me water colour. The first operation before water color is to fix the new sheet of paper on a for example wood board with tape, make it wet with a sponge and let's dry.
Is the drying time under constraint that limit the warping? If so, is attaching the ply on a thick board of same or larger dimensions with enough double side tape a solution?
Christian