Hello André
Which kind of aluminum foil did you test? Is the aluminum for kitchen suitable?
Yes. Agreed. Unfortunately bandwidth and efficiency seem not going in the same way. A compromise has to be done. It is what I got from the "theoretical" approach.
Christian
Yes, lightweight household foil. It increases stiffness because it does not stretch. Compression resistance is provided by the epoxy adhesive.
If you use skins of CF/epoxy instead of aluminum, you should be able to get even better efficiency than EPS alone. For, say, a 3 mm EPS (or XPS) core with 0.23 mm CF/epoxy skins, the increase in stiffness (B) due to the stiff skins should (theoretically) more than compensate for the increase in density.
My best estimate of B/mu^3 for solid EPS/XPS is about 70, compared to about 140 for the CF/PS/CF composite, as described. Since aluminum is heavier than CF/epoxy, using aluminum skins (.23 mm) instead drops B/mu^3 down to about 30 or lower, depending on how much adhesive it takes to stick the aluminum to the foam.
I'm not sure what bandwidth you'd get, however. with the CF/PS/CF since I haven't tried it.
Eric
Eric I battled to get lightweight CF over here. I think the lightest I could source affordably was 200g/sqm at 0.2mm thickness. I'm speaking off the top of my head as my computer is elsewhere. But
I can easily source alu foil at 0.06mm at the supermarket.
I think it's the thickness that makes it as light as CF even though the density is higher. Breaking strength is obviously not an issue, but I'm not sure if there's much difference in elasticity between Alu and CF of similar mass.
I have looked at importing spread tow CF at 60g/sqm, but the MOQs are not affordable right now.
I can easily source alu foil at 0.06mm at the supermarket.
I think it's the thickness that makes it as light as CF even though the density is higher. Breaking strength is obviously not an issue, but I'm not sure if there's much difference in elasticity between Alu and CF of similar mass.
I have looked at importing spread tow CF at 60g/sqm, but the MOQs are not affordable right now.
Andre
Looking at your post and the results leads me to this idea:
Take a BMR driver and insert it in the hole in the panel the way you did, keep the surround but fix the BMR diaphragm to the panel at points in the manner you used (but maybe with something other than hot melt, perhaps bridging fins or similar)... The edge of the BMR diaphragm may need some selective reinforcement as it's not as tough/stiff as polycarbonate, and the panel will need to be lightweight. It would be an interesting experiment. I also just happen to have a handful of these drivers ... Just need the time 🥴
Eucy
I've mentioned this stuff before... May be worth a try:
https://ctech-llc.com/en/products/glass-fibre-tissue/
Eucy
Andre,
200g/sqm is about what I've used. so far. It does get more expensive to get thinner. I'm looking at some 90 g/sqm spread tow but haven't pulled the trigger yet.
CF/Epoxy and Aluminum do have a similar elastic modulus, but Elastic Modulus is normalized for thickness, not mass. So the same weight of aluminum will be just over half as stiff as CF/epoxy.
Eric
Hello Eric
My understanding (probably because I was just at the same thinking) is to use household aluminum which is thinner (50µm?). Among the advantages of this aluminum (let's suppose it works!), it is the availability and cost but also the fact the epoxy won't go through it which should simply the process, control and reduce the quantity of it. Seems to me close to what I can do with wood and veneer. Am I wrong?
Christian
You may be right Christian,
A quick search reveals that typical household aluminum foil is really only about 17µm! I didn't realize it was that thin. From an efficiency (B/µ^3) perspective, that could be very good indeed, and possibly much, much better than wood and veneer, even possibly better than CF/PS/CF.
For sandwich composites, the highest (thoeretical) values of B/µ^3 are usually for very thin skins, often thinner than are actually practical or even possible. That is the case for CF/epoxy skins, where about 0.1 mm is about the thinnest that is practical, but while thinner would theoretically be better. So in that sense, the availability of very thin aluminum foil could give it an advantage over CF/epoxy as a skin in a skin/core/skin composite. There are a couple possible pitfalls however:
a) Does the weight of adhesive required offset the potential stiffening gains?
b) Is the skin so thin that on the compression side of the beam/plate that it buckles, and hence, beam theory stiffening is no longer applicable?
c) Is the skin so thin that it fails to "harden" the surface sufficiently to transmit high frequencies?
It seems worth a try. I'll have to run the numbers with a balsa core too.
Eric
Good questions.
Among the possibilities I have in mind, there is the kind of aluminum below. It said reinforced. We could expect the resin fills the small change of height. Easy to find here in almost all the supermarkets.
Carbon fleece - 8g/m2
https://www-carbix-se.translate.goo..._sl=sv&_x_tr_tl=en&_x_tr_hl=sv&_x_tr_pto=wapp
//
https://www-carbix-se.translate.goo..._sl=sv&_x_tr_tl=en&_x_tr_hl=sv&_x_tr_pto=wapp
//
One fundamental factor is being overlooked here
All of these skins being discussed are unstressed in the layup stage.
You can get very significant stiffening from very light stressed skins
That's where yellow trace and similar tissues shine
Soak yellow trace, apply a light coating of diluted PVA or latex, apply the trace, work out bubbles and apply the same diluted adhesive on the face as you work. Weigh down to prevent curling, let dry and repeat for the other side.
Try it .. You'll be very, very surprised just how stiff the panel becomes, for very low weight gain, sure solely to the shrinkage of the paper, something that Kraft paper is to heavy to achieve.
This process kept a lot of aircraft in the air in days past.
Even strips of this will do the job, you don't need to cover the full face
Eucy
All of these skins being discussed are unstressed in the layup stage.
You can get very significant stiffening from very light stressed skins
That's where yellow trace and similar tissues shine
Soak yellow trace, apply a light coating of diluted PVA or latex, apply the trace, work out bubbles and apply the same diluted adhesive on the face as you work. Weigh down to prevent curling, let dry and repeat for the other side.
Try it .. You'll be very, very surprised just how stiff the panel becomes, for very low weight gain, sure solely to the shrinkage of the paper, something that Kraft paper is to heavy to achieve.
This process kept a lot of aircraft in the air in days past.
Even strips of this will do the job, you don't need to cover the full face
Eucy
Good questions.
My take on it is that the adhesive is a big contributer, weight-wise, to the final density of the panel. (a above)
But the skins must be adhered anyway, so make the glue part of the solution:
Use an adhesive that cures hard, (such as resin or PU) and not one that cures soft or stretchy (such as PVA cold glue).
A soft-curing glue is OK on the tension side of the beam, but will not be of benefit on the compressive side of the beam (b above) and this is where a hard-curing expoy resin is ideal. It does not compress, and even ultra-thin alu foil does not Buckle. The foil is there to provide tensile strength where epoxy will crack.
(c) I'm not sure if the final hardness of the skin makes much difference to transmission of HF which I think is more a function of the thickness and softness of the core. And low density EPS is a PIA in this respect. Therefore I make it as thin as possible so that HF is not damped by the amount of soft material in the middle.
Thanks Eucy,
But the thing about a BMR is that it's a Balanced-Mode Radiator. The diaphragm is constructed in such a way that it bends in specific modes that deliver its amazing bandwidth, and the surround is an integral part of that diaphragm. Once we start messing with the mass and compliance of that surround then that bandwidth will be destroyed.
I have been thinking about constructing a larger radius BMR using exciters, but there are physical limits which force BMRs to the (small) sizes that are commercially available.
I've tested 22g/sqm CF tissue.
But it is simply tissue, with short fibres running in random directions. It's almost impossible to lay up since it acts like, ermm... like wet tissue paper, and it bunches up no matter what you do. Or it wraps itself around a roller.
I did eventually get some of the stuff down, but I think the result was not much better than naked resin by itself without the layer of binding fibre.