[FullRangeMan]

.Pure carnauba could be applied with heat instead of solvent, maybe an advantage.

I can be wrong but, apply a kind of oil in the panel will made it no harder, just heavier, as a oil is a heavy liquid.

I have applied over 10 years a shining fixative varnish spray in the woofers paper/polipropilene cones (formula=Resin acrylic+Treated Aliphatic Hydrocarbon), also it do not made the cone any harder but prevent attacks by humidity, mold and sunwear.
There is two types of spray can paint, a opaque and a shining, both transparent, very low price.

[sedge]

 While sitting here at the computer reading these posts I have been thinking about nxt ,and how they say you must place exiters around the panels in certain places to cause maximum exitation [spl]so that the hole of the panel vibrates at the same time .
podium on the otherhand mentioned something about it acting  as an acoustic horn ?
podiums exiters  are positioned in a centrol line down the panel , I would expect the sound waves to move  out and away from the centre much like the stone in the pond .The wave front is more natural  and eminates from a thine  line source.This is a hybrid speaker and I like  how it sounds,long live whatever it is called !.

[bobloblob]

I think the statement from NXT refers to smaller panels.  In that case, they may be trying to get piston-generated bass frequencies by moving the whole panel in the same plane - which would have to be quite rigid.  By placing them in line, you should have bending-wave generation of the bass, transitioning to dml mode at higher frequencies.  I think spacing between exciters may be critical for the frequencies where this transition occurs.

Ziggy seems to have found a good compromise in his speakers on where and how this occurs, as has Podium.

[mkstat]

.

[bobloblob]

If you are looking at coating materials for the balsa, you might check out water-base polyurethane.  It tends to penetrate less than most other finishes.  Also, there are cross-linkers you can add which will increase its hardness to nearly the level of catalyzed polyester (aka boat resin).  If you want something that will penetrate and make the wood harder, there is a product called Wood Hardener, made by a company in the U.S., Minwax.  It is formulated to penetrate old rotten wood and set up inside, making it much harder.  I have poured the stuff on an old timber and watched a puddle of it get completely absorbed by the timber, and not just in obvious voids.  It should work on cardboard, too, except it will not pass through the glue where the cardboard is glued together.  It should stiffen the balsa, but will probably increase the mass a little, so I don't know what the tradeoffs may mean.  There are also some penetrating epoxies for the same purpose.  I think you may want to glue two thin sheets of balsa together, crossing the grain, so that they will reinforce each other, otherwise you will have a panel that is very flexible along the grain.  I don't think you will get the results you want from balsa when it is used in large areas.  For one thing, this honeycomb look of the grain is in the wrong direction to give the reinforcement you need.  Hard to beat hexagons and triangles for strength, but they need to be perpendicular to the sheet, I think - at least in this case.

I think the reason the sound level is relatively stable is that, like line arrays and large panels, particularly the narrower ones, the sound is spreading as a cylinder, rather than speading from a point, so the drop-off with distance is much less, and why they use line arrays at large concerts.

Part of the reason for the clarity - and there are other reasons, I know - is that, like with other panels, you are listening in the near-field.  For typical cone drivers, you have to get very close to be in the near-field.  With these, everywhere in the room is most likely in the near-field.

When I tipped my hat for you for this project, I didn't mean that you develped it along with Podium, sorry for that misunderstanding, just that you started this thread and had the get-up-and-go to step out into new territory - even without the background - where no one else was, and I appreciate that sort of gutsiness - or loopiness, which is probably a better way to view those of us in this hobby.  Hang in there.

[el`Ol]

If you are looking at coating materials for the balsa, you might check out water-base polyurethane.  It tends to penetrate less than most other finishes.

sounds interesting
@Fullrangeman: I mean carnauba wax, not carnauba oil.

If you want something that will penetrate and make the wood harder, there is a product called Wood Hardener, made by a company in the U.S., Minwax.  It is formulated to penetrate old rotten wood and set up inside, making it much harder.

Probably not so interesting. The balsa is hard enough by itself, the task of the coating is to increase damping and get rid of resonances.

I think you may want to glue two thin sheets of balsa together, crossing the grain, so that they will reinforce each other, otherwise you will have a panel that is very flexible along the grain.  I don't think you will get the results you want from balsa when it is used in large areas.  For one thing, this honeycomb look of the grain is in the wrong direction to give the reinforcement you need.  Hard to beat hexagons and triangles for strength, but they need to be perpendicular to the sheet, I think.

What's easily available for hobbyists (just shipping won't be so easy and inexpensive for Podium-size sheets) is 3-layer plywood with fibers perpendicular to eachother. This is what I mean. It would probably be a big advantage if there was at least one layer with fibers perpendicular to the sheet surface (end grain wood) like Goebel use. I have no idea where to get something like that.
BTW, when I remember correctly I have read in a hifi test magazine many years ago that the Goebel also has layers of glass fiber. I believe that the end grain wood layers mainly have the task of damping here.

[bobloblob]

One of the problems with balsa is that it will damp out high frequencies like foam.  Knock on a block of balsa and you will see what I mean.   I think you may run into the same problems as with Gatorfoam, where the exciters are producing treble, but it is coming off the back, rather than passing through the material.  This is why Nomex works so well.  The honeycomb material between the sheets is very rigid, and the high frequencies pass through to the front sheet very easily.  A honeycomb shape is useful here because it has the most useful shape for strength to volume.  It is very efficient, needing less material and leaving more space open, thus keeping the mass low while transfering energy.  If you could use the balsa on end and also in a honeycomb pattern, perhaps even a simple corrugated pattern, you would have something, but even then you would need to harden the balsa to prevent high-frequency absorption.  The three-layer is good stuff, but much heavier than balsa, and may bend even more easily than layered balsa.

Carnauba wax is great stuff, I've used it often, and I think it is the hardest of the natural waxes, but I don't think it has the type of strength you need.  The front and back sheets need to be tough, and as rigid as can be, without being brittle, in order to bring the dml operation into as low a frequency range as possible.  Correct me if I am wrong about that.  What would be wanted I think would be large single sheets of graphene on both sides.  These could be bought for only several billions of dollars in the next few years.  Until then, compromise, compromise, compromise.

For what it's worth, and that's not much, I am in the process of moving, and have neither the money nor the time right now to experiment with the panels, though I bought a bunch of the $5 exciters.  What I spout off here may or may not be useful, so please don't take it as gospel.  I look forward to the results you and others are getting in spite of any nay-saying.  I am learning a lot.

[el`Ol]

One of the problems with balsa is that it will damp out high frequencies like foam.  Knock on a block of balsa and you will see what I mean.

When I knock on a 500x100x1 mm balsa sheet its sound is somewhere between normal wood and metal. In fact one has to distinguish between the behaviour along the fibers and perpendicular to the fibers. Once I did an experiment with one layer balsa excited by a magnetostatic drive. In the measurement the highs were there, but it sounded what I call foggy, something I also associate with the Podium Sound or paper dual cones. Interestingly with the dayton exciter on three layer balsa this foggyness was not there. But I didn?t do measurements.
When the damping is enhanced by coating, will it sound foggy again?

[FullRangeMan]

The Balsa wood I see here is light but very soft or fragile, maybe after a spray of varnish or sealer it hardens.
A GatorFoam panel with Balsa wood in the center could be good...  (polyethylene layer+Balsa+polyethylene)

[el`Ol]

The Young's modulus of balsa along the fibers (max value) is higher than that of polyethylene.
http://www.matbase.com/material/wood/class4-5-10-years/balsa/properties
http://www.engineeringtoolbox.com/young-modulus-d_417.html

I had a look at 2.5 mm polyethylene sheets recently and they looked suitable for DML. Not as dead as acrylate and not as alive as Polystyrene. But I don't expect the linear behaviour of wood or honeycomb material.

[FullRangeMan]

Do you think it is possible make a sandwith with PE+balsa+PE??

[bobloblob]

Maybe the balsa I have is a lighter version.  When I knock on it, it is nearly as dead as styrofoam.  If it is layered across the grain and glued, it may harden some from the glue, and the layering will increase its stiffness too.  While single sheet balsa is rigid in smaller pieces, it is very flexible across the grain in larger sheets.  Along the grain it remains fairly stiff.  If it works for you, that's all that matters, though.

One of the problems with the material used between the front and back sheets is that there can be a deadening effect from creating what amounts to constrained layer damping properties like that used in sound isolation.  When the material in the middle of a layer resonates at different frequencies from the front and back materials, sound frequencies don't travel through them well.  With thin sheets front and back, this may be less of an issue, but I think it would be best to find a really hard but light material for the middle layer.  That's the reason behind Nomex.  If you can get balsa that is less spongy, that would be great.

There was stuff that was used in the surfboard industry to fill dings - tiny glass bubbles that seemed nearly as light as the air that filled them, and extremely hard.  Not expensive either.  You do need to wear a dust mask when working with the stuff, though.  Perhaps a layer of this in an appropriate medium trapped between two sheets might be worth looking at, using something like thin strips of balsa for ribbing.  Then again, perhaps the middle layer could be merely thin strips of balsa used perpendicular to the front and back sheets, all layed out in some sort of grid, in a rough approximation of Nomex.

[jeffac]

What about 6 mm light aircraft plywoods such as poplar "Lite" Ply or the better quality but somewhat heavier and more expensive spruce/birch plys. Weight/density of Lite ply seems to be in the order of 3 x that of balsa, but in comes in large sheets. Just a thought. Could be varnished with natural resins for additional surface rigidity.

http://www.aircraftspruce.com/catalog/wppages/liteply.php
http://www.airfieldmodels.com/information_source/construction_materials_for_model_building/plywood.htm
http://www.aircraftspruce.com.au/WOOD.pdf
http://sites.google.com/site/theskydartteam/references

[el`Ol]

Do you think it is possible make a sandwith with PE+balsa+PE??

Sandwiches with a damping balsa middle layer are common. But this is usually end grain balsa (fibers perpendicular to the plate surface). Concerning linearity / neutrality I would prefer aluminium or titanium as cover layers.

[FullRangeMan]

Sandwiches with a damping balsa middle layer are common. But this is usually end grain balsa (fibers perpendicular to the plate surface). Concerning linearity / neutrality I would prefer aluminium or titanium as cover layers.

Alcan have various laminate boards with aluminium, as Alucobond, all ALU board names are aluminum sandwich, but they are all havier than Gatorfoam.
A big gray titanium naked or laminate plate could be great, except for the price.

[Angaria]

Potentially useful list of materials http://books.google.com/books?id=a93mICgjuzMC&lpg=PA103&ots=7421kLbM2_&dq=fiberbrite%20sheet&pg=PA103#v=onepage&q=&f=false

the price on fiberbrite stuff is right.  http://www.americanfibertech.com/webpages/050_category.asp

[el`Ol]

A big gray titanium naked or laminate plate could be great, except for the price.

At the Highend in Munich this year I heard the Blumenhofer Fun 17 and I really liked it. Later I discovered by googling that it uses a carbon fiber / polymer sandwich cone which is very well-damped and suitable for simple crossover. Strangely this speaker has a very open sound, no comparison to the muffled sound of polypropylene, bextrene or the polymer mix used by Harbeth. Here is the stuff:
http://www.victrex.com/en/products/aptiv-films/aptiv-films.php
Could be a hot candidate for laminating on titanium. BTW, what do these titanium sheets cost? Are they more expensive than aramide honeycomb?

[FullRangeMan]

Could be a hot candidate for laminating on titanium. BTW, what do these titanium sheets cost? Are they more expensive than aramide honeycomb? 

By what I know TI is very expensive, primarily it is a military metal and for the army money is no problem.
Could be fine if a single layer TI panel could do the job, a sandwich is a more complex issue.

[el`Ol]

Visaton say on their discussion-forum that pure metal sounds awful.

[FullRangeMan]

Visaton say on their discussion-forum that pure metal sounds awful.

Thankyou for this info, it spare us time and money.