thin curved and braced plywood for cabinets

[kifeinthesink]

"The curved walls will be really stiff. I bet that two layers (i'd use plywood) of 4 mm bonded together with a west-style epoxy and additional layers of epoxy inside and out bent over an irregualr plywood skeleton will outperform the thick lead damped box you are proposing (and you might actually be able to lift it)."

this was a comment make by Planet 10 in a thread a few years back.

I've been thinking of ways to make a cabinet with a shape resembling a fat aircraft wing and the method he explains here seems like it would do the trick. I never thought of using very thin plywood because I was stuck in the mode of thinking that thick and heavy = good. I would not have tried the type of material that planet 10 mentions but his endorsement is enough to make it worth looking at.

Assuming the cabinet is very well braced does this seem like a suitable method of building a cabinet. Im planning a small speaker, either a MT or MTM so the panels will not be large.

Here's the original thread if anyone wants to see it in context.
link

[GM]

Greets!

Obviously, if monocoque construction can handle the vibrational stress of supersonic/space craft, it will work quite well for an audio speaker. Using wood just increases its damping factor.

GM

[lufbramatt]

Well, during the construction of my last pair of speakers, which use 5 layers of 4mm plywood over a rigid framework, i tested the speakers out with 2 layers to see what they sounded like, just because the boxes were totally covered with wood. There was a very strong resonance at about 700hz that made some tracks unlistenable. The sound was generally a bit loose and boomy, and i wasnt very impressed. From what i heard id say that 2 layers of 4mm ply simply arnt enough.
Certainly, once id got all 5 layers on the sound improved beyond all recognition and the cabinets became much more inert.
However, these cabinets were glued together with vast amounts of ordinary PVA white glue, I can imagine the adhesive has a major effect on how the layers interact with each other, so using an epoxy may have sonic benefits.

The (nearly) finished article- still need polishing and waxing
(weird reflection on front corner is my fone on the table, not a big dent!)

[kifeinthesink]

thanks for that info lufbramatt. Did you use any damping material when you tested out the 2 layer cabinet?

[Landroval]

A guy from a finnish forum is building floorstanders with curved sides and he ended up with a strong skeleton of braces and 4 layers of 4mm plywood on the sides.

[planet10]

Originally posted by lufbramatt
a rigid framework,

What did the framework look like?

West system epoxy is often used with really thin cedar... basically the cedar is there to act as a "holder" for the epoxy.

dave

[nicknz]

Hi,

I'm just building a small pair of 2way active speakers and they are using 2 layers of 4mm ply, the slight twist being there is a layer of 15mm liear pvc foam sandwiched in between

The advantage off the ply/foam/ply is that the resulting panel is very nearly as stiff as a solid wood panel of the same thickness, but less than half the weight, the result is that the fundamental resonance is almost doubled If you want to go a little more high tec you can throw some carbon cloth into the laminate stack

This is fairly standard boatbuildind technology, which is my industry. The laminate can be made into curved panels very easily as the foam can be molded with the use of a hot air gun.

This really is the DIY equivalent to the old celestion SL6i's (or SL600s?) which had cabinets made of Aerolam, to achieve the same high stiffness, low weight properties

regards
nick

[noah katz]

"Obviously, if monocoque construction can handle the vibrational stress of supersonic/space craft, it will work quite well for an audio speaker."

Planes are designed for strength, not sound quality of their airfoils.

Re curved panels, a lot depends on the curvature, more (smaller radius) gives greater stiffness.

[Nemophyle]

it's a question i asked myself too, because i'm so sick of cutting 22mm MDF sheets

can the masters put some clear halogen light on this subject ?

[GM]

Greets!

Planes are designed for strength, not sound quality of their airfoils.

Actually, this isn't true per se since if they're not 'tuned' properly they will either self destruct over time due to vibrational fatigue or shatter rather abruptly from various high stresses, though I was thinking about fuselage construction when I posted this. Regardless, with greater stiffness through high Fs materials and bracing there's less need for damping since acoustic energy falls at 1/f and the stiffer the work 'platform', the greater the electro-mechanical efficiency of the driver(s), up to a point of diminishing returns of course. The wood in between the epoxy 'shell' described in the original post adds a relatively small amount of stiffness and mostly damping, further reducing the need for any add on damping.

Bottom line, the ideal for either is to have no 'sound' quality, i.e. it's mechanical attributes fall outside the desired performance 'window', either above or below, and for wide BW apps such as speaker cabs or planes capable of breaking the sound barrier, etc., 'above' is the most cost effective.