Combining constrained layer damping and bracing

[rick57]

I’ve cut 9 mm ply into pieces, and am about to assemble my first speaker project (training wheels for more serious projects) using Usher drivers in a two way, in a box 400 H * 240 W *300 D mm (16* 10* 12 in).

I’ve have read quite a bit about different approaches, eg from a US speaker god
“no un-braced box panel area should be larger than 4 inch squared for 3/4 inch thick wood panels”.
I haven’t time to go that far, but was thinking of slightly staggered front to back bracing at about 100 mm (4 in) intervals, creating panels about 100 * 300 (4* 12 in).

I want to combine bracing and constrained layer damping (probably liquid glue, plasterboard & two layers of bituminous sheets, maybe + masonite).

Should I take the easier approach of assembling the inner layers of the CLD inside the box, then brace across the inner layer?

Or, should I brace the outer box, and then do the CLD layers in between the braces – which with smaller pieces would take maybe twice as long?

Thanks

[rick57]

Someone to answer a dumb newbie question?

[joe carrow]

I'd say that you want to brace the inner box, then build box around the box. Seriously though, 9 mm seems to be some seriously thin plywood!

I think that for ply of that size, you want to have a LOT of bracing- have a look at the matrix stuff B&W uses. In my opinion, that's what you need to immitate for such thin ply.

Also, I'm not quite sure I can visualize with building CLD inside of a braced box. The important thing with a contstained layer damping system is that there is no solid connection between the internal box and the external box- no wood touching! That means that if the internal box flexes a little bit with a bass note, the flexing deforms the bitumen (or sand) and this deformation is a very lossy way to transmit the mechanical energy to the outer box (and thus the air). The viscous layer turns vibrations to heat.

Anyhow, if it comes down to actual construction techniques, i don't know a whole lot about saws and nails. Good luck with your project, it sounds like it's like you're determined

[rick57]

Yes 9 mm is pretty thin, but is to be combined with sheetrock (plasterboard) and extra plywood.

>I'd say that you want to brace the inner box, then build box around the box.
I was going to go from the outside in - box, bracing, then between the braces with CLD - any other opionions?

[kfr01]

I think using thicker wood to begin with would be a much more worthwhile exercise.

I agree with what Joe said.

[rick57]

Thanks guys, if it’s two - nil, I’ll probably double the thickness of the outer box.


And the question of the best (easiest?) way to both brace (not up to B&W levels!), and incorporate CLD - any opinions??

[rick57]

I recall now (from a couple of years ago) that the idea of getting away with a thin box was from UK makers eg Rogers, who IIRC used thin boxes combined with heavy linings.

I was thinking of updating a thin box with bracing & CLD.

Being only a bookshelf sized speaker, I’m not worried about it getting too heavy.

I’m really trying to optimise sound quality for minimum (slow) construction time!

Thanks

[rdf]

Originally posted by rick57
I recall now (from a couple of years ago) that the idea of getting away with a thin box was from UK makers eg Rogers, who IIRC used thin boxes combined with heavy linings.

I owned a pair of JR149s. The cabinets (working from memory here) were ~ 1/16" aluminum formed into a cylinder and the interior face lined with self-adhesive 1/4" bituminous damping pad. I never considered it a success, at low to moderate levels the cabinet vibrations could very easily be felt.

If it helps any, and I'm by no means qualified in the field, this is just what I've read, CLD generally means sandwich construction of a very thin layer of damping between two hard materials, typically one of which is the target for damping. Most of the material I found dealt with industrial machinery and ship building. My suggestion would be to play around glueing up a couple of mock panels and experiment, for example sandwich 1/8" linoleum floor tile between two pieces of your 9 mm wood and see (hear) what happens. You might even be better off glueing a hard material, like 1/8" aluminum, between the wood panels but that gets expensive fast.

One handy tip I do have CLD experience with: polyurethane glue. Awesome stuff, foams to fill voids and dries super hard, super light and super damped. For a speaker project I'm half-considering as the external skin of a cabinet two layers of 1/16" aluminum separated by plastic screen soaked in polyurethane glue. The glue would be the actual damping substrate of the CLD construction.

[rick57]

Re CLD: If polyurethane glue dries hard, that wouldn’t ‘work’.
The inner material needs to be able to move, as it does dissipating energy and generating a degree of heat.
When googling on CLD, as originally it’s an industrial approach, you need to add eg +speakers.

An aluminum cylinder is very different from the normal BBC style Brit speakers.
I’ll research the BBC style Brit speakers when time allows, and be more specific about how they worked. I think Linkwitz advocated a similar approach c. 1980

[rdf]

Jim Rogers designed the JR149 loudspeaker, that's why I thought it's the one you meant.
Polyurethane glue doesn't dry brittle or crystaline like model airplane glue for example, it can be bent and trimmed with a knife. A very interesting adhesive, bonds anything.