Suggestions for ult. sandwich

[Christof]

Hey folks
I'm in the process of building a pair of Chalcedony speakers from Selah and would like to make the walls and baffles from the best materials possible. Does anyone have any ideas for the best material combination which would have a total thickness of no greater than 1.75"

One sandwich I thought of sandwich, hold the mayo:

.5"mdf + bitumen layer + .25" acoustic cork + bitumen layer + .5" MDF.....all held together with thickened West System Epoxy

peace
Christof

[Rocky]

MDF/epoxy/lead/epoxy/MDF.

The bitumen you suggested will not do much good in mechanical dampening AFAIK, you need mass... in other words "lead"..

3mm lead is usually a nice thickness I think..

[Christof]

Lead sandwich...I like the idea, infact I saw lead sheets at the plumbing supply store today.

I just had a thought, what if I made a 0.5" cavity b/t two layers of 0.75" mdf and filled the cavity with lead shot and epoxy. If the shot is spherical and I fill the entire cavity than I should have the equivilent of, say 0.25" solid lead b/t the mdf.

Christof

[Rocky]

I'm not sure how the lead shots would perform in transforming the box's vibration energy into heat.. I'd go for sheets i think..

[joe carrow]

Rocky, this pertains to your post about fabricating a horn, as well:

Mass does not provide any mechanical damping. I think Till already started this in the other thread.

What provides mechanical damping is friction; viscous, dry, or other. Adding mass decreases a panel's (or horn's) resonant frequency, decreasing mass increases it. Increasing stiffness increases resonant frequency, decreasing stiffness decreases it.

Take a look over here for someone using a bitumen layer:

http://users.skynet.be/accupulse/manger_head.html

Another example of what people have done to increase friction is building a double walled enclosure and filling the space in between with sand. The inner panel vibrates, and the the vibration is dissipated in the form of heat in the sand layer before it can vibrate the outer layer.

A viscous layer of bitumen dissipates vibrations better at a high frequency- it would be wise to have a stiff inner panel, a viscous damping layer, then a heavy outer panel-- basically what they did with the accupulse there.

I don't claim that I could say what scheme would work best- but I'm very interested in this approach, and finding out what benefits it can have for sound quality. Some day I would like to try a few things, including ceramics.

And Rocky, good luck with the horns! It sounds like an awesome project. Like someone else said, it seems like FEA would be the surest way to know without building. I'd recommend the knuckle test. (or in this case, the brass knuckle test )

Christof- not to forget you. That also sounds like a lot of fun! I think that the cork is interesting, but I would use something thicker than half inch mdf. I seem to remember Bob Brines has been using some sort of ceramic plate on the inside of some of his new enclosures... i forget.

Good luck on both of your projects.

[Rocky]

Thanx for correcting me on this, Joe.

[454Casull]

An interior layer of either aluminum honeycomb or open-celled aluminum foam, soaked in liquid bitumen until the cells are filled. Then a simple exterior lay of carbon fiber/epoxy pre-preg composite to provide strength.

The bitumen provides both the mass and the damping.

BTW, you can substitute something like this:

http://www.latexite.com/pr-aptgrade.html

for the liquid bitumen.

[Greg B]

Well, it's all kinda overkill, but sand filled panels work well. You could mix in some lead shot. I don't think filling it in epoxy is necessarily a good idea. Besides the practical problem of heat generated, flash curing and cracking; the damping would be more effective if the panels aren't coupled. However, I would recommend birch ply instead of MDF because of its higher modulus.

Birch or appleply laminated to particleboard is pretty good, and good enough IMO. Plastic resin glue is a little easier to deal with and less toxic than epoxy, and works well for laminating.

Watch your cutouts with a baffle that thick. It *will* pay to sculpt away the backside so there isn't a tunnel. The tunnel can really muck up the mids.

GB

[bzdang]

Lead is incredibly toxic to children under 6 years old. If you have young kids, avoid bringing any lead into your home if possible, handle your electronic solder with disposable gloves and use a work surface that they can't touch.

http://www.cdc.gov/nceh/lead/lead.htm

[NH7RO]

I've heard of some affixing ceramic tiles to the inside walls (or backsides of dipoles) for added damping BUT is ceramic all that good for such a purpose? All the tiles that I pick up at Lowe's and rap my knuckles on have a ringing sound rather than a dull thud, so what's the reason for choosing ceramic instead of something else?
Jeff