Phenolic

[Chris7]

I saw in another thread that people are considering phenolic as a baffle material. I'm curious what makes phenolic attractive for this application?

I am not a materials engineer, but I thought the advantage of MDF was that it had a moderate modulus of elasticity and a relatively high density, both factors combining to make it well damped. The MOE of phenolic is about 4 times that of MDF, and its density is about 2-2.5 times that of MDF. It seems to me that based on those numbers, phenolic would be less well damped and more prone to ringing, with the fundamental higher in frequency. In any case though, it seems to me that using two sheets of baltic birch plywood laminated together gives you roughly the same MOE as phenolic and roughly the same weight, but with the advantage of being much easier to source. For an open baffle speaker, surely the extra thickness wouldn't make a difference?

[timber_mg]

Damping is related to the visco-elasticity of a material so it is possible to have stiffness as well as damping in some cases, though rare and generally easier achieved by a sandwich (CLD). Perhaps it is because one can cast such baffles?

Often phenolic is used as a composite with weaves/papers such as Nomex which is Kevlar paper with a phenolic matrix I believe.

[noah katz]

"The MOE of phenolic is about 4 times that of MDF, and its density is about 2-2.5 times that of MDF. It seems to me that based on those numbers, phenolic would be less well damped and more prone to ringing, with the fundamental higher in frequency."

Higher stiffness and mass both reduce the amplitude to an input, other things being equal.

The other thing is damping, which I imagine is quite a bit higher for phenolic than MDF, even more so than plywood.

[Chris7]

Higher stiffness and mass both reduce the amplitude to an input, other things being equal.

Doesn't higher stiffness increase the amplitude of the fundamental resonance?

The other thing is damping, which I imagine is quite a bit higher for phenolic than MDF, even more so than plywood.

This is where I am not clear on how things work. Intuitively, damping requires some "give" in the material combined with mass, the former factor which works against phenolic. However, I confess I don't have a deep understanding of materials.

[JonMarsh]

Many types of phenolics, depending on the fiber used and the binders. In general, it's denser, heavier, and much stiffer than MDF. Some types are reasonably machinable, some an absolute bear (I speak from experience).

Wilson Audio uses it exclusively in their enclosure designs, which are acknowledged to be among the most strong and inert. Some other former speaker designers I know are rather in favor of it, other than the cost. There's a reason Wilson's speakers are so expensive beyond just markup and expensive Scan Speak drivers.

[Wilk]

Would "Corian" work well at all. It is really dense, and machines real nice, and it doesn't require any finishing other than basic wet sanding. I have thought about using this several times? I don't have any tech specs right here with me, but I could get them easily enough if it was a real possiblity.

[joecarrow]

I've heard of phenolic in one notable DIY project before- here's a link: http://users.skynet.be/accupulse/manger_head.html
Note- they use constrained layer damping.

I've also heard of Corian being used before, but I don't remember any links that stand out in my mind. It should work alright.

The stuff is absurdly expensive for most DIY stuff- you'd better be spending $1k per cabinet in drivers, and have an exquisitely treated room with optimal speaker placement and a well considered listening position before you go all the way to phenolic resin. It might be great, but it's a matter of diminishing returns. If the cabinet walls aren't the weak link, then you won't notice the improvement.

[Chris7]

Would "Corian" work well at all. It is really dense, and machines real nice, and it doesn't require any finishing other than basic wet sanding. I have thought about using this several times? I don't have any tech specs right here with me, but I could get them easily enough if it was a real possiblity.

The tech specs of Corian are here:

http://www.corian.com/corian/documents/pdf/C-1-2004.pdf

On paper it looks better than phenolic. Roughly the same modulus of elasticity, but significantly higher density than most conventional phenolics. Probably easier to source too, being a common building material.

[joecarrow]

Forgot to mention- higher MOE increases resonance frequency, higher density decreases resonance frequency. Mass and damping decrease amplitude.

You can think of it as a spring-mass system where MOE approximates the spring constant, density approximates the mass, and internal damping approximates the damper. Having something that's dense and stiff might ring like a bell if it is not well damped.

If it's a subwoofer enclosure, there's a good chance your resonance frequency can be well above any excitation within the bandwidth. If your sub goes up to 80 hz, and the cabinet resonates at 300+ hz, then you would probably be in good shape. If your midrange enclosure rings at 1khz, then you're in trouble. You'd probably want to pile on some big blobs of viscous stuff to increase mass and damping.

[Wilk]

Yea, I was asking cause I know where to get corian vs phenolic. The cost on it is a little high. Not quite as bad as you are making it sound, but high enough. I would still defintly consider it though for the right design at the right time. I really wanna a good LCR setup for my HT room in the future made outa Corian. Evil Twin has been quitely talking about a design that has my eye for a high gloss corian box. Time will tell.

I am not sure about the possible damping/ringing issues would play out, just have to experiment I quess.

[Martyn]

Joe is correct...

It might seem a little counter-intuitive at first (until you think about it), but high density isn't necessarily what you want in an enclosure material. If you want a stiff enclosure with a high natural frequency, you should use thick, low density material with a high elastic modulus. Note that halving the density and doubling the thickness will give you much greater stiffness for the same mass. Of course, there are practical limits to the thickness that can be tolerated - it might be interesting to experiment with carbon-reinforced foamed concrete, but its WAF would be poor.

Just for fun, a few months ago I ran some comparison calculations on the modes of vibration and natural frequencies of rectangular flat plates. I drew the following conclusions:

Doubling the thickness doubles the natural frequency.
Doubling E increases fn by ~40%.
Halving the density increases fn by ~40%.

This isn't the whole story because there's damping, inertia, and sound transmission loss to consider too.

[PMazz]

I made up a couple of Corian baffles for PJay years ago but I think they were mostly cosmetic.

If anyone's considering working with stuff like this, make sure you have a powerful router (2HP min- 3 better) and your 1/2" shank bits are sharp.




Look up countertop fabricators and look for "solid surface" especially. Small pieces like the stuff above can probably be salvaged from dumpster diving but many places will give away or sell small cut offs. The last time I was at my fabricator's shop he was working some 2" thick phenolic for sink tops. You needed a crane to move it! It's the latest rage right now for kitchens too so you guys may be able to find it around in different thicknesses.

Pete

[ThomasW]

Lots of phenolic here

http://www.usplastic.com/catalog/category.asp?catalog%5Fname=USPlastic&category%5Fname=79&Page=1

[PMazz]

This is the stuff I'm talking about.

Richlite

[Brian C.]

Good luck trying to get ahold of some of that stuff.

Their head office and factory are about 3 miles away from me. I've been there, got the runaround, was referred to the regional rep., and they really have no desire to deal with you unless you're one of their "certified fabricators".

Although, there are two different product lines from Richlite... industrial, and architectural. The industrial product is easier to come by, but only comes in one color (looks like parchment paper). The price? A 4'x8' sheet of 3/4" is only $1100.00. The architectural version is even more.

Here's another option (a similar, competing product), though I think the prices are going to be about the same...

Paperstone

[jdybnis]

I really like how the Paperstone products look. Also they are made with a water-based resin so they might be less toxic to work with.

[noah katz]

"Doesn't higher stiffness increase the amplitude of the fundamental resonance?"

No, the amplitude decreases with the square of freq.

"This is where I am not clear on how things work. Intuitively, damping requires some "give" in the material combined with mass,"

You're not too far off; most stiff materials have very low internal damping.

But a lot more damping can be gotten by using a separate damping element, like the CSD (constrained layer damping).

"The tech specs of Corian are here:

http://www.corian.com/corian/documents/pdf/C-1-2004.pdf

On paper it looks better than phenolic. "

According to that the modulus is 1.5 Mpsi, 1/20th of steel.