Making a dense nonresonant material

[Few]

While experimenting with ways to fill some gaps in a fiberglass speaker baffle I've been working on forever, I came up with a combination that might be of use to others as well. I combined some two part polyurethane with sand and ended up with a very dense material that seems, at least by the knock test, to be highly nonresonant. It would be easy to mold into a variety of shapes including an acoustically inert speaker baffle.

I half filled (actually a bit less than half full) a plastic cup with the two parts of the polyurethane and poured sand into the mixture before it started to thicken. For no good reason I put the mixing stick into the mix and held it there while everything cured. After the polyurethane hardened I peeled away the plastic cup, which was easy because the polyurethane did not adhere to it at all, and ended up with a "sandsicle." The hardening process took about fifteen minutes and was only mildly exothermic. The photo below shows the result.



The polyurethane tends to foam up if any moisture gets into the mixture, so it's important to use dry sand.

I used TAP Quick-Cast polyurethane because I had it on hand, but I've just ordered something that I think should be similar but considerably less expensive. It's Casting Resin from AeroMarine Products.

I've seen others mix sand with epoxy, but the polyurethane is cheaper and I find it a bit easier to use. Since the sand makes up more than half of the final volume, the cost for a speaker baffle might not be out of reach. The best part is that the result feels like a highly damped version of Corian. Since it's designed for casting in molds, it'd be quite easy to mold a baffle with the driver cutouts and smooth contours built right in.

Few

[Cal Weldon]

Hi Few, Interesting stuff, thanks for sharing.

Have you done other kinds of tests on it? Like what happens when you drop it on the floor or what kind of thud do you hear when you bang it on the table? How about destructive testing as in cutting with a knife or saw or putting it in a vise and squeezing it? I love to fool around with materials like that so I am very curious.

[Conrad Hoffman]

Maybe you can cast the excess into a small block and use it as a sanding block! I knew someone who made abrasive blocks out of 2-part RTV and silicon carbide compound. IMO, this is a similar road to what various commercial turntable manufacturers have done over the years using various fillers. I'd think with some experimentation you could get optimal damping- people seem to dislike materials that are both too live and too dead.

[ChrisA]

Originally posted by Few
.... I combined some two part polyurethane with sand and ended up with a very dense material that seems, at least by the knock test, to be highly nonresonant. It would be easy to mold into a variety of shapes including an acoustically inert speaker baffle....

My dad used to be into HiFi back when I was was a kid. So this is 50's and 60's He was telling me about a speaker maker back then who did high end speakers and cast the boxes out of concrete, about 3 inches thick. Heavy but were absolutly 100% "dead". Another method from that era was to build a double wall box with an inch or so between the walls then after placing the speaker in the listening room, fill the space with dry sand. Sand between two sheets of plywood is likely even more dead than concrete. This was all before modern materials like MDF were available.

There are lighter bulk fillers that do about as well as sand. The one I like is "Micro Balloons". THese are little hollow glass spheres. A bucket of these weights about as much as an empty bucket. No much more than air, as the glass is very thin. You can mix these with any resin. Epoxy is nice but cheap polyester works. I like to add chopped fiber for more strength. The result is a hard dense foam that is very strong. I've tested it with a ball peen hammer. You can adjust the mist to make it any strength you need and even combine mixes on one part.

The big problem with cast parts is that you have to make one wood and/or clay or plaster part, then paint polish and wax it then make a mold from it. It's worth it if you make many parts from the mold but not otherwise.

One of the best uses for "micro" slurry is to combine it with other material. For example lay down one ply of glass, carbon or kevlar cloth then a thick layer of micro balloon slury and then a skin of cloth. The result of very, very ridged and light. Almost like a modern surf board but with stringer skins. No exotic tools. Just scissors, paint brushes, wooden sticks.

[Few]

Cal: Sorry I'm not set up for quantitative torture tests, but I did determine that it wasn't clear whether the concrete wall in my basement or the sandsicle was going to come out the winner. If I hit the edge of the sandsicle against the wall, I could manage to dislodge a very small chip from the sandsicle. Otherwise, it takes a hell of a lot of abuse. My vise didn't even leave a mark on the surface when I squeezed as hard as I dared, and I certainly didn't see any distortion of the shape.

When I hit the sandsicle against the concrete wall it sounds something like hitting the wall with a rock, but with more damping. I wouldn't call it a thud, but it doesn't create the "click" associated with something very hard and undamped either. Sorry I can't do a better job characterizing the stuff.

Conrad: I seem to vaguely remember an article in Speaker Builder many years ago in which someone reported on an epoxy/sand/pulverized rubber mix. I've found epoxy to be pretty expensive if you need a lot of volume, which is why I was exploring other options. It seems to me that the polyurethane is less brittle than epoxy so I think it contributes some of the properties that the rubber did in the epoxy/sand/rubber mix.

ChrisA: Thanks for the ideas. The loose sand approach definitely leads to a dead material. This polyurethane/sand concoction has the advantage of being self supporting like concrete, but with better damping. I wouldn't pitch it as the answer to all speaker construction problems, but I do think it would be very well suited to some applications.

I've got some microballoons on order with my next batch of polyurethane. I'm happy to have the mass of sand in the particular filler application that got me started down this path, but I wanted to try microballoons instead of the talc I've used previously to thicken epoxy for other uses. I'm looking forward to experiencing the light as air effect first hand.

I've been doing some molding projects by building the mold directly instead of building a plug and then making a mold from it. For some situations it works well and it saves a step. I like the fact that you can choose something easy to shape when building the mold and then choose other properties for the final product.

Thanks for the interest.
Few

[cuibono]

Chris A, where do you get your poly resin? I'm in the same area as you, and I'd like to buy it locally.

[Conrad Hoffman]

Micro-ballons are really neat- in a jar they act just like liquid once you get them moving. One thing- never breath them. Wear a dust mask. I've used urethane casting materials (TASK9) to make replacement parts like gears, and they work great. You can replicate things to a few thousandths of an inch in a silicone mold The only downside I found was poor temperature resistance. Not a problem in the living room, but easily a problem in, say, a hot UPS truck. What works great for the diy-er might not be a good commercial solution, thus the use of epoxies.

[ChrisA]

Originally posted by cuibono
Chris A, where do you get your poly resin? I'm in the same area as you, and I'd like to buy it locally.

It's really easy to find. The quickest place, although not the cheapest is West Marine. They also sell the "west" (no relation, different companies) epoxy system. You can pick up any of the west system fillers and mix with cheaper poly. The West epoxy is first rate, thier poly is generic. There is a WM on PCH at the end of 190th by King Harbor. The other bigger WM store is near MDR. (To make this relivent to you non-locals WM was a web site.)

If you need more than a small amount or don't want "generic" resin and know what to ask for (i.e. "5 gal drum of Isophalic polyester") there is a plastics shop on Normandy on about 200th in Torrance "South Bay Plastics". They sell to both home and industrial users.

I used to build racing kayaks and was in there all the time. Now the stuff I build is smaller and I'm a fan a West System epoxy.

[Cal Weldon]

Originally posted by Few
[B]Sorry I can't do a better job characterizing the stuff.

[B]

Not at all, I think you did a great job, thank you.

[Few]

Glad it was useful Cal.

I started to post something on one of the other threads that addresses the issue of damping cabinets but didn't want to chime in several months after the original discussion took place. Since I'm pitching the polyurethane/sand mix as being of interest because of its density and inherent damping, maybe it'll be appropriate to make my points about Q values here instead. For those unfamiliar with the parameter, Q factor is short for quality factor. High Q resonances ring for many oscillations while low Q resonances are said to be more damped--they die off after undergoing fewer oscillations. But I'm getting ahead of myself...

There seems to be a persistent point of confusion concerning the behavior of high Q resonances. In an old thread concerning cabinet constructions damped cabinets are described as sounding muddy when compared to cabinets with high frequency weakly damped (therefore high Q) cabinet resonances. Undamped and stiff cabinets are said to sound better because:

...the resonances are high Q, if they do get excited they quickly release the energy, so that it is not stored up and released with a smooshing./masking affect on the low-level details.

This caught my eye because high Q resonances actually take longer to decay than low Q ones. In fact, it is a defining feature of a high Q oscillator that it stores energy instead of dissipating it as heat.

The quality factor can be expressed as the number of cycles an oscillating systemgoes through before the amplitude dies off to a small fraction of its original amplitude (I've seen that fraction expressed as 1/e^(2 pi) but haven't verified that). A high Q oscillator therefore undergoes more oscillations before the amplitude dies away. Of course if two systems have the same Q and one has a higher resonance frequency than the other, the higher frequency system will stop oscillating sooner, but it'll be no less resonant.

The point of damping an oscillating system is to dissipate its vibrational energy as heat rather than to allow it to show up as an organized vibration. Statements that low Q speaker cabinets are bad because they store up energy and release it later as audible vibrations seem to me to be inconsistent with this pretty firmly established fact.

It is true that high Q resonances have narrower bandwidth and are therefore less likely to be excited by a continuous sinewave that's somewhat off resonance. Music contains many transients, though, and these necessarily contain a broad range of frequencies leaving few spectral gaps for a high Q cabinet resonance to hide in. In other words, the cabinet's high Q resonance may have a narrow bandwidth but the signal exciting it most likely will not; there's still likely to be significant frequency overlap between music signals and the resonance.

So much for the high Q (haiku?) lecture. Sorry if I'm guilty of hijacking my own thread.

Few