Sorbothane to replace felts?

[J.Harris]

A question for all the techies- has anyone ever used sorbothane as a replacement material for piston valve felts? To those not familiar, sorbothane is a visco-elastic material engineered for shock absorption and vibration dampening. It comes in several durometers (hardnesses) and would seem to be an ideal material for synthetic felts.

I recently (yesterday) purchased a pre-owned instrument that has the Baer PVAK system installed. The horn plays terrifically but the return stroke of the valves seems a little too loud to me. I was wondering if the sorbothane might be a viable alternative to the brass bushing/o-ring set-up. No offense intended to Mr. Baer- just looking to improve further upon a successful innovation.

Thanks for your comments.

[imperialbari]

Wouldn’t sorbitol give an even sweeter bounce?

K

[J.Harris]

For a better description and specs- you can go to http://www.sorbothane.com" target="_blank" target="_blank .
The stuff resembles a rubber but without the bounce back tendencies. It can be "cast" into various shapes but most likely would be most useful supplied as a thin sheet. Probably the most common use for this stuff, other than in industrial applications, has been as a shoe insole. I assume that using a thin piece on top of a plastic or brass washer may be the best application as it would allow minimal compression of the sorbothane while retaining its vibration dampening qualities. You can order a 12"x12"x1/8" sheet from edmunds scientific for $20-$30.
Hope that gives you enough info to form an opinion. Thanks- Jason

[J.Harris]

Anyone else have any input about this idea? Thanks- Jason

[Brucom]

. . . the stuff that I use...It is available in about eight densities, but the middle density offers the best compromise of quietness, . . .

So, what's your favorite stuff called, and which is the middle number in a set of eight?

[pulseczar]

I've used sorbothane to help quiet the lower stack of a saxophone. It was the perfect compromise between quiet action and a crisp response on the upstroke. I haven't used it for brass instruments but I'll definitely try it next time I get the chance.

[Rick Denney]

So much of this is a matter of feel that it would hard for anyone to render an opinion unless they tried it. Since nobody seems to have tried it, there is ample opportunity for someone willing to experiment.

The compressive stress of their hardest material is 30 psi at 20% strain. Let's ponder that. The bearing surface of a valve cap may be about .4", and the hole for the stem .2". The bearing surface therefore has an area of 0.09 square inches. Let's call it 0.1 square inches and be generous.

That means that the Sorbothane will compress 20% with a three-pound force. If the washer is .16" thick (as is the case on my Holton), that means a three-pound force will change the valve alignment by .03". That's enough to be a problem. Considering the dynamic, etc., I don't think that force is too high for the downstroke, especially for those of us with ham hands.

But for the upstroke, I doubt a piston will put more than a few ounces of force. If the strain is linear (it is during compression, but not during relaxation, which is the damping effect), that strain would be more like a couple of thousandths. That's within acceptability, I would expect, especially for most instruments that have that much error in the way the ports are aligned.

And the damping (note: NOT "dampening", which is what I do to the inside of my tuba with my warm, humid breath) is good at its own resonant frequency, even when that frequency is quite high.

So, I would think it a good choice for the rebound bumper, if you can buy it in the right range of thicknesses (which I bet isn't that easy, especially in small quantities).

Rick "predicting the main problem being one of availability" Denney