Engineered tuba revisited


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Posted by Eric on December 19, 2003 at 04:05:31:

Scanning the archives, I stumbled across a thread entitled "The engineered tuba" where folks discussed how the tuba would look had it been invented in the present day. I've been thinking about that topic and here's what I've come up with:

The modern way to make complex tuba-like shapes is to mold them, so my tuba body would be made in two pieces sandwiched together. It would form a nautilus-like spiraling tuba, perhaps even a round spiral. The material could be any composite like carbon fiber or fiberglass, vacuum formed plastic, or even brass where the metal is propelled onto the mold using explosives. I would particularly like to try making a tuba from porcelain or that concrete MIT students use for their concrete canoes.

Piston valves are very complex to make and demand a lot of precision. Rotor valves don't seal as well as pistons, have sharp bends, and the shape of the path through the rotor can cause an acoustical discontinuity. The easiest valve to make is an old fashioned disk valve where the rotor is a disk with two parallel 180 degree bends of tubing that switch between 4 tubes in a square connected to the flat face the disk rotates against. The bearings could be simple oilite sleeves or even miniature ball bearings. The rotor could be made from aluminum or magnesium for light weight, although with its ease of forming, thin brass might be lightest. The simplest spring would be a torsion spring, basically a long, straight piece of piano wire along the axis of the valve and attached to the valve tubing (which is parallel to the axis of the valve). A 6 port valve can be configured to engage one loop of tubing when rotated 30 degrees one direction and a different loop of tubing when rotated the opposite direction. With a suitable linkage, a full double tuba could be made using 3 6 port valves plus a traditional 4 port 4th valve instead of the usual 3 8 port valves plus a 6 port change valve used on double french horns. Another advantage is that all the valves and tuning slides can use identical U-crooks, maybe even cast crooks with a more acoustically ideal shape than round.

Instead of conventional valve oil, the valve seal would be provided by a neodymium ring magnet around each port with ferrofluid (magnetic oil) permanently suspended by magnetism around the periphery of the port. Ferrofluid seals are commonly used around rotating shafts for high vacuum work. The flat ports of the disk valve make finding a suitable magnet much easier than for normal valves. Such powerful magnets might cause noticeable drag in a fast spinning metallic valve rotor due to eddy current losses, so a nonconductive rotor might be better.

For the valve actuation I would like to try something akin to bicycle brake cables connected to a moveable fixture (oxymoron?) with the valve buttons. Since all the valve tubing is parallel, the fixture could simply clamp to the tubes and slide and rotate to whichever position is most comfortable for the player.

This wouldnt be a lap instrument, it would have a permanent leg that rests on the floor and adjusts for sitting or standing, or collapses completely for transport.

So, what do you think?

-Eric Bamberg




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