Air leaks at receiver/shank interface. Problem?

[cheburashka]

I've been going through my old BBb Mahillion, looking for leaking tubing junctions and cracks and soldering loose braces. One small crack in the tubing that exits the valve cluster was really watering down the tone. I finally got everything soldered tight with one exception--the seal between the mouthpiece shank and the receiver leaks a bit. This is especially noticable when I use a Kellyberg. I assume that the receiver is out of round by a bit. Is this going to have much of an effect on my sound? Is there anything I can do about it, short of taking it to a good tech (who's 180 miles away. . .) ?

[Donn]

I haven't played around with Lexan, but maybe if you heat the receiver enough, the Kellyberg will soften up and make a perfect fit? Then you can follow up with a report on the incredible, revolutionary tone quality achieved with perfect-fit shanks. Maybe insert the mouthpiece after heating, lest the shank get heated through and sag or something.

I routinely wrap my mouthpiece shank with a thin plastic film (from a plastic sack or something), because I'm hoping for a better seal, a little protection to the shank finish, and anyway it helps keep the mouthpiece from falling out accidentally. You've probably thought of that already. Ironically my Kellyberg is the one mouthpiece that doesn't get that treatment, because the plastic doesn't cling to it and I expected it to conform slightly better on its own.

[Chuck(G)]

Is there anything I can do about it, short of taking it to a good tech (who's 180 miles away. . .) ?

I'll bet that a quick twist of Dan Schultz's #5 Jarno taper reamer would fix that right up...

[Alex C]

If you want to try something that will work but not affect your instrument, get some white plumber's tape (teflon tape) and make a wrap around the mouthpiece shank once.

It usually works best if you wrap it about a quarter inch from the end of the shank. Once you've inserted the mouthpiece into the receiver you can tell where it's making contact; the tape will still be white where it's <b><i>not</b></i> in contact and almost clear where it <b><i>is</b></i> in contact.

You may have to make a double wrap around the shank to make a complet seal, but start with a single wrap. You will notice a difference immediately.

I wouldn't use a ream on the receiver.

[Dan Schultz]

Is there anything I can do about it, short of taking it to a good tech (who's 180 miles away. . .) ?

I'll bet that a quick twist of Dan Schultz's #5 Jarno taper reamer would fix that right up...

You're right, Chuck!

[iiipopes]

Well, it depends on the receiver. If the receiver is just worn, a little help to recenter it would be fine. If it is an odd size or an historical/collectible receiver that may be difficult or impossible to duplicate, probably not. Unfortunately, of course, the ultimate correct fix is to do just that: have it taken off and resoldered. But just like putting a rubber band around a spit key with a broken spring, the tape will work until you can make an appointment when you are going to be there anyway for another reason.

BTW: please don't heat up the lexan. One miss and you will melt it, ruining the horn, ruining the receiver, and setting off noxious fumes.

[cheburashka]

That's one of the things I was wondering about. If you cushion the metal/metal contact (or the lexan/metal contact) wouldn't that deaden the sound wave? Wouldn't even something as simple as plastic wrap do the same? It would be so much easier if this was all intuitive, i.e. air flow rather than sound wave transmission.

I'm going to try a light lapping of the shank/receiver, using a mouthpiece that already has some wear to the shank, and a fine brass polish. The other thing I'm noticing is that each mouthpiece has a "sweet spot" where it settles in at its best.

[Donn]

Intuition is an understatement. Lots of people here seem to believe that mouthpiece vibration is an a factor, but lacking any explanation for it in terms of physical phenomena, I think it's also reasonable to believe that it isn't - that the tuba's voice is solely due to air flow (suitably modulated.)

Check it out for yourself, you may come to a different conclusion than I. Just don't take what people here say at face value, or you'll end up buying a titanium mouthpiece.

[windshieldbug]

The importance of having the correct mouthpiece can not be overstated. The choice can affect your entire life. Just ask OJ.....

"If the 'piece don't phit, then you must acquit!"

[cheburashka]

I believe the proper rhyme would be:

If the piece don't fit,
You'll sound like. . .a guy with a leaky mouthpiece interface. . .it.

[Rick Denney]

But I believe it's more important for cheburashka to get his leadpipe rounded in order to stop the leaks. I would try to accomplish this using metal.

I don't say that all plastics are bad for this purpose, but polymers with low stiffness, (= low Young's module), like epoxy, generally are. Other polymers can function good acoustically (like Kelly lexan mouthpices, which I have not tried yet), but I don't know if these polymers can be used for "bodywork".

Firstly, teflon tape will not acoustically isolate the mouthpiece. It is thinned to the molecular level where there is contact, and only fills in the gaps where there is no contact.

Secondly, epoxy is quite stiff as plastics go, with a modulus of elasticity in the range of 2000 ksi even without the filler. Glass-filled epoxy has a stiffness modulus of up to 4300 ksi. That's in the range of about 10% of steel.

Lexan, on the other hand, is only about 10% as stiff as epoxy, with a flexural modulus of about 295 ksi.

Neither polymer, however, has a linear relationship between stress and strain that is required to have a true Young's Modulus as you have with most metals.

Rick "who thinks that if an epoxy repair made the horn dead, it was because of the big blob of epoxy inside the tube, or the air leak that still remained" Denney

[Rick Denney]

Sounds reasonable, I remember plenty of epoxy on the lead pipe back in 1982.
Do you know what properties (for example crystallinity) that makes a polymer (or compsite) acoustically suited in a mouthpiece? Are they different from property requirements in the rest of the tuba?
Lexan obviously work as mouthpiece material, but would lexan in the lead pipe ruin the sound?

We just don't know enough about the subtle interaction between the vibration of the material and the vibration of the air within it. We know that whatever interaction there is must be very subtle, but we also have an abundance of anecdotal evidence of that effect that can't be ignored. Just because we can't explain, quantify, or even think of how to measure it doesn't mean it isn't there. And the human evaluation counts, too--tubas are musical instruments and have to fit their players like a familiar glove. Feel counts for most players.

There are good players who say the Lexan mouthpieces work fine, and others who insist that they are completely dead. I have a feeling that the latter group has more trouble with the shape than the material, but I'm speculating and maybe I'm just not good enough to notice the difference. But it casts doubt on your premise that being okay for the mouthpiece might not make it okay for the leadpipe, because not everyone agrees that Lexan is okay for the mouthpiece. Mouthpieces are relatively thick and stiff.

To me, resonance is controlled by stiffness and mass. I'm thinking of structural stiffness, not unit stiffness that I reported in my previous message. Structural stiffness is the application of unit stiffness to a given design. Thus, a leadpipe of Lexan, which is perhaps 2% as stiff as brass, might be made more stiff by making it much thicker (and it would be at least five times thicker than brass). That would only close the gap a bit, of course. The advantage to polycarbonate is not its stiffness, but rather its strength and toughness, which makes it extremely durable. Epoxy is generally even more durable, but a little harder to work with.

Elasticity is also a key measure, and plastics are, by definition, not particularly elastic like most metals. I'm an engineer not a materials scientist, so I don't look at the molecular reasons why that is, I just look at the material characteristics, and listen for a ring when I tap it with a fingernail. With most plastic, I get a thud instead of a clank (or ring), so at high frequencies it's quite well damped. That may or may not be important. At low frequencies, I think it might be plenty elastic enough.

There are tubas made of carbon composite outer branches that use carbon fiber fabric buried in epoxy resin. It's much lighter, and less stiff, but much stronger than brass. Again, opinions vary on the musical effect. In general, I think it works, but that doesn't mean it's the same. Fiberglass sousaphones are a minimal example using a similar composite material that is much less stiff and strong.

Brass has been used because it was an easy material to fashion into a tuba 170 years ago when it was invented. It still is, especially considering that tubas are not made in sufficient quantities to warrant the sorts of production equipment that makes plastic a practical alternative.

So, my answer to your question is a definite maybe.

Rick "not sure of anything" Denney