Caution with Removable bells

[timothy42b]

I just doubt a very small leak that far down the bell loses much air.

Of course there is air moving very slowly down the horn, and a sound wave moving much faster - I'm pretty sure the air flow speed is essentially zero compared to the sound wave. Possibly an impedance mismatch causes a reflection to the sound wave simultaneously with a leak. I dunno. I know on a trombone an open spit valve affects some notes but not others. I think a leak there causes turbulence and an impedance problem. The flare of a bell is an impedance change, at different points for different frequencies.

I've measured static pressure in trombone crudely with a DIY manometer, years ago. The differential pressure between a point just past the mouthpiece (I inserted mouthpiece into a tee-fitting that sealed to the trombone) and the outside air was about 1 inch of water, or about 0.032 psig. A tuba would have to be even less. (The leak at the end of the bell is VERY large.) This measurement would be easy to do on any tuba with bits, just use a PVC tee from the big box store. Probably cost less than a dollar USD.

A bullet hole in a tuba? Until I looked at some photos on this site, I had never seen a tuba bell that wasn't heavily dented. Those still seem to play.

[Rick Denney]

It's not about air flow leaks, but pressure leaks. The sound wave is a series of pressure fronts whose spacing determines the resulting vibration of the ear drum and the perceived pitch. Each harmonic is a different series of pressure fronts that are superimposed on each other. The higher the frequency, the steeper is the increase in pressure for a given amplitude (that steepness is called slew, and when removed from the time-series of the waveform can be treated like a transient.) Some of them may be strong enough and sharply enough defined to create sufficient pressure to push through a tiny opening in a bell stack. This has the effect of changing the impedance--that frequency will be undermined so some extent. The question is to what extent. Given that the pressure front where the bell stack is 6 or 7 inches in diameter is huge, a tiny leak won't undermine much of it before it has passed. And that's assuming the leak is sufficient to affect a frequency that is important in the sound. Hence, the small effect. A water key is an opening that is a significant percentage of the diameter of the tubing, and therefore affects a much wider range of frequencies (lower frequencies have shallower transients that the water drain will still pass) and also will affect them more profoundly. In the smaller tubing, the static pressure (which is the result of air flow) will also cause a noticeable leak.

Trumpets have much higher frequencies in the harmonics, and therefore much steeper transients and higher instantaneous pressures. But I doubt a pinhole in the trumpet bell area would have much effect compared to further upstream in the bugle.

Rick "static pressure is only part of the story" Denney

[Donn]

I do wonder though if air flow or static pressure could have something to do with stability of pitch. If you buy the notion that air flow is considerably less in a saxophone, than a tuba, and that the saxophone's pitch resonance is much more easily destabilized by a leak. Let's see, where's my ophicleide - oh dang, I never did get one!

[iiipopes]

I agree from experience with the necessity of a good fit for a detachable bell. I used to have a sawn-off St. Pete upright bell attached to the Miraphone tenon on my BBb 186 (taken off the stock recording bell) before I converted the bell. 1st ledger line Eb - 1st valve was always iffy. I don't know if it was a leak, or if the extra mass of the tenon/receiver interfered with a node or anti-node, but I can tell you that once the Besson bell was retrofitted the intonation of the Eb solidified, as did some other notes.

OTOH, I purchased another tenon for the stock recording bell, so I could have both, had the recording bell assembled, and when I used the recording bell, I didn't have the intonation problems with Eb. But I did have the typical 5th-partial flat notes, that were not so bad with the upright bell. I attribute this to dumb luck: with the St Pete bell, it was too short, and I had to use a longer main tuning slide. With the recording bell, I used the stock tuning slide. I attribute the "fixing" of the flat 5th partials with the upright bell to having just enough cylindrical tubing in the right place - about two additional inches on the main tuning slide, that it helped sharpen those notes.

When I changed out to the Besson bell, its tail was slightly smaller in diameter than the Miraphone, so when my tech trimmed it, it came out to exactly the same height as with the cobbled St. Pete bell, so I used the longer tuning slide. Again, the 5th partials were much improved, with B nat, 2nd space C, and Db good, and mid-line D only slightly flat, and with no adverse consequences to the intonation of any other notes. I consider myself very, very lucky, indeed!