Posted by John Swensen on December 22, 2000 at 17:05:32:
In Reply to: And the (mouthpiece) beat goes on... posted by Rick Denney on December 22, 2000 at 16:28:55:
Regarding the backbore, it is actually the ratio of the cup volume to the throat area that determines the sharpness/flatness characteristics in the high range, while the cup volume (mostly) affects the flatness/sharpness in the low range (bigger cup volume, flatter lower range), at least, according to Arthur H. Benade (Fundamentals of Musical Acoustics). Benade would adjust mouthpieces for particular instruments by reaming out the throats and backbores of mouthpieces, taking acoustical measurements and play-testing as he went along.
The most relevant characteristic of a mouthpiece, relative to its pitch, is its "popping frequency", the tone you get when you slap the rim end down on your open palm, holding it there; higher popping frequency, sharper tuning in the higher range (and when playing louder/brighter). A non-destructive way to change the popping frequency is to paint some fingernail polish in the throat (to reduce its area). It can be removed with some acetone or fingernail polish remover. Bits of clay might be pressed into the cup (away from the throat) to reduce its volume, raising the popping frequency (as well as reducing the cup volume). Playing around with clay and/or nail polish may give you enough insight to predict what reaming out a Helleberg would do for your York's intonation.
The backbore has some effect on the pitch as well, although I don't understand it, nearly as well.
Also, there has been some research with simulated mouthpiece forms on trumpets, which tends to agree with Benade's reports, especially when the paper's ambiguities are clarified by the author (for example, the different cup shapes all have different volumes, so the effect of cup shape may be dwarfed by changes in cup volume).