Bach 10 vs Kelly 31
The Kelly 31 has a wider cup diameter, the Kelly 31 features a deeper cup, the Bach 10 has a larger throat opening.
Cup Diameter
The Kelly 31 has a cup diameter of 17.53 mm / 0.6902 in compared to 16.80 mm / 0.6614 in on the Bach 10 — a difference of 0.73 mm / 0.0287 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 31 has a — cup while the Bach 10 has a very deep cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Bach 10 has a wider throat (4.50 mm / 0.1772 in vs 3.80 mm / 0.1496 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
The Bach 10 is described as unspecified, and the Kelly 31 as Lexan polycarbonate. Material affects mass, warmth, and how quickly the piece responds.
Which is better?
Neither mouthpiece clearly dominates the other — the right choice depends on your embouchure, playing style, and the sound you're after. Try both if possible.