Bach 11 vs Kelly MC
The Kelly MC has a wider cup diameter, the Kelly MC features a deeper cup, the Bach 11 has a larger throat opening.
Cup Diameter
The Kelly MC has a cup diameter of 16.76 mm / 0.6598 in compared to 16.55 mm / 0.6516 in on the Bach 11 — a difference of 0.21 mm / 0.0083 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly MC has a very deep cup while the Bach 11 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Bach 11 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 11 is described as unspecified, and the Kelly MC 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.