Bach 18 vs Kelly 11
The Kelly 11 has a wider cup diameter, the Kelly 11 features a deeper cup, the Bach 18 has a larger throat opening.
Cup Diameter
The Kelly 11 has a cup diameter of 18.29 mm / 0.7201 in compared to 16.15 mm / 0.6358 in on the Bach 18 — a difference of 2.14 mm / 0.0843 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 11 has a — cup while the Bach 18 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Bach 18 has a wider throat (4.50 mm / 0.1772 in vs 3.66 mm / 0.1441 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
The Bach 18 is described as unspecified, and the Kelly 11 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.