Bach 8 vs Kelly 51D-EU
The Kelly 51D-EU has a wider cup diameter, the Kelly 51D-EU features a deeper cup.
Cup Diameter
The Kelly 51D-EU has a cup diameter of 25.40 mm / 1.0000 in compared to 24.75 mm / 0.9744 in on the Bach 8 — a difference of 0.65 mm / 0.0256 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 51D-EU has a — cup while the Bach 8 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Bach 8 is described as unspecified, and the Kelly 51D-EU 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.