Bach 15D 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 compared to 24.40 mm on the Bach 15D — a difference of +1.00 mm. 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 15D has a shallow cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Bach 15D 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.