Bach 15 vs Kelly 51D-EU
The Kelly 51D-EU has a wider cup diameter, the Kelly 51D-EU features a deeper cup.
The internal diameter of the cup, measured at the widest point inside the rim.
Mouthpiece A
Bach 15
• Baritone / Euphonium
Cup diameter24.40 mm / 0.9606 in
Best suited for
Orchestral playing — large cup for full, dark tone
High-volume projection — open throat for power
vs
Mouthpiece B
Kelly 51D-EU
• Baritone / Euphonium
ProfessionalCup diameter25.40 mm / 1.0000 in
Best suited for
Orchestral playing — large cup for full, dark tone
Professional euphonium
Cup Diameter
The Kelly 51D-EU has a cup diameter of 25.40 mm / 1.0000 in compared to 24.40 mm / 0.9606 in on the Bach 15 — a difference of 1.00 mm / 0.0394 in. A wider cup generally produces a fuller, darker tone but requires more air support.
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.