Bach 12E vs Kelly 12C
The Bach 12E has a wider cup diameter.
Mouthpiece A
Bach 12E
• Trombone
Cup diameter24.50 mm / 0.9646 in
Cup depthShallow
Rim width—
Throat diameter5.85 mm / 0.2303 in
Backbore—
Finish—
Material—
Best suited for
High-volume projection — open throat for power
vs
Mouthpiece B
Kelly 12C
• Trombone
LeadCup diameter24.13 mm / 0.9500 in
Cup depthShallow
Rim width—
Throat diameter—
BackboreS
Finish—
MaterialLexan polycarbonate
Best suited for
Lead trombone; smallest cup
Cup Diameter
The Bach 12E has a cup diameter of 24.50 mm / 0.9646 in compared to 24.13 mm / 0.9500 in on the Kelly 12C — a difference of 0.37 mm / 0.0146 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a shallow cup depth, so the sound character from depth alone will be very similar.
Material
The Bach 12E is described as unspecified, and the Kelly 12C 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.