Bach 4 vs Kelly 5G-EU-SS
The Bach 4 has a wider cup diameter, the Kelly 5G-EU-SS features a deeper cup.
Cup Diameter
The Bach 4 has a cup diameter of 26.00 mm compared to 25.40 mm on the Kelly 5G-EU-SS — a difference of +0.60 mm. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 5G-EU-SS has a — cup while the Bach 4 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Bach 4 is described as unspecified, and the Kelly 5G-EU-SS as 316 Surgical Stainless Steel. 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.