Kelly 31 vs Yamaha HR-29C4
The Kelly 31 has a wider cup diameter, the Kelly 31 features a deeper cup, the Yamaha HR-29C4 has a larger throat opening.
Cup Diameter
The Kelly 31 has a cup diameter of 17.53 mm / 0.6902 in compared to 17.08 mm / 0.6724 in on the Yamaha HR-29C4 — a difference of 0.45 mm / 0.0177 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 31 has a — cup while the Yamaha HR-29C4 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Yamaha HR-29C4 has a wider throat (3.98 mm / 0.1567 in vs 3.80 mm / 0.1496 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
The Kelly 31 is described as Lexan polycarbonate, and the Yamaha HR-29C4 as unspecified. 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.