Kelly MC vs Yamaha HR-31D4
The Yamaha HR-31D4 has a wider cup diameter, the Kelly MC features a deeper cup, the Yamaha HR-31D4 has a larger throat opening.
Cup Diameter
The Yamaha HR-31D4 has a cup diameter of 17.28 mm / 0.6803 in compared to 16.76 mm / 0.6598 in on the Kelly MC — a difference of 0.52 mm / 0.0205 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly MC has a very deep cup while the Yamaha HR-31D4 has a deep cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Yamaha HR-31D4 has a wider throat (4.50 mm / 0.1772 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 MC is described as Lexan polycarbonate, and the Yamaha HR-31D4 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.