Bach 16 vs Yamaha HR-31D4
The Yamaha HR-31D4 has a wider cup diameter, the Bach 16 features a deeper cup.
Cup Diameter
The Yamaha HR-31D4 has a cup diameter of 17.28 mm / 0.6803 in compared to 16.25 mm / 0.6398 in on the Bach 16 — a difference of 1.03 mm / 0.0406 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Bach 16 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
Throat diameters are essentially identical at 4.50 mm / 0.1772 in, so resistance feel will be comparable.
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.