Bach 18 vs Yamaha HR-35C4
The Yamaha HR-35C4 has a wider cup diameter, the Bach 18 has a larger throat opening.
Cup Diameter
The Yamaha HR-35C4 has a cup diameter of 18.08 mm / 0.7118 in compared to 16.15 mm / 0.6358 in on the Bach 18 — a difference of 1.93 mm / 0.0760 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a medium cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Bach 18 has a wider throat (4.50 mm / 0.1772 in vs 3.98 mm / 0.1567 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
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.