Bach 12 vs Yamaha BB-SELF
The Yamaha BB-SELF has a wider cup diameter, the Bach 12 has a larger throat opening.
Cup Diameter
The Yamaha BB-SELF has a cup diameter of 32.86 mm / 1.2937 in compared to 32.75 mm / 1.2894 in on the Bach 12 — a difference of 0.11 mm / 0.0043 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 12 has a wider throat (8.33 mm / 0.3280 in vs 8.00 mm / 0.3150 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.