Bach 12C vs Yamaha SL-51BL
The Yamaha SL-51BL has a wider cup diameter, the Yamaha SL-51BL has a larger throat opening.
Cup Diameter
The Yamaha SL-51BL has a cup diameter of 25.23 mm / 0.9933 in compared to 24.50 mm / 0.9646 in on the Bach 12C — a difference of 0.73 mm / 0.0287 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a shallow cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Yamaha SL-51BL has a wider throat (6.92 mm / 0.2724 in vs 5.85 mm / 0.2303 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.