Yamaha AH-38D4 vs Yamaha EP-51DS
The Yamaha EP-51DS has a wider cup diameter, the Yamaha EP-51DS has a larger throat opening.
Cup Diameter
The Yamaha EP-51DS has a cup diameter of 25.24 mm / 0.9937 in compared to 18.90 mm / 0.7441 in on the Yamaha AH-38D4 — a difference of 6.34 mm / 0.2496 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a deep cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Yamaha EP-51DS has a wider throat (7.11 mm / 0.2799 in vs 5.25 mm / 0.2067 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.