Bach 9 vs Giardinelli 5D
The Giardinelli 5D has a wider cup diameter, the Giardinelli 5D has a larger throat opening.
Cup Diameter
The Giardinelli 5D has a cup diameter of 25.00 mm / 0.9843 in compared to 24.72 mm / 0.9732 in on the Bach 9 — a difference of 0.28 mm / 0.0110 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 Giardinelli 5D has a wider throat (6.00 mm / 0.2362 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.