Bach 9D vs Kelly SCREAMER-UK
The Bach 9D has a wider cup diameter, the Bach 9D has a larger throat opening.
Cup Diameter
The Bach 9D has a cup diameter of 16.05 mm / 0.6319 in compared to 16.00 mm / 0.6299 in on the Kelly SCREAMER-UK — a difference of 0.05 mm / 0.0020 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 Bach 9D has a wider throat (3.66 mm / 0.1441 in vs 3.56 mm / 0.1402 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
The Bach 9D is described as unspecified, and the Kelly SCREAMER-UK as Lexan polycarbonate. Material affects mass, warmth, and how quickly the piece responds.
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.