Bach 9D vs Kelly SCREAMER
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 — 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 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.