Is it the player or is it the instrument?
Robert W. Pyle

Previous measurements of trombone tones (J. Acoust. Soc. Am., Vol. 125, No. 4, Pt. 2, April 2009, p.2597) showed considerable player-to-player variability in the degree to which the radiated spectrum changes with different bell alloys. This might arise in the following way. Consider two very different players, A and B. Player A is able to produce the desired timbre, independent of the bell alloy, by altering the embouchure as needed. Given a choice of instruments, player A will presumably pick the one that most easily produces that timbre. Player B, on the other hand, always blows each instrument as freely as possible, allowing the instrument to determine the tone quality, and then chooses an instrument with the desired timbre. It is plausible to think that the characteristics of the sound pressure in the mouthpiece cup will show greater variability (with changes of bell alloy) for player A than for player B due to the embouchure adjustments by player A. This hypothesis will be tested by simultaneous measurement of sound pressure internally within the mouthpiece and externally on the bell axis. The relationship between the internal and external sound pressures, to the extent that it is independent of the player, may reinforce (or, perhaps, contradict) common beliefs among players and instrument builders about the effect of alloy on tone color.