  In a simple model, the reed of the clarinet is mechanically loaded by the
series combination of the acoustical impedances of the instrument itself and of
the player's vocal tract. Here we measure the complex impedance spectrum of
players' tracts using an impedance head adapted to fit inside a clarinet
mouthpiece. A direct current shunt with high acoustical resistance allows
players to blow normally, so the players can simulate the tract condition under
playing conditions. The reproducibility of the results suggest that the
players' "muscle memory" is reliable for this task. Most players use a single,
highly stable vocal tract configuration over most of the playing range, except
for the altissimo register. However, this 'normal' configuration varies
substantially among musicians. All musicians change the configuration, often
drastically for "special effects'' such as glissandi and slurs: the tongue is
lowered and the impedance magnitude reduced when the player intends to lower
the pitch or to slur downwards, and vice versa.
