There’s not just one, it depends on the style,
the performer and his instrument.
Like the one that’s the favorite
of trumpet players, you know,
the one with the crumpled face
and the pained look of focus
just before he blasts high C.
Every note in the upper range
becomes a new source of agony.
Then there’s the face
of philosophical perplexity,
the one used by trombone players
when they reach higher than they should,
eyebrows lifted against the hairline,
chin extended and tucked into the throat,
usually during a technical lick  in numerous positions.
Of course, there are the sax players
and their ballads, eyelids nearly closed,
head in a languorous droop
that sometimes lolls back
and swivels side to side
to help kick in an arousing vibrato.
And then the drummer
with his classic wild man look,
crazy faced with the fixed grin
and scary stare, like he’s about
to lurch off his seat, unlike
the piano player, the aristocrat
with his proud, confident posture,
convinced that for the next few hours
he and his ensemble own your soul,
how he notices you’ve immersed yourself
in the excitement and emotion of the music,
with your intense squint and locked grin,
that empathetic grimace
especially obvious when your head bobs feverishly
in a contagious yet effusive sign of approval.