This mechanism, available optionally on S. Fox Eb, D and C clarinets, gives an extra pitch boost for the altissimo register, particularly the E-F# region.
It an also be retrofitted to existing clarinets.
The vent hole, located on the back of the instrument, is normally open but is closed whenever the left hand first finger ring is depressed. It requires no change in fingering; the player is virtually unaware of its presence.
The placement and size are
such that the vent has no effect on high C# and D, but it progressively
sharpens the notes above (using fingerings for which the left hand first
finger is lifted), to a maximum of approximately 20 cents. It has
no impact on the throat tones or left hand middle F/high C.
On S. Fox C clarinet
Retrofitted to Boosey & Hawkes Eb clarinet