The single-reed and mouthpiece arrangement of clarinets and saxophones (Fig. 7) acts as a pressure-controlled valve that allows energy into the instrument for the initialization and maintenance of oscillations in the downstream air column.
A single-reed woodwind mouthpiece.
The inherent non-linear behavior of this mechanism, which is attributable to such aspects as the flow characteristic through the reed aperture and the reed's displacement when it hits the mouthpiece facing, is most obviously demonstrated by the fact that a nearly static pressure applied at the upstream side of the system is converted into acoustic energy on the downstream side at a number of harmonically related frequencies.
The flow and reed movement are controlled by the difference in pressures on the upstream and downstream sides of the reed channel,
The upstream pressure is typically assumed constant or slowly varying and tends to force the reed toward the mouthpiece lay.
Negative pressure in the mouthpiece tends to reinforce this action, pulling the reed toward the lay, while positive pressure in the mouthpiece, if acting alone, tends to push the reed away from the mouthpiece lay.
The single-reed valve is initially open but can be blown shut against the mouthpiece lay by an appropriate pressure difference .
The movement of the reed controls the volume flow through the reed channel and into the mouthpiece.