Three simple configurations of PC valves

Figure 21: Simplified models of three common configurations of pressure-controlled valves.

1. $(-,+)$: the valve is blown closed (as in woodwind instruments or reed-pipes of the pipe organ).
2. $(+,-)$: the valve is blown open (as in the simple lip-reed models for brass instruments, the human larynx, harmonicas and harmoniums).
3. $(+,+)$: the transverse (symmetric) model where the Bernoulli pressure causes the valve to close perpendicular to the direction of airflow.