An air column driving the reed kills the sound by opening on rarefaction

Evidence

Suppose the air column (blue pendulum) is higher frequency (cases C and c). Case C has the air column strongly excited, which is what we want in order for the instrument to sound. The air column is driving the reed above its natural frequency; thus the force on the reed and reed displacement are close to 180 degrees out of phase. This means that when the force on the reed (pressure in the mouthpiece) is high, the reed is closed (moved toward the force). It will open when the mouthpiece pressure is low, forcing air into a rarefaction, killing the air column resonance. This will not do; case C does not work. — Eric J. Heller, Why You Hear What You Hear- An…, 1610