Open reeds must vibrate below the air column’s playing frequency to be stable.

Evidence

We just concluded that blown closed reeds must have free vibration frequencies well above playing frequencies. It follows from these arguments that blown open reeds must be the opposite: they must freely vibrate below the playing frequencies. This corresponds to cases C and c. In the “mostly air column” mode, case C, which we hope will work, the lips are indeed driven by the air column above their natural resonance frequency, so that displacement and forces are 180 degrees out of phase, that is, the lips are displaced in opposition to the high pressure in the mouthpiece. They open when the pressure is high in the mouthpiece. This is self-sustaining, since the condition of resonance is satisfied, that is, amplitude (pressure) being added to preexisting amplitude. This is indeed what happens (see figure 16.23, bottom). — Eric J. Heller, Why You Hear What You Hear- An…, 1615