Vocal resonance cannot bend notes up, only lip pressure can.

Evidence

A note cannot be bent up with vocal resonance, since no case works. The only candidate would be case C, which is mostly reed plus air column pushed up. But the pressure is high in both the oral cavity and the mouthpiece at the same time—thus the steady airflow when the reed is open cannot reinforce both resonances (vocal tract and reed plus air column). Of course, lip pressure on the reed could be used to bend the note up, an entirely different mechanism. — Eric J. Heller, Why You Hear What You Hear- An…, 1627

Categorization

• XREF: The asymmetry (down-bend works, up-bend doesn’t) parallels the phase constraints seen across coupled-oscillator systems.