Vocal tract coupling reinforces the reeded air column by lowering its resonant frequency.

Evidence

We are looking for a mode that has been pushed down in frequency and is mostly reed plus air column (because we want the instrument to sound). It also has to satisfy the in-phase criteria of the pressure given in the previous paragraph. There is a case able to satisfy these requirements—case a of figure 10.9. It has been lowered in frequency compared to before the coupling, and it is mostly blue pendulum motion (reed plus air column). The blue pendulum swinging to the left corresponds to high pressure in the mouthpiece, and the red pendulum swinging left corresponds to low pressure in the oral cavity. Viewed from either the vocal tract or reed perspective, the resonance in one is reinforced by the other. — Eric J. Heller, Why You Hear What You Hear- An…, 1625