Buzzing lips generate many harmonics that resonance then shapes before radiating.

Evidence

The first job of the lip-reed drive is to supply power at the fundamental and many harmonics thereof The abrupt open and shut action of the lip reed is far from sinusoidal. Many harmonics are generated as the lips are buzzed. The harmonic content of the lip buzz will be greatly modified by resonance and dissipation before the final sound emerges from the bell. — Eric J. Heller, Why You Hear What You Hear- An…, 1539