Trumpet lips open on high pressure because notes are played above resonance

Evidence

In play, the lips open at high pressure in the mouthpiece. Releasing pressure into a chamber already high in pressure means more work is done by the air source; this fits the maximum work on resonance idea. Below the resonance frequency for free lip vibration, low pressure on the outside encourages the lips to open, as is intuitive. All the notes played on the trumpet, however, lie above this resonance, so we must expect a 180-degree phase shift, meaning the lips open when the pressure is high outside the lips, agreeing with the principle of maximum power being supplied to the trumpet resonance controlling the playing frequency (since adding air to the mouthpiece at its maximum pressure does maximum work; see figure 16.23). — Eric J. Heller, Why You Hear What You Hear- An…, 1543

Categorization

• DEFINE: Explains the 180-degree phase shift in lip motion relative to mouthpiece pressure, a key mechanism for how brass instruments self-oscillate.
• XREF: Connects to the maximum work on resonance idea developed earlier in this book’s chapters on oscillation and resonance.