A mouthpiece raises impedance to match the lips to the air column, boosting power

Evidence

The first effect of the mouthpiece an overall increase in impedance is caused by the cup and constriction of the mouthpiece. This enhances power and loudness. Explanation: The small cup builds pressure during each opening of the lips, causing source loading/proximity resonance. The mouthpiece thus plays a similar role to the reproducer chamber in a gramophone. Benefit: The lips are a vibrating source much heavier and with much higher impedance than air in a tube, and the increased impedance at the mouthpiece provides better impedance matching to the tube, allowing more power to go into making sound for the same effort. — Eric J. Heller, Why You Hear What You Hear- An…, 1491

Categorization

• XREF: The impedance-matching explanation connects to how the gramophone reproducer and the earlier source-loading discussions in this book’s chapter on vocal cords / string instruments work.
• DEFINE: Explains ‘source loading’ and impedance matching as mechanical coupling between a high-impedance vibrating source and a tube.