Sound pressure level depends on oscillation amplitude, not average pressure

Evidence

An important fact: when the instrument is played, there is consistently higher average pressure (supplied by lungs, diaphragm, and so on) in the vocal tract than in the mouthpiece. The sound pressure level (SPL) is based not on the average pressure somewhere, but on the oscillations above and below average pressure. Thus the SPL could be higher or lower in the vocal tract than in the mouthpiece. In spite of the oscillations, the pressure in the oral cavity is always higher than in the mouthpiece. — Eric J. Heller, Why You Hear What You Hear- An…, 1604

Categorization

• DEFINE: Clarifies that SPL measures oscillation, not average pressure.
• CANDY: Tidy fact: a wind player’s mouth can be at higher average pressure than the mouthpiece yet still produce lower SPL.