Sound-radiation efficiency depends on piston size relative to wavelength, not absolute size

Evidence

Note that we have not been speaking of any absolute size, such as 10 cm, but rather the relative size of the piston face compared to the wavelength. Thus, a piston face only a few centimeters across can be extremely efficient at radiating sound in the high end of the audio range, above 5000 or 10,000 Hz. But the same piston would be very inefficient at radiating sound at 20 or even 500 Hz. The wavelength at 500 Hz is more than 68 cm. — Eric J. Heller, Why You Hear What You Hear- An…, 660

Categorization

• DEFINE: Clarifies the conceptual basis of acoustic impedance and why size matters only relative to wavelength.
• ADDENDUM: Directly supports the trees-as-infrastructure piece’s physics-free framing, though it is really more of an acoustic-impedance gem — relevant to why small sound sources behave differently.