Passive devices and even walls can amplify sound without power

Evidence

As was stated in a scholarly paper by A. G. Webster in 1919, when such phonographs were still in regular use, “it is well known that very little sound is emitted by the phonograph … with the horn taken off, although in the former case the motion of the diaphragm is exactly the same.” This is an important point. The action of the horn is not somehow to make the diaphragm move more vigorously; rather, the diaphragm motion is the same. More power is produced because of the presence of an enclosing chamber and the horn that leads from it. The former resonantly loads the source; the latter gets the sound out with minimal reflections. — Eric J. Heller, Why You Hear What You Hear- An…, 539 (#corroborates, via linked claim)

Categorization

• XREF: connects to: Curved surfaces concentrate sound locally while draining power elsewhere in the room; A reflecting wall doubles total sound power output from any source; Loudspeakers and rooms function together as a single audible system (+4 more)

Broader Connections

• A horn amplifies sound without increasing diaphragm motion #corroborates (0.85) — Eric J. Heller, Why You Hear What You Hear- An…, 539
• Curved surfaces concentrate sound locally while draining power elsewhere in the room #complicates (0.82)
• Loudspeakers and rooms function together as a single audible system #expands (0.80)
• A reflecting wall doubles total sound power output from any source #qualifies (0.78)
• Room boundaries alter a sound source’s radiated power, not just its perceived sound #expands (0.78) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2134
• Walls absorb sound by converting organized acoustic energy into heat through friction #exemplifies (0.72)