Sound can be amplified without any external power source

Evidence

Since the invention of the powered amplifier about a century ago, it has been natural to assume that amplification of sound requires some sort of powered amplifier—plugged into a wall or a battery, for example. This is not true. An Edison reproducer is surely an amplifier; it draws its power from using part of what would otherwise have been lost as friction to generate sound. Even a wall can induce amplification. — Eric J. Heller, Why You Hear What You Hear- An…, 506

Categorization

• CANDY: A gramophone horn or even a wall amplifying sound with zero electricity is a fun counterintuitive tidbit for casual conversation.
• QUESTION: The acoustic-impedance matching idea—harvesting otherwise-lost frictional energy—deserves a follow-up on how wattage-free amplification redirects energy rather than creating it.
• SEED: Angle: ‘amplification without amplifiers’ — how passive acoustic horns, speaker cones, and even building walls harvest ambient mechanical energy rather than adding electrical power.