Echoes are the defining confinement mechanism behind all resonances.

Evidence

Whether they return immediately or after some longer delay, echoes are harbingers of resonance. Resonances correspond to maximum power supplied by a driver. The role of echoes (another word for confinement) is built into the very definition of resonant structures (see chapter 9). — Eric J. Heller, Why You Hear What You Hear- An…, 1075

Categorization

• DEFINE: Frames echo as synonymous with confinement, tying the impulse-response picture directly to the definition of resonance.