The fixed energy of an impulse forces increasing echoes to weaken proportionally.

Evidence

When objects are large or complex, or high Q, an impulse may yield a bounty of echoes, their numbers increasing perhaps exponentially as time progresses after the impulse, although their individual strength has to diminish correspondingly, since there is a fixed amount of energy supplied by the impulse. (The sound from a single tap cannot grow ever louder thereafter. Eventually, the energy arriving back at a given spot has to settle down, when averaged over some short time interval. The more echoes there are, the weaker each must be.) — Eric J. Heller, Why You Hear What You Hear- An…, 1076

Categorization

• XREF: Connects to reverberation physics — exponential echo growth balanced against 1/energy-per-echo, the tradeoff behind concert-hall late reflections.