Resonance arises when echo round-trip time equals an integer number of wave periods.

Evidence

For the free edge, the same-sign reflection means a proximity resonance, since both peaks will contribute with the same sign to the cosine transform for a cosine of sufficiently low frequency. Higher resonance conditions are found at frequencies f, 2f, … , where f = 1/T, and T is the return time for the first echo. This coincides with the idea of resonance as the constructive addition of wave amplitude at a source. For example, the peak at f has a wavelength λ = c/f = cT, which means that at frequency f = 1/T, the distance of the round-trip, c · T, is one wavelength—that is, in-phase addition. — Eric J. Heller, Why You Hear What You Hear- An…, 1071

Categorization

• DEFINE: Ties echo return time to the classic resonance condition of constructive wave addition.