Reflections cannot introduce frequencies not present in the original sound

Evidence

Can echoes change the pitch of the original sound? An object can reflect high-frequency sound with greater or lesser efficiency than low-frequency sound, but it can do nothing to introduce new frequencies that aren’t already present, unless the object itself is set to vibrating at these new frequencies. This is theoretically possible, through nonlinear response. It is certainly not a common occurrence, and is not expected to happen for most audible sounds, which are, after all, very feeble conveyers of energy. — Eric J. Heller, Why You Hear What You Hear- An…, 2629

Categorization

• DEFINE: Clarifies the physical limits of echo: reflection is frequency-spectrum-filtering only, never creation, except via nonlinearity.