Echo
A distinct reflected sound wave received at the listener's ear with sufficient time delay after the direct sound.
Governing formula
d = v t / 2
SI unit
m
In depth
Human hearing requires at least ~0.1 second delay between direct and reflected sound to distinguish an echo, requiring a reflecting wall at least ~17 meters away in air.
Examples in the real world
Shouting across a steep mountain canyon and hearing your voice bounce back a second later.