But echo suggests a still deeper mystery. Acousticians will explain that the reflection of a sound off a distant surface is simply a case of the original wave bouncing back, the angles of incidence and reflection being equal. In order to understand this effect one may project a mirror image of the original sound deep behind the surface, at exactly the same distance and angle from the surface as the original. In other words, every reflection implies a doubling of the sound by its own ghost, hidden on the other side of the reflecting surface. This is the world of alter-egos, following and pacing the real world an instant later, mocking its follies.

Schafer, Murray R, The Soundscape- Our Sonic E…, loc. 717