To understand what an echo is, you first have to understand what sound is. In Webster’s Fourth Edition College Dictionary, sound is “vibrations in air, water, etc. that stimulate the auditory nerves and produce the sensation of hearing.” Vibrations through the air can be thought of as oscillation of molecules. As the molecules oscillate, they pass energy on to surrounding molecules, and those molecules pass energy on to other surrounding molecules. This is how sound travels, and the oscillation of the molecules is often referred to as sound waves.

An echo happens when the sound waves reach a surface, bounce off of it and travel in the opposite direction. For optimum echoes, the surface should be perpendicular to the waves, and as frictionless as possible. In places like the Grand Canyons, you can hear many echoes because the sound waves bounce off of surfaces, then others bounce off of other surfaces, and some will bounce back to you, but at different times.