Sound travels in waves, in the same way that waves of water travel in the ocean. When you make a sound, the sound waves travel through the air. You will first hear the sound when those waves reach your ear.
However, if sound waves hit an obstacle such as a building or the walls of a cave, they will bounce off the air because such objects do not absorb sound. Instead, they reflect or bounce the sound back, and you will hear it a second time. This second sound is called an echo.
If it is only an obstacle that reflects the sound, you will hear only one echo, a simple echo. But if many surfaces reflect the echo bouncing from surface to surface, like in a valley or canyon surrounded by mountains, you hear the sound of the echo over and over again, but it gets weaker with each repetition. This repeated echo is called a compound echo or reverberation.
Heavy clouds close to the ground can also reflect sounds and create echoes.
Echoes can measure how far a person is from the surface that the echoes bounce off, as the sound waves travel at about 300 m per second.
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