This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
Nearly all ancient peoples had pretty, poetic stories about the echo. Some, very likely, thought merry little fairies really lived in rocky caves and valleys, to shout and laugh back at them. No one really knew what an echo was until wise men discovered that sound travels in waves, just as water and light and electricity travel. (See Acoustics, Wave-Motion and "Sound Waves and the Telephone.") If a wave of water is stopped by a break-water or a cliff, it is thrown back into the sea. So if a sound is stopped by a wall it is thrown back to our ears. A big empty building, especially if windows and doors are closed so the sound waves cannot escape, is a fine place for echoes Best of all is a rock-walled glen in hilly country. The sound very seldom comes back just as it was made. When a wave of water is stopped it is broken up and thrown back in spray. So a sound wave when stopped, is broken into an airy, shattered echo. It really seems as if some mocking sprite calls back from a fairy grotto in the rocks.