Echoes
In some places in valleys in the mountains, or in towns where the streets are shut in by buildings or in large halls and rooms, the sounds we make are repeated and come back to our ears. When we shout against a tall rock, the sound of our voice returns to us ; and in a quiet street in a town you can often hear the sound of your footsteps repeated as you walk along. Such repeated sounds are called “echoes”. Some echoes are very clear and exact; they repeat not only a single shout but whole sentences. If you say, “What are you doing?” the echo answers quite distinctly in the same words and in your own voice – “What are you doing?”
What is an echo ? In the old days people believed it was the voice of a fairy or nymph that lived in the mountains, and that mocked people when they shouted by repeating their words. The old Romans had a myth about a nymph called Echo, whom the goddess Juno punished by making it so that she could not speak unless someone first spoke to her, and then only to repeat the words she heard. But of course these are only fairy-tales, and we know better.
What we call “sound” Is caused by certain vibrations in the air which, striking on the drum of the ear, make it vibrate at the same speed, and which are converted by the brain into “sound”. This means that if there were no ears to hear there would be no sound, though the vibrations that cause sound would still be there. These vibrations travel as waves through the air at the rate of 1,090 feet a second.
Contrast this with the speed of light-waves which travel at the rate of 186,000 feet a second!
Now these sound-waves can be reflected or refracted. An illustration will make this clear. Sometimes a boy at play throws a ball against a high wall. The ball strikes the wall and bounces back into his hands. Something like this happens when an echo answers you. You throw the sound of your voice at a tall rock or wall, and the sound bounces back, so to speak, from the wall or rock into your ears. Sound-waves, as we have said, do not travel very fast through the air ; so it takes a little time for them to reach the rock or wall and come back again to your ears. That is why you hear your words repeated a little time after you have said them. So an echo really consists of sound waves or vibrations that are reflected or thrown back from some solid obstacle.
