Echoes> Have you ever walked in a big open space with a tall wall, a building or a cliff on one side, and yelled out? “ECHO!” You probably heard back “. . .Echo!” It sounded like someone over there was yelling back at you - in your own voice! What was going on?
Of course, that really was your own voice you heard a little bit after you yelled. The sound waves from your voice took a small amount of time to travel to the wall. Then they bounced backwards and returned to your ears, taking more time. So because sound took time to travel there and back again to your ears, you heard your own voice after you had stopped yelling. An <#5652,5110><!echo>!
How did the wall bounce back the sound? The sound <#5675,6><!waves> from your voice were <#5679,2><!vibrations> of air that squeezed and relaxed, squeezed and relaxed as they traveled forward. When they hit the wall, they pushed against the heavy wall. But it didn’t move. So the air just pushed and sprang back, and started moving in the opposite direction. It bounced off the wall! Sound waves bounce, or reflect, off many large, solid objects.
Some materials don’t reflect sound nearly as well as others. You’ll get a very nice, strong echo off a hard surface such as a marble cliff. But you won’t get nearly as good an echo off a group of trees (though sometimes you can get a tiny echo). Some materials absorb sound energy, preventing an echo from bouncing back at you. Even in a large room, you can often hear the difference between echoes off a marble, a wooden, or a cloth-covered wall.
