An echo is a reflected (反射) sound. Stand in a very large, empty hall with nothing on the walls. Then shout. The sounds you make will hit the walls and come back to you. The walls reflect the sounds. These reflected sounds are called "echoes".
We can use reflected sound to make sound travel farther. Hold a watch about fifty centimeters from your ear. You cannot hear it. The sound of the watch travels a short way in all directions. lt does not travel as far as your ear. You can make it travel to your ear by using a roll of cardboard (纸板筒).
