Do you hear the echo of your voice faster in the short tube (25 m) or the long tube (50 m)? That’s right, you hear it faster in the short tube. Twice as fast, in fact!
When you shout, you make a sound. That sound travels as waves through the tube, right to the end. There it bounces back and reaches your ear again. Then you hear your voice, but with a delay. We call this the “echo”. In the short tube, you will hear this echo sooner, because the sound does not have so far to travel. It only has to cover half the distance.
You only hear an echo if the distance is long enough. If the sound bounces back too quickly, you hear it (virtually) at the same time as the sound that you made. That’s why you don’t hear an echo in shallow wells, for example, but you do in very deep wells.
Echoes to look inside a body
How can you look at a baby that is still in its mother’s belly? It’s possible with echoes.
The doctor sends sound waves — that you can’t hear — through the body of the mother (and the baby). These waves come up against various tissues in the body. These bounce the sound waves back. A computer then converts these echoes into video images.
We call this technique “echography” or ultrasound. As well as babies, you can use it to look at your heart, bladder, kidneys, blood vessels and more.