Have you ever noticed a change in the “whistle” a train makes as it rushes by? If you listen to a passing train, you’ll notice that the sound gets higher and then lower as the train goes past you.
Actually, the whistle makes the same sound all the time. The sound seems to change because the train is passing you.
The sound spreads out in all directions from the train. But because the train is moving, each sound wave starts a little ahead of the place where the last one started. This makes the sound waves ahead of the train bunch up so that more of the waves reach your ear every second. And the more waves that reach your ear in a second, the higher the sound.
But behind the train, the waves are spread apart. As the train speeds away, fewer waves reach your ear each second – so the sound gets lower.
Most things never catch up with the sound waves they make. But some jet planes do. Supersonic planes can fly faster than sound travels. When they fly this fast, they slam into the waves of air they have made. This creates a tremendous air wave called a shock wave. The shock wave spreads out behind the plane in a funnel shape. Travelling at the speed of sound, the shock wave crashes into the air and the ground. This makes a huge exploding noise called a sonic boom.
Picture Credit : Google
