Have you ever noticed that during a lightning storm you can see the flash of lightning before you hear the thunder? This is because light travels faster than sound. The speed of sound in freezing air (32 °F.) is about 1,090 feet per second or 743 miles per hour. The speed of sound increases as the temperature rises, about a foot a second faster for each degree. At 68 °F., the speed of sound in air is about 1,130 feet per second or 765 miles per hour.
Sound travels through the air in waves similar to those produced when you drop a stone into a pond. One of the people who studied sound and air waves was an Austrian professor of physics, Ernest Mach. About 1870, he photographed cannon shells flying through the air in order to discover what happens to an object as it speeds through the air. He found that the moving object produced shock waves. The object pushes against the molecules in the air. As one molecule is pushed, it in turn pushes the others near it. Imagine a long line of boys standing one behind the other. The last boy in the line gets pushed. As he moves forward, he pushes the boy in front of him. This happens all the way down the line. This is how sound and shock waves are produced.
As the plane goes through the air, it creates sound waves. The plane itself displaces air about it as it speeds forward.
As the speed of a plane approaches the speed of sound, it is pushing rapidly against the molecules in the air and creating shock waves. As the plane reaches the same speed as sound, these waves pile up and form an invisible barrier. When the plane exceeds the speed of sound, it must "crash" through this barrier. As it does, it creates a thunderlike sound. You will see the plane before you hear its motor, just as you see lightning before you hear the thunder.
