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When you knock off something it vibrates making the air vibrate which causes the sound of the bang. The air vibrates to your eardrums and sends a message to the brain and the brain tells you. That is sound.
The Sound Barrier.
Sound is: - at sea level - app. 1223km /hr (760 mph)
Chuck Yeager.
Chuck Yeager was born in Febuary the 13, 1923. He was born in Myra in West Virginia. He grew up in Hamlin. When he finshed High School, he joined the U.S. Army Air Corps. He fought in World War II. Chuck's plane was shot down over enemy lines in 1943,but with the help of the French he made it to Spain, which was neutral. Chuck was not allowed to fly again, but he made an appeal to Genral Dwight D.Eisinhower and he was allowed to fly agian in World War II. He stayed in the Airforce teaching youngsters to fly. In 1947 he was asked to test a rocket-powered X-I fighter plane. Yeager broke the sound barrier on Oct. 14th 1947.
The sound barrier is a speed barrier that judges plane's actions by the speed they are going. To break the sound barrier you have to go 1,220 kph. The shock wave that is made by an airplane moving at a supersonic speed makes a shocking change in air pressure. The fastest plane ever is the SR-7I blackbird. This is one of the most difficult barriers to go through because the high-speed flight makes heat through friction for eg. Rub your hands together really hard you will feel that they are getting hotter. That is friction. Because of that, the surface of the plane's temperature gets higher then the safety limit for the structure of the aircraft.
The speed of sound.
Slow-flying planes create air pressure disturbances that move at the speed of sound, traveling well in front of the plane. The airflow adjusts and disturbances disperse. Planes flying at the speed of sound experience a dramatic increase in their drag because disturbances accumalate instead of disperse. The airplane has almost caught up with pressure waves it is creating with its foward thrust. Planes flying faster than the speed of sound cause powerful shockwaves because airflow has no time to adjust for them. The amazing photo above shows the actual moment when this happens!