An experiment with sound.

Let's suppose that a plane is flying past an observer at half the speed of sound.
Mark off twenty equal spaces on a sheet of paper, ten before the observer and ten after. The plane covers this distance at a constant speed.But where is the plane, when the observer hears the sound it's making?
The sound made ten spaces away is heard when the plane is half that distance away (5). The sound made nine spaces away is heard when the plane is half that distance away (4.5). Etc.
The plane flies towards the observer and then away from him. Here, the sound made by the plane one space away is heard when the plane is one and a half spaces away. Two spaces corresponds to three, etc.
As the plane approaches the observer at half the speed of sound, the sound it makes during a flight of ten spaces is heard during a flightof only five spaces. The sound is heard at twice the speed it was made.
As the plane goes away at the same constant speed, the sound it makes during ten spaces is heard during fifteen spaces. The sound is heard at two thirds the speed it was made.
Sound speeded up is pitched higher.
Sound slowed down is pitched lower.
If you do the same experiment with a plane flying at the speed ofsound and with one flying faster than sound, you'll find that these results also coincide with the observed (heard) phenomena.