The Doppler Effect describes how moving sound waves can shift frequencies for either the observer, the source, or both, depending on the motion of each. You’ll find Doppler shifts with waver, sound, and light waves. For sound waves, if the source is moving toward the observer, the observer will hear higher pitch sounds. And if the source moves away from the observer, then they will hear lower pitch sounds. The frequency at the source didn’t change, only what the observer perceives is difference because there’s motion between them. It’s a shift in the apparent frequency.

[am4show have=’p9;p58;’ guest_error=’Guest error message’ user_error=’User error message’ ]

If the source is moving really fast – we’re talking faster than the wave itself can move – then the waves get bunched up in front of the source, like the water waves at the bow of a boat in the water, and form a shock wave. Aircraft that approach the the speed of sound (Mach 1) form shock waves at specific points on the aircraft. As an aircraft moves faster than Mach 1, it will move ahead of the waves it creates.

The white cloud you see around the jet is related to the shock waves that are forming around the aircraft as it moves into supersonic speeds. You can think of a shock wave as big pressure front, which creates clouds. In this case, the pressure from the shock waves is condensing the water vapor in the air.

Click here to go to next lesson on Air Horn.

[/am4show]