Given information
Given:
The difference in pitch of approaching and retreating cars' engine noises can be used to calculate vehicle speeds on a racetrack.
Explanation:
When the source of sound waves moves relative to a stationary listener, the Doppler effect occurs, generating a frequency shift. (If the sound source is stationary but the listener is moving, this can also happen.) The Doppler effect can be observed in the diagram below. The siren of a police car sends out sound waves in all directions. The sound waves clump together in front of the car and spread out behind it because it is moving ahead (to the left). The frequency of sound waves that are closer together is greater, whereas the frequency of sound waves that are further apart is lower. The pitch of a sound is determined by the frequency of sound waves. Sound waves with a greater frequency produce higher-pitched sounds, whereas sound waves with a lower frequency produce lower-pitched sounds.
Step-by-step explanation
As the car approaches, the frequency of the engine sound is Doppler shifted upward, as illustrated in
. [source moving toward stationary observer]
As the car travels away, the frequency of the engine sound is Doppler shifted down, as illustrated in
.[source moving away from stationary observer]