The wavelength of the sound wave becomes longer as the car moves away. A longer wavelength means that not as many wave cycles can pass in a given time interval, so the frequency is lower, and thus the pitch of the sound is much lower .
The wavelength of the sound wave becomes longer as the car moves away. A longer wavelength means that not as many wave cycles can pass in a given time interval, so the frequency is lower, and thus the pitch of the sound is much lower .