When an object is moving away from you while making a sound the pitch of the sound is perceived to be lower.
This phenomenon is described as the Doppler effect. Doppler effect explains the change in the observed frequency of a wave when there is a relative motion between the wave source and the observer.
The pitch perceived by an observer depends on the frequency of the sound wave. A high frequency corresponds to a high pitch. when a body such as an ambulance siren, or police car siren is moving away from the observer, the frequency observed and the pitch of the sound drop.
