The siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer. Astronomer John Dobson explained the effect thus:
according to the doppler effect the sound of a police car siren is
start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer