When a fire engine goes by, why does the siren’s sound change?

Have you ever stood on a city sidewalk and heard the distant wail of an approaching fire engine? As the fire truck comes into view, the sound of its siren becomes louder and more frantic, the wail higher and higher pitched. Then, as it passes, the opposite effect occurs: The sound of the siren drops in pitch, getting lower and lower as the vehicle vanishes into distant traffic. But the fire truck’s driver hears no such change; to him, the siren that he flipped on 20 blocks ago has sounded at a steady pitch.