As the police car approaches us, the pitch increases, and when it moves away from us, the pitch decreases. When the car approaches us, the sound waves of the siren are ‘compressed’ more towards us, resulting in higher pitch. When the car is moving away, the sound waves get stretched; as a result sound fades away. This effect can be noticed only by an outside observer; the driver hears the same pitch of sound at all times. This phenomenon is called the Doppler effect, which is named after its discoverer.