When an ambulance passes by the sirens pitch changes. The sounds becomes higher-pitched as it approaches and then the pitch lowers as it gets further away. This change in pitch is known as the Dopp?

The change in pitch of a siren as it approaches and moves away is known as the Doppler effect. This effect occurs because the sound waves emitted by the siren are compressed as the ambulance approaches, leading to a higher pitch, and stretched as it moves away, resulting in a lower pitch.