The Doppler Effect: Why Ambulance Sirens Sound Different Approaching vs. Receding

The change in siren pitch as an ambulance passes is the Doppler effect. When a sound source moves toward an observer, sound waves compress, increasing frequency (higher pitch). As it moves away, waves stretch, decreasing frequency (lower pitch). This phenomenon applies to all wave types, including light and water waves.

Key Points

• Approaching: higher pitch/louder perception.
• Receding: lower pitch/softer perception.
• Not limited to sound; observed with light (redshift/blueshift) and water waves.