Approaching
Sound waves are compressed, so the observer hears a higher pitch.
Physics Learning Tool
Doppler Effect of Sound Simulator
Watch and hear how an ambulance siren changes pitch as it moves toward, passes, and moves away from an observer.
Big idea: When the ambulance approaches, sound waves bunch up and the siren sounds higher. After it passes, waves spread out and the siren sounds lower.
Ambulance passing an observer
Move the ambulance or press Start Motion. The observer hears a higher pitch before the ambulance passes and a lower pitch after it passes.
Approaching: higher pitch
Wavefronts are closer together in front of the moving ambulance, so the observer hears a higher frequency.
-180 m
25 m/s
700 Hz
343 m/s
What observer hears755 Hz
Pitch change+55 Hz
Wavelength in front0.45 m
Wavelength behind0.53 m
Approaching: f' = f × v / (v - vs)Use this before the ambulance reaches the observer.
Receding: f' = f × v / (v + vs)Use this after the ambulance passes the observer.
What to notice
The actual siren frequency stays the same. The observed frequency changes because the source is moving relative to the observer.
What students should remember
Passing moment
The pitch changes quickly when the ambulance moves past the observer.
Moving away
Sound waves are stretched out, so the observer hears a lower pitch.
Practice keywords
Use this Doppler Effect of Sound Simulator to explore sound waves, ambulance siren pitch, observed frequency, source speed, wavelength, higher pitch, and lower pitch.
Doppler Effect of Sound Simulatorsound wavesambulance sirenobserved frequencypitch changewavelength