Doppler Effect of Sound Simulator

Use this Doppler Effect of Sound Simulator to see why an ambulance siren sounds higher as it approaches and lower after it passes an observer.

How to use the Doppler effect simulator

Move the ambulance

Use the position slider or press Start Motion to let the ambulance pass the observer.

Change the speed

A faster ambulance creates a bigger pitch difference between approaching and moving away.

Adjust the siren

Change the actual siren frequency and compare it with the frequency heard by the observer.

Try the siren preview

Press Play Siren to hear an ambulance-style siren shift as the ambulance moves.

Core idea

The actual siren frequency does not change. The observer hears a different pitch because the ambulance is moving. Sound waves bunch up in front of the ambulance and spread out behind it.

Formulas used in the tool

Approaching source

f’ = f × v / (v – v_s)

The observed frequency is higher when the ambulance moves toward the observer.

Receding source

f’ = f × v / (v + v_s)

The observed frequency is lower when the ambulance moves away.

Front wavelength

λ_front = (v – v_s) / f

Wavefronts in front of the ambulance are closer together.

Back wavelength

λ_back = (v + v_s) / f

Wavefronts behind the ambulance are farther apart.

Ideas for practice

Set the ambulance speed to 0 m/s and notice that the observed frequency equals the actual siren frequency.
Increase the speed and compare the approaching and receding pitch shift.
Pause the ambulance before and after the observer and explain the wave spacing.
Change the speed of sound and see how the observed frequency changes.

Frequently asked questions

What is the Doppler effect?

The Doppler effect is the change in observed frequency when a sound source and observer move relative to each other.

Why does an ambulance siren sound higher as it approaches?

The ambulance moves toward the sound waves it emits, so the wavefronts in front are compressed and reach the observer more often.

Why does the pitch drop after the ambulance passes?

After passing, each new wavefront starts farther away from the observer, so the waves arrive less often and the pitch sounds lower.

Does the siren itself change frequency?

No. The siren emits the same frequency. The change is in the frequency heard by the observer.