Physics » Mechanical Waves and Sound » Summary of Doppler Effect

Summary and Main Ideas


  • The Doppler effect is a change in observed frequency due to the relative motion of a source and an observer.

  • The following equation can be used to calculate the frequency of the wave according to the observer or listener:


  • If the direction of the wave from the listener to the source is chosen as positive, the velocities have the following signs:

    Source moves towards listener

    \({v}_{S}\): negative

    Source moves away from listener

    \({v}_{S}\): positive


    Listener moves towards source

    \({v}_{L}\): positive

    Listener moves away from source

    \({v}_{L}\): negative

  • The Doppler effect can be observed in all types of waves, including ultrasound, light and radiowaves (explored in another tutorial).

  • Sonography makes use of ultrasound and the Doppler effect to determine the direction of blood flow.

Table: Units used in Doppler effect

Physical Quantities
QuantityUnit nameUnit symbol
Frequency (\(f\))hertz\(\text{Hz}\)
Speed (\(v\))metres per second\(\text{m·s$^{-1}$}\)

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