Physics » Mechanical Waves and Sound » Summary of Doppler Effect

# Summary and Main Ideas

## Summary

• 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:

${f}_{L}=\left(\frac{v+{v}_{L}}{v+{v}_{S}}\right){f}_{S}$

• 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 Quantity Unit name Unit symbol Frequency ($$f$$) hertz $$\text{Hz}$$ Speed ($$v$$) metres per second $$\text{m·s^{-1}}$$

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