Physics » Sound and the Physics of Hearing » Doppler Effect and Sonic Booms

# Summarizing Doppler Effect and Sonic Booms

## Summary

• The Doppler effect is an alteration in the observed frequency of a sound due to motion of either the source or the observer.
• The actual change in frequency is called the Doppler shift.
• A sonic boom is constructive interference of sound created by an object moving faster than sound.
• A sonic boom is a type of bow wake created when any wave source moves faster than the wave propagation speed.
• For a stationary observer and a moving source, the observed frequency $${f}_{\text{obs}}$$ is:

$${f}_{\text{obs}}={f}_{s}\left(\cfrac{{v}_{w}}{{v}_{w}±{v}_{s}}\right),$$

where $${f}_{s}$$ is the frequency of the source, $${v}_{s}$$ is the speed of the source, and $${v}_{w}$$ is the speed of sound. The minus sign is used for motion toward the observer and the plus sign for motion away.

• For a stationary source and moving observer, the observed frequency is:

$${f}_{\text{obs}}={f}_{s}\left(\cfrac{{v}_{w}±{v}_{\text{obs}}}{{v}_{w}}\right),$$

where $${v}_{\text{obs}}$$ is the speed of the observer.

## Glossary

### Doppler effect

an alteration in the observed frequency of a sound due to motion of either the source or the observer

### Doppler shift

the actual change in frequency due to relative motion of source and observer

### sonic boom

a constructive interference of sound created by an object moving faster than sound

### bow wake

V-shaped disturbance created when the wave source moves faster than the wave propagation speed

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