- Describes the change in the transmitted frequency of a sound wave (ft) compared to the reflected (observed) frequency of the wave (fr) that occurs due to the relative motion between the observer and the source
- If the source and the observer are stationary any waves reflected back will be of the same frequency (fr = ft)
- If the source is moving toward the observer it causes waves to be more closely packed together, so the observer witnesses a higher frequency wave (fr > ft)
- Conversely, if the source is moving away from the observer it causes waves to be more loosely packed together, so the observer witnesses a lower frequency wave (fr < ft)
- The change in the observed frequency from that originally transmitted is known as the ‘Doppler shift’ (fd = fr – ft)
- It is proportional to the relative velocity between the source and the observer
- It does not matter whether it is the source or the observer which is moving
- An example in everyday life is the change in pitch of a siren as a police car passes by
