Sound Waves
4.0 Doppler's Effect
4.0 Doppler's Effect
When a source (of light and sound) and observer separated by a distance are at rest relative to the medium, the observer receives the same number of waves per second as emitted by the source.
However if there is motion of source and observer relative to the medium (line of sight) the number of waves received per second is usually different from the emitted and frequency or pitch of the source appears to be different.
This apparent change in frequency or pitch due to motion of source and observer relative to the medium along the line of sight is called doppler's effect.
Example: The sound of vehicle's horn changes as the vehicle moves past you.
Expression for the apparent frequency of sound in different cases with the following assumption.
- The velocity of the source, observer and the medium are along the line joining the position of the source and the observer.
- The velocity of the source and the observer is less than the velocity of sound.
- The velocity of sound is always positive.
Special notation
$f =$ Actual frequency
$f '=$ Observed frequency (apparent frequency)
${\overrightarrow v _w=}$ Velocity of sound
${\overrightarrow v _s}=$ Velocity of source
${\overrightarrow v _o}=$ Velocity of observer
