 Doppler shift

 
The change in frequency of a wave emitted by a moving source is calculated from the component of its velocity moving towards or away from the observer (or listener in the case of sound). The ratio of emitted and detected frequencies is given by c/(v+c), where c is the wave velocity and v is the radial source velocity.
 
The speed of sound in air is ~343 m/s, and of light in a vacuum is 299792458 m/s (or 1 c  use the input unit menus!). This calc can be used e.g. to determine redshifts in nonrelativistic situations, or the pitch change of a passing vehicle siren. Doppler shifts are used in many apparatus such as police radar, spectroscopy, and medical analyses. Note that you do not need to enter a source frequency to determine the pitch change or proportional frequency change.  