This is the Doppler equation.

f ' is the frequency observed by the observer. f is the frequency of the source. v is the speed of sound in the medium v _{o }is the magnitude of the component of the velocity of the observer, relative to the medium, along the line from source to observer. v _{s }is the magnitude of the component of the velocity of the source, relative to the medium, along the line from source to observer.

The **upper** sign in the numerator is for the
case in which the observer is moving **toward** the
source.

The **upper** sign in the denominator is for the
case in which the source is moving **toward** the
observer.

The **lower** sign in the numerator is for the
case in which the observer is moving **away from **the
source.

The **lower** sign in the denominator is for the
case in which the source is moving **away from **the
observer.

Rather than memorize the preceding four sentences; it is better to remember that a velocity of either object toward the other increases the observed frequency and that a velocity of either object away from the other decreases the observed frequency; and; to choose the + and - signs to cause the Doppler equation to yield these results.