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 vo is the magnitude of the component of the velocity of the observer, relative to the medium, along the line from source to observer. vs 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.