Source, sound, observer are in same direction f' = f $ ( $ v-v~^o #_ v-v~^s $ )
if the direction of observer or source changes, then sign changes.
If freq. Appears be double the freq. of source, for a stationary observer, then v~^s = $v#_2
If for a stationary observer freq. Appears to be half the real freq. Then
v~^s= -v