Source Moving, Observer Stationary

With the observer stationary ($v_o=0$), the general Doppler formula reduces to $f' = f_0\,\frac{v}{v \mp v_s}$ If the source moves TOWARD the stationary observer at speed $v_s$, the denominator becomes $v-v_s$ (smaller than $v$), so $f'=f_0\,\dfrac{v}{v-v_s} > f_0$ -- the observer hears a higher pitch than the source's true frequency. If the source instead moves AWAY from the stationary observer at speed $v_s$, the denominator becomes $v+v_s$ (larger than $v$), so $f'=f_0\,\dfrac{v}{v+v_s} < f_0$ -- the observer hears a lower pitch. This is exactly what is heard as an ambulance or a train approaches a stationary platform, passes, and recedes: the siren's or whistle's pitch is heard rising continuously as the vehicle approaches (source mov …