Observer Moving, Source Stationary

With the source stationary ($v_s=0$), the general Doppler formula reduces to $f' = f_0\,\frac{v \pm v_o}{v}$ If the observer moves TOWARD the stationary source at speed $v_o$, then $f'=f_0\,\dfrac{v+v_o}{v} > f_0$ -- a higher pitch is heard, because the moving observer intercepts wavefronts at a faster rate than a stationary observer would. If the observer instead moves AWAY from the stationary source at speed $v_o$, then $f'=f_0\,\dfrac{v-v_o}{v} < f_0$ -- a lower pitch is heard, the observer now intercepting wavefronts more slowly, in effect chasing after them. It is worth noticing carefully that the algebraic FORM of the correction differs between this case and the source-moving case above: when the source moves, the speed $v_s$ enters in the DENOMINATOR (altering how closely wavefronts are packed in space); when the observer moves, the speed $v_o$ enters in the NUMERATOR (altering only the rate of interception, not the wavefronts' spacing in the medium itself). This genuine asymmetry between a moving source and a moving observer is a direct consequence of the fact that sound requires a material medium (air) whic …