When a source moves toward a wall: the wall receives frequency $f' = fv/(v-v_s)$ and reflects it. A stationary observer between source and wall hears two frequencies: direct $f_d = fv/(v+v_s)$ (source receding) and reflected $f_r = fv/(v-v_s)$ (effectively approaching). Beat frequency = $f_r - f_d = fv \cdot 2v_s/(v^2-v_s^2)$. If $v_s \ll v$: beat frequency $\approx 2fv_s/v$.