(1) Source moves toward stationary observer: $f' = fv/(v-v_s)$. (2) Observer moves toward stationary source: $f' = f(v+v_o)/v$. (3) Source at speed of sound ($v_s = v$): $f' \to \infty$ (shock wave / sonic boom). (4) Source and observer move together at same velocity: $f' = f$ (no Doppler shift). (5) Source moves perpendicular to line joining source and observer: no Doppler shift in classical physics. (6) Echo/reflection: wall acts as stationary observer, then re-emits at received frequency.