A source of sound emitting at a frequency $f$ is moving towards a wall with speed $v_{\mathrm{b}}$. If $v$ is the speed of sound then the beat frequency heard by a stationary observer behind the source is

• A

  $\frac{v_{\mathrm{b}}}{v-v_{\mathrm{b}}} f$
• B

  $\frac{v_{\mathrm{b}}}{v+v_{\mathrm{b}}} f$
• Correct answer
  C

  $\frac{2 v_{\mathrm{b}} v}{v^2-v_{\mathrm{b}}^2} f$
• D

  $\frac{2 v_{\mathrm{b}}^2}{v^2-v_{\mathrm{b}}^2} f$

Hence, the correct answer is option: 'C'.