$A$ car travelling at a speed of $54 \ km/h$ towards a wall sounds a horn of frequency $400 \ Hz$. The difference in the frequencies of two sounds,one received directly from the car and the other reflected from the wall,noticed by a person standing between the car and the wall is (speed of sound in air is $335 \ m/s$):

• A
  $35.9 \ Hz$
• B
  $20 \ Hz$
• C
  $70 \ Hz$
• D
  $35.9 \ Hz$ (Wait,let's calculate: $v_s = 54 \ km/h = 15 \ m/s$,$v = 335 \ m/s$,$f = 400 \ Hz$. Direct sound frequency $f_1 = f = 400 \ Hz$. Reflected sound frequency $f_2 = f \times \frac{v}{v - v_s} = 400 \times \frac{335}{335 - 15} = 400 \times \frac{335}{320} = 418.75 \ Hz$. Difference $= 418.75 - 400 = 18.75 \ Hz$. Since $18.75 \ Hz$ is not in options,let's re-evaluate. If the observer is between the car and wall,the direct sound is $f_1 = f \times \frac{v}{v - v_s}$ and reflected is $f_2 = f \times \frac{v}{v - v_s}$. The difference is zero. Wait,the observer is stationary. Direct sound $f_1 = f \times \frac{v}{v - v_s}$. Reflected sound $f_2 = f \times \frac{v}{v - v_s}$. The difference is $0$. Let's re-read: 'person standing between the car and the wall'. The car is moving towards the wall. The person hears direct sound from the car (source moving towards observer) and reflected sound from the wall (wall acts as a source moving towards observer). Both frequencies are the same. Difference is Zero.