A source of sonic oscillations with frequency $n=1700$ Hz and a receiver are located on the same normal to a wall. Both the source and receiver are stationary, and the wall recedes from the source with velocity $u=6.0$ cm/s. If the beat frequency registered by the receiver is (0.6)x Hz. Then the value of x to the nearest integer is?. The velocity of the sound is equal to $v=340m/s.$

As both source and receiver are at rest the frequency of sound waves which directly reach to receiver, will be 1700 Hz and the frequency of sound which wall will receive as a moving observer is 
     $n_1=n_0\left[\frac{v}{v+u}\right]$
         The wall now behaves as a moving source of frequency $n_1$ which when received by receiver, the frequency observed is
     $\begin{array}{l}{n}_2=n_1\left[\frac{v-u}{v}\right]\\ =n_0\left[\frac{v-u}{v+u}\right]\end{array}$
         $\begin{array}{l}=1700\left[\frac{340-0.06}{340+0.06}\right]\\ =1700\times \frac{339.94}{340.06}\\ =1699.4Hz\end{array}$  
      Thus beat frequency received by detector is  $\Delta n=1700-1699.4=0.6Hz$