Waves

The velocity of car is 36 km/h =10 m/s. The appeared frequency due to Doppler effect is f = $\left(\frac{v+v_{r}}{v+v_{s}}\right) f_{0}$ where v is the velocity of waves in the medium; $v_{r}$ is the velocity of the receiver relative to the medium, which is positive if the receiver is moving towards the source; vs is the velocity of the source relative to the medium; positive if the source is moving away from the receiver. Therefore, the frequency reflected from the wall is f' = $\left(\frac{320}{320-10}\right)$ × 8 kHz = 8.25 kHz The frequency observed by the car driver is f" = $\frac{320+10}{320} f'$ = $\frac{330}{320} \times 8.25$ kHz = 8.5 kHz