The velocity of car is 36 km/h =10 m/s. The appeared frequency due to Doppler effect is f = (
v+vr
v+vs
)f0 where v is the velocity of waves in the medium; vr 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' = (
320
320−10
) × 8 kHz = 8.25 kHz The frequency observed by the car driver is f" =