Question

A person standing on a road sends a sound signal to the driver of a car going away from him at a speed of 72 km h−1. The signal travelling at 330 m s−1 in air and having a frequency of 1600 Hz gets reflected from the body of the car and returns. Find the frequency of the reflected signal as heard by the person.

Answer 1

The frequency of reflected signal as heard by the person is 1417 Hz.

Explanation:

Given,  

• The velocity of Sound (Air) v = 330 $ms^-^1$

• Frequency of signal emitted by the source $n_0$ = 1600 Hz

The velocity of Source $v_s = 72\ kmh^-^1 = 72 \times \frac {5}{18}$

= 20 $ms^-^1$

As the sound gets reflected therefore:

velocity of source ($v_s$) = velocity of observer($v_L$)

The velocity of sound heard by the observer is -

n = $\frac {v + v_L}{v - v_s} \times n_0$

On proper substitution -

n = $\frac {330 - 20}{330 + 20} \times 1600$

= 1417 Hz