Question

A driver in a stationery car blows a horn which produces monochromatic sound waves of frequency 1000 Hz normally towards a reflecting wall. The wall approaches the car with a speed of 3.3 ms^-1 –

Assume speed of sound in air is 340ms^-1

1. The frequency of sound reflected from wall and heard by the driver is 1020 Hz
2. The frequency of sound reflected from all and heard by the driver is 980 Hz
3. The frequency of sound reflected from wall and heard by the driver is 1040 Hz
4. The frequency of sound reflected from wall and heard by the driver is 960 Hz

Answer 1

Correct Answer - Option 1 : The frequency of sound reflected from wall and heard by the driver is 1020 Hz

Before reflection, the stationary car was source and wall was observer and observer is moving towards the source. After reflection, the wall becomes the source and the man in stationary car becomes observer; now the source is moving towards the stationary observer. The apparent frequency is now given as

\(f' = f\frac{{v + {v_{wall}}}}{{v - {v_{wall}}}} = 1000\frac{{\left( {340 + 3.3} \right)}}{{340 - 3.3}} = 1020\;Hz\)