Question

A train is moving at a speed of 72 kmph towards a station sounding a whistle of frequency 640 Hz. What is the apparent frequency of whistle as heard by a man standing on the platform when train approaches to him?

Answer 1

The phenomenon called Doppler effect is the reason for an apparent frequency of the sound.

• Due to the motion of source or listener, the frequency of sound produced can vary.
• A situation with a stationary listener and a moving source where source approaches the listener can be studied using the formula

                                f ' = f ($\frac{c}{c-v}$)

        f -   original frequency of sound

        f ' - Frequency of sound for the listener

        c  - Speed of sound in air at room temperature = 337.5 m/s

        v  - Speed of the source

•         f' = 640 ( $\frac{337.5}{337.5-20}$)  
•            = 680.31 Hz