in still air. The Wind then starts blowing towards the petrol pump at 
Calculate the wavelength, speed, and frequency of the horn’s sound for a man standing at the petrol pump. Is this situation completely identical to a situation when the observer moves towards the truck at 
sand the air is still?For the standing observer:
Frequency is given as 
Velocity of sound is given as 
Speed of the wind is given as 
Because there is no relative motion between the observer and the truck, the observer will hear the horn at 
..
The wind blows in the observer’s direction at 
.
Effective velocity of the sound will be, 
The wavelength 
of the sound can be calculated as
For the observer running towards the train :
Speed of the observer is given as 
We know,
The apparent frequency of the sound as the observers move towards the truck is :
= ![v_{H}\left[\left(v+v_{0}\right) / v\right]](https://www.learnatnoon.com/s/wp-content/ql-cache/quicklatex.com-15615ab099e145f5fc270dbbeb0e15ff_l3.png "Rendered by QuickLaTeX.com")
=![200[(20+340) / 340]=211.764 \mathrm{~Hz}](https://www.learnatnoon.com/s/wp-content/ql-cache/quicklatex.com-b34b1fbda33351f7af999344dbfb9751_l3.png "Rendered by QuickLaTeX.com")
As the air is still the effective velocity of sound is still 
.
As the truck is stationary the wavelength remains 
.
Hence, the two cases are not completely identical.