What physical change occurs when source of sound moves and observer is stationary?

What is the physical change occurs when the source of sound is stationary but the observer moves?

In case of a moving source of sound approaching an observer.

A train standing at the outer signal of a railway station blows a whistle of frequency 400 Hz in still air .The train begins to move with a speed of 10ms^(-1) towards the platform. What is the frequency of the sound for an observer standing on the platform?(sound velocity in air = (330ms^(-1)) )

What is the apparent change in frequency when source and observer move with same velocity in the same direction?

When a source of sound moves towards stationary observer,the wavelength of sound received by the observer

What is the cause of Doppler's effects when the observer is in motion?