Question

When a sound source approaches you, how does the pitch change?

Answer 1

Doppler Effect.

Explanation:

An increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other. The effect causes the sudden change in pitch noticeable in a passing siren, as well as the red shift seen by astronomers.

Taken from here.

Example of it is an ambulance approuching to you. From far away you can hear the sound changing as it approuches to you. This is beacuse the wavelenght is getting shorter as it comes towards you and the wavelenght increases as it passes you.

Hope I helped :)