Any sound source produces sound waves
The speed at which these waves travel depend on what medium they travel through
Sound travels at 340 \[m^s\] in air
The equation c=fλ is extremely useful
When the sound source in stationary, the waves have a uniform wavelength \[\lambda\] between them
When the source is moving, the ripples grow closer together in the direction of motion of the source
Because the frequency and wavelength are inversely proportional...
When the source is moving away from you, the wavelength becomes larger...
So, when an sound source moves closer it sounds louder