When does the Doppler effect happen?

Answer 1

The doppler effect happens when the source of a sound is moving. When it is moving towards you, it sounds louder and can have a different pitch and when the source is moving away, its a quieter more "far-off" sound. This is because when the source is moving, the sound waves are getting compressed in front of it due to relative velocity. Once the sound wave is made, it's speed is independent of its source. This is also how a sonic boom happens. At super sonic speeds, the sound barrier is broken which is the build up of all the sound waves created.

Answer 2

The Doppler effect occurs when there is relative motion between a source of waves (such as sound or light) and an observer. As the source and observer move closer together, the wavelength appears shorter and the frequency higher (resulting in a higher pitch for sound waves). When they move farther apart, the wavelength appears longer and the frequency lower (resulting in a lower pitch for sound waves).