A person riding on a bus hears the sound from a horn on a car that is stopped.

What is different about the sound when the bus is approaching the car compared to when the bus is moving away from the car?

The Doppler effect occurs when a source of waves and/or observer move relative to each other, resulting in the observer measuring a different frequency of the waves than the frequency that the source is emitting.

Let use our case study.

It would be higher pitched when approaching due to the Doppler effect.

The sound waves as you approach them, travel faster relative to you so they pass more frequently which is higher pitched. when you travel away from the source you are out-speeding these waves so they sound slower and thus, lower-pitched.

When a car approaches bus, the sound waves that reach person on the bus have a shorter wavelength and a higher frequency. The person hear a sound with a higher pitch. When the car moves away from bus, the sound waves that reach the person have a longer wavelength and lower frequency. The person hear a sound with a lower pitch.