SIMPLIFIED: "17.8: The Doppler Effect"

Link to article: https://phys.libretexts.org/Bookshelves/University_Physics/University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/17%3A_Sound/17.08%3A_The_Doppler_Effect 

Note: The image above was taken from the original article.

The article linked above is about the Doppler effect. You may have heard this name before. When is it used in life? Have you ever noticed how a siren from an ambulance, police car, or a fire truck gets super high pitched when the car gets near you and then that sound gets lower pitched as it goes away until you stop hearing it? That is because of the Doppler effect. 

I’ve simplified an article on sound waves on this website, but in that article, we assumed that the thing making sound was a stationary object. However, the Doppler effect deals with things that make sound while they are moving, like these police cars. When a police car is moving TOWARDS you, there are new sound waves constantly being created. However, the police car had already created sound waves before. Therefore, when the police car is moving towards you, the new sound waves that it is creating are closer to the waves that it had created before, causing more waves to reach your ear. Since there are more waves over the same amount of time, the frequency will increase, causing you to hear a higher pitched sound. 

On the other hand, when a police car is moving away from you, the waves are not as close together anymore. Since you experience fewer sound waves over the same amount of time, the frequency of the wave is going to be lower, and you will therefore hear a lower pitched sound.