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# Science_Lab_Yellow / Lesson 10: How Do Moving Sounds Change?

 How Do Moving Sounds Change What will we be learning today?   In this lesson, we are going to learn all about how do moving sounds change.

 How Do Moving Sounds Change?   An echo is a copy of the original sound. Both the original sound and the echo have the same pitch. However, have you ever heard a siren blaring as a police car sped past you? If you listened carefully, you may have noticed that the pitch of the siren changed as the police car sped by. As the car came toward you, the siren was higher in pitch. As it sped away from you, the pitch was lower.

 Approaching Sound As the blaring siren approaches, its sound waves crowd together. There are more sound waves reaching your ear each second than there would be if the police car were standing still. The frequency of the sound increases. The pitch is higher.

 Sound waves from the moving police car bunch together (1) as the car approaches the listener. They spread apart (2) as the police car moves away from the listener.

 Departing Sound   As the siren moves away from you, its sound waves spread apart. There are fewer sound waves reaching your ear each second than there would be if the police car were standing still. The frequency of the sound decreases. The pitch of the siren moving away from you is lower.

 Change of Frequency   This change in frequency (and pitch) as a source of sound moves toward or away from you is known as the Doppler effect. It is named for the 19th century Austrian scientist Christian Johann Doppler, who first described it.

 Many radar (radio detection and ranging) devices use the effect to find the speed of objects. Patrol cars detect changes in frequency as a way of detecting speeding vehicles.

 Knowing how different materials reflect and absorb sound waves can be helpful at school or home. Your school library probably has stacks of books and other sound-absorbing materials. They help keep the library quiet so you can read and study. Whether you are a student deciding what furniture to put in a room to make it quiet or an architect deciding how to design a building, you must know how different materials affect sound waves.

 Let's Review What We Learned Today!   Now complete your worksheet!