What is sound?

1. Have you ever wondered how different sounds are made? What is sound?

2. Vibration • The back-and-forth motion is called a vibration. • What can you notice if you place your fingers against your throat while you talk or hum? • Vocal cords in your throat vibrate when air moves past them. This allows you to speak. • Consider the bell of an alarm clock. When the alarm goes off, the bell vibrates. How does the sound reach your ear?

3. Sound Waves • Think about what happens when an ocean wave rolls under a floating object. • In some ways an ocean wave is like a sound wave. • A sound wave is a wave that transfers sound through matter and spreads outward in all directions.

4. Energy Transfer • Study this picture. The blue dots show what happens to air particles when the bell rings. • Energy from a vibration causes air particles to move. • Air particles bump into one another. • Some air particles are crowded and some are spaced apart. • The air particles move back and forth.

5. Quick Check • Can a sound wave move through water? Explain. • When you pluck a string, it vibrates and makes a sound. How can you stop the sound?

6. You know that sound travels through air. Sound can also travel through solids, liquids, and other gases. How does sound travel?

7. Echoes • An echo is a repeated sound that occurs when sound waves bounce off a surface. • Dolphins use echoes to navigate and find prey. The sounds they make are reflected by underwater objects, like fish.

8. The Speed of Sound • Sound does not travel at the same speed through all materials. • Sound travels: • slowest in a gas (air) • faster through a liquid (water) • fastest through a solid (glass or metal) • Sound cannot travel through a vacuum • A vacuum does not contain matter. • Without matter there can be no sound waves.

9. The Human Ear • What happens when a sound wave reaches your ear?

10. Quick Check • A friend shouts from the far side of an empty gym? You hear the word several times. Why? • Why can’t sound travel through outer space?

11. If all sounds come from vibrations, why don’t they all sound the same? How does sounds differ?

12. Try tapping your desk and then slapping your desk. • Were the sounds the same? • Do you think the sound waves you produced were the same?

13. Wavelength and Frequency • The wavelength is the distance from the top of one wave to the top of the next wave. • In sound waves, wavelength is the distance from one area of crowded particles to the next. • Frequency is the number of vibrations a sound source makes in a given amount of time. • When you strike a small bell, it vibrates quickly. • The vibrations produce sound waves with a high frequency.

14. Pitch • The frequency of a sound wave determines its pitch. • Pitch is the highness or lowness of a sound. • High sounds have high frequencies (beat of a mosquito’s wings) • Low sounds have low frequencies (croaks of a toad)

15. Amplitude and Volume • The amount of energy in a sound wave is related to its amplitude. • Sound waves with high amplitude are made by objects that vibrate with a lot of energy. • Amplitude affects the volume, or loudness, of sound. • As an airplane takes off, sound waves with high amplitude fill the air creating a loud sound. • When you whisper, your vocal cords vibrate just a little. The sound waves have low amplitude and the volume is low.

16. Quick Check • You tune a guitar. One string makes a sound that is too low. How can you fix this? • Why do flutes and tubas make different sounds?

17. What is sonar? • Sonar is a technology that uses sound waves to detect underwater objects. • Sonar stands for SOundNavigation And Ranging. • It works by sending out sounds and receiving echoes.

18. Using Echoes • A sonar device measures the time difference between the sound and its echo and then makes an image of the object. • How do we use sonar? • Sailors use it to measure how deep the water is. • Fishers use sonar to find schools of fish. • Scientists use sonar to map the ocean floor. • Others use it to search for shipwrecks.

19. Quick Check • Why might it be important to locate shipwrecks underwater? • Why doesn’t sonar work well in air?