Chapter 16: Sound

1. Chapter 16: Sound Section 1: The Nature of Sound Section 2: Properties of Sound

2. Section 1: The Nature of Sound • Sound & Longitudinal Waves • Remember, sound begins with a vibration. • How sound travels: • Sound is a disturbance that travels through a medium as a longitudinal wave.

3. Sound is a disturbance that travels through a medium • as a longitudinal wave.

4. How Sounds Are Made As the drum vibrates back & forth, it creates compressions & rarefactions in the air.

5. How Sounds Are Made When a person speaks or sings, the vocal cords vibrate. The vibrations produce longitudinal sound waves in the air.

6. Sounds in Solids & Liquids • Example: • When you knock on the door, the particles of the door vibrate. • The vibration creates sound waves that travel through the door. • When the sound waves reach the other side of the door, they make sound waves in the air on the far side.

7. When sound waves enter a room through an open door, they diffract (spread out).

8. Sound can only travel in there is a medium to transmit the compressions & rarefactions. • In outer space, there are no molecules to compress or rarefy. • The energy of the original vibrations has nothing through which to travel. • So, sound does not travel through the vacuum of space.

9. How Sound Bends • When sound waves hit a barrier with a small hole in it, some of the waves pass through the hole (Similar to the diffraction shown in a harbor). • When sound waves go through a doorway, they spread out. • Because of diffraction, you can hear sounds from around corners. • Waves passing a corner spread out as they pass.

10. The Speed of Sound • Speed of sound depends upon the elasticity, density, and temperature of the medium. • Speed of sound at room temp. is about 342 m/s.

11. Speed of sound depends on the medium through which it is traveling!

12. Elasticity = the ability of a material to bounce back after being disturbed. • If a medium is very elastic, its particles easily go back to their original positions. • Sound travels more quickly in mediums that have a high degree of elasticity because when the particles are compressed, they quickly spread out again.

13. Which one of these would sound travel faster in? Some substances are more elastic than others. Sponges & rubber bands are more elastic than modeling clay. .

14. Solid materials are usually more elastic than liquids or gases, so compressions & rarefactions travel very well in solids. • Liquids aren’t very elastic • Gases are generally very inelastic & are the poorest transmitters of sound.

15. Density • Speed of sound depends on how close together the particle of the substance are. • In materials of the same state of matter – solid, liquid, or gas- sound travels more slowly in denser mediums. • The denser the medium, the more mass it has in a given volume. The particles of a dense material do not move as quickly as those of a less dense material. • Sound travels more slowly in dense metals, such as Pb or Ag, than in Fe or steel.

16. Temperature • In a given medium, sound travels more slowly at lower temperatures and faster at higher temperatures. • At 20 ºC, speed of sound in air = 340m/s. • At 0 ºC, speed of sound in air = 330 m/s. • At higher altitudes the air is colder, so sound travels more slowly.

17. Moving Faster than Sound Oct. 14, 1947, Chuck Yeager became the first person to fly faster than the speed of sound. He flew at a high altitude in order to do this. Why?

18. Moving Faster than Sound Oct. 15, 1997, Andy Green became the first person to drive a land vehicle faster than the speed of sound. Why did he choose to drive in the desert?

19. Section 2: Properties of Sound

20. Intensity • When a sound wave carriers a large amount of energy, the molecules of the medium move a greater distance as the waves pass by, and the sound wave has a greater amplitude. • The intensity of a sound wave is the amount of energy the wave carries per second through a unit area. • Measured in watts per square meter (W/m2)

21. Loudness • Sound waves of higher amplitude have a greater intensity because they carry more energy per second through a given area. • Though intensity & loudness aren’t the same thing, the greater the intensity of a sound wave, the louder it is. • Loudness describes what you actually hear. • Measured in decibels (dB). • Sounds louder than 100dB can cause damage to your ears.

23. A loudspeaker gives out sound by vibrating cones of material. The greater the amplitude of vibration, the greater the volume, or loudness, of the sound.

24. Frequency • Most people can hear sounds with frequencies between 20 Hz & 20,000 Hz. • Sounds with frequencies above the normal human range of hearing are called ultrasound. • Sounds with frequencies below the normal human range of hearing are called infrasound.

25. Pitch • The pitch of a sound is a description of how high or low the sound seems to a person. • The pitch of a sound that you hear depends on the frequency of the sound wave.

26. The key farthest to the left on a piano is attached to the longest string. This key plays the note with the lowest pitch.

27. Resonance • All objects vibrate naturally. • Frequency of the object depends on the shape & type of the object. • If the frequency of sound waves exactly matches the natural frequency of an object, the sound waves can add to the object’s vibrations. • Resonance occurs when the frequency of the sound waves & the natural frequency of the object are the same.

28. Some musical instruments can produce notes w/ vibrations that match the natural frequency of a crystal glass. If the note is sustained, the amplitude of vibration can cause the glass to shatter.

29. The Doppler Effect • Doppler Effect = the apparent change in frequency as a wave source moves in relation to the listener. • If the waves are sound waves, the change in frequency is heard as a change in pitch.

30. The Doppler Demonstration: • Doppler put a musical band on an open flatcar of a train. • He stood on the ground nearby. As the train approached him, the notes the musicians played seemed to be a higher pitch. • As the train passed, the notes seemed to drop in pitch.

31. Doppler repeated the experiment, but this time he stood on the train and had the musicians play while they were seated on the ground. • Doppler heard the same changes in pitch as the train he rode approached and passed the band • The effect was the same regardless of who was moving, the band or Doppler.

32. Changing Pitch • As a sound source moves toward the listener, the waves reach the listener with a higher frequency. The pitch appears to increase because of the Doppler effect.

33. As the police car speeds by, the pitch of the siren seems to change. Ahead of the car, the sound waves are piling up, so the pitch is higher. Behind the car the waves spread out, so the pitch is lower.

34. When a plane flies faster than the speed of sound, waves pile up to form the shock wave known as the sound barrier.