Chapter 16.1 The Nature of Sound

1. Chapter 16.1 The Nature of Sound Pg. 540-545

2. Question to think about… • If a tree falls in a forest and no one hears it , does the tree make a sound?

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

4. Making Sound Waves • A sound wave begins with a vibration • Sound waves are made up of compressions and rarefactions • Example: • Striking a metal gong • When the gong is struck it vibrates • The vibrations disturb nearby air particles • Each time the gong moves to the right it pushes air particles together creating a compression • Each time the gone moves to the left, the air particles bounce back and spread out, creating a rarefaction

5. How Sound Travels • Sound waves carry energy through a medium without moving particles of the medium along • Each particle of the medium vibrates as the disturbance passes. • When the disturbance passes your ear you hear the sound • A common medium for sound is air, but sound can travel through many other mediums

6. Interactions of Sound • Sound waves reflect off objects, diffract through narrow openings and around barriers, and interfere with each other

7. Sound Waves and Reflection • A reflected sound wave is called an echo • The harder and smother the surface, the stronger the reflection • In rooms where there are soft materials you don’t hear an echo because the soft materials absorb the sound

8. Sound Waves and Diffraction • Sound Waves do not always travel in a straight line • Sound waves can also diffract, or bend, around corners

9. Sound Waves and Interference • Sound waves may meet and interact with each other • When sound waves meet constructive or destructive interference can occur

10. The Speed of Sound • All sound travels through the air at the same speed • At room temperature (20ᵒC) sound travels through the air at 343 m/s • The speed of sound is not always 343 m/s because sound waves travel at different speeds in different mediums • The speed of sound depends on the characteristics of the medium that sound travels through • 1. Elasticity • 2. Density • 3. Temperature

11. Speed of Sound: Elasticity • Elasticity is the ability of a material to bounce back after being disrupted • The elasticity of a medium depends on how well the medium’s particles bounce back after being disrupted • In a more elastic medium the particles bounce back more quickly • In a less elastic medium the particles bounce back slower

12. Speed of Sound: Elasticity • The more elastic the medium, the faster sound travels • Sound travels well in solids because they are usually more elastic • Particles of solids do not move very far , so they bounce back and forth very quickly as the compressions and rarefaction of the sound waves pass • Most liquids are not very elastic and sound does not travel well • Sound travels very slowly in gases because gases are not very elastic

13. Speed of Sound: Density • Density is how much matter, or mass, there is in a given amount of space, or volume. • Sound travels more slowly in denser mediums • The particles of dense materials do not move as quickly as those of less dense material.

14. Speed of Sound: Temperature • Sound travels more slowly at lower temperatures than at higher temperatures • This is because at low temperatures, the particles of the medium move more slowly • At low temperatures the particles are harder to move and return to their original positions more slowly. • Example: @ 20ᵒC  343 m/s @ 0ᵒC  330 m/s

15. Speed of Sound: Temperature • Since the temperatures are lower at higher altitudes, sound travels more slowly at higher altitudes