Chapter 16.1 The Nature of Sound

Pg. 540-545

Question to think about…

• If a tree falls in a forest and no one hears it , does the tree make a sound?

Sound Waves:

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

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

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

Interactions of Sound

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

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

Sound Waves and Diffraction

• Sound Waves do not always travel in a straight line

• Sound waves can also diffract, or bend, around corners

Sound Waves and Interference

• Sound waves may meet and interact with each other

• When sound waves meet constructive or destructive interference can occur

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

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

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

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.

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

Speed of Sound: Temperature

• Since the temperatures are lower at higher altitudes, sound travels more slowly at higher altitudes