The Nature of Sound

Objectives

• What is sound?

• What physical properties of a medium affect the speed at which sound travels through it?

What is a Sound Wave?

• Sound waves carry energy through a medium without the particles of the medium traveling along.

What is the medium through

which sound waves travel?

What is a Sound?

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

How are sound waves made?• Air is made up of tiny particles

• Vibrations generate a disturbance in the molecules in the air

• The force of the disturbance pushes the molecules closer together generating a compression

• In between vibrations, the molecules spread out and rarefactions are created.

Sound Waves

Sound Waves

Sound Waves

Pluck a guitar string………. • Plucking the string

causes vibrations

• The vibrations cause compressions and rarefactions

• The sound wave travels through the air in longitudinal wave form

SING!• Vocal cords are like a guitar string

• When you speak, air is forced past your lungs and rushes past your voice box

• Larynx

• 2 folds of tissue vibrate creating sounds

Let’s try it!

How else can sound travel?• Sounds can travel

through solids and liquids

• Knock

• This causes vibrations in the medium

• The vibrations generate sound waves

How far can the vibrations travel?

• You can put your ear on a train track and hear the train coming from miles away.

• Why?

Limitations of Sound:• Sounds can

travel only if there is a medium through which to transmit the compressions and rarefactions

Sound in Space• Sounds can not

travel through outer space

• No molecules in space to compress or rarefy

• There is no matter in space

How does sound bend?•Diffraction – sound

waves can bend and spread around a corner

• Sound waves bend out (diffract) and spread when they hit a barrier or a hole in a barrier

The speed of sound in different media:• The speed of sound depends on the

physical properties of the medium it travels through.

• Air at room temperature – 342 m/s

• The speed sound can travel through a medium depends on:– Elasticity– Density– Temperature

•Elasticity – the ability of a material to bounce back after being disturbed

• Speed depends on how well the particles in the medium bounce back after being disturbed

• The more elastic, the faster sound travels• Examples:

– Solids are more elastic than liquids– Gases are not very elastic at all

Density• Density – how much matter, or mass,

there is in a given amount of space or volume

• The speed of sound depends on how close together the particles of the substance are.

Density Continued• Sound travels more slowly in denser

mediums

• More dense means more mass per volume

• The particles of a dense material do not move as quickly as those of a less dense material

Temperature• Sound travels more slowly at

lower temperatures than at higher temperatures

• At low temperatures, the particles of a medium are sluggish

• Sound waves move and return to their original positions more slowly than they would at high temperatures.