Book O - Waves

Unit 3: Energy On the Move

11.1: The Nature of Sound

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• Every sound is produced by an object that vibrates.

• For example, your friends’ voices are produced by the vibrations of their vocal cords, and music from a carousel and voices from a loudspeaker are produced by vibrating speakers.

What causes sound? 11.111.1The Nature of SoundThe Nature of Sound

• Sound waves are longitudinal waves.

Sound Waves 11.111.1The Nature of SoundThe Nature of Sound

• As you know from the last lesson -- a longitudinal wave is made up of two types of regions called compressions and rarefactions.

Sound Waves 11.111.1The Nature of SoundThe Nature of Sound

• You’ll see that when a radio speaker vibrates outward, the nearby molecules in the air are pushed together to form compressions.

Sound Waves 11.111.1The Nature of SoundThe Nature of Sound

• As the figure shows, when the speaker moves inward, the nearby molecules in the air have room to spread out, and a rarefaction forms.

Sound Waves 11.111.1The Nature of SoundThe Nature of Sound

• As long as the speaker continues to vibrate back and forth, compressions and rarefactions

are formed.

Sound Waves 11.111.1The Nature of SoundThe Nature of Sound

•As the drum vibrates back and forth, it creates compressions and rarefactions in the air – just like the radio speaker.

• Compressions and rarefactions move away from the speaker as molecules in the air collide with their neighbors.

Traveling as a Wave

The Nature of SoundThe Nature of Sound

• A series of compressions and rarefactions forms that travels from the speaker to your ear.

• This sound wave is what you hear.

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Traveling as a Wave

The Nature of SoundThe Nature of Sound

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•When you pluck a guitar string, it vibrates back and forth, creating compressions and rarefactions.

•These compressions and rarefactions travel through the air as longitudinal waves similar to the longitudinal waves that you saw travel along a spring.

Traveling as a Wave

The Nature of SoundThe Nature of Sound

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•Your vocal cords act like vibrating guitar strings. Whenever you speak or sing, you force air from your lungs up through your voice box, or larynx.

Traveling as a Wave

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•Your larynx consists of two folds of tissue called vocal cords, shown below.

•The forced air rushes by your vocal cords, making them vibrate.

Traveling as a Wave

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•As your vocal cords move toward each other, the air between them is compressed.

•As they move apart, the air spreads out, or is rarefied.

•Like vibrating guitar strings, your vocal cords produce compressions and rarefactions in the air.

A Model for Transmitting Sound 11.111.1The Nature of SoundThe Nature of Sound

• A line of people passing a bucket is a model for molecules transferring the energy of a sound wave.

A Model for Transmitting Sound 11.111.1The Nature of SoundThe Nature of Sound

• When the people are far away from each other, like the molecules in gas, it takes longer to transfer the bucket of water from person to person.

A Model for Transmitting Sound 11.111.1The Nature of SoundThe Nature of Sound

• The closer the particles, the faster they can transfer energy from particle to particle.

• The bucket travels quickly down the line when the people stand close together.

• Most sounds you hear travel through air to reach your ears.

Moving Through Materials 11.111.1The Nature of SoundThe Nature of Sound

• If you’ve ever been swimming underwater and heard garbled voices, you know that sound also travels through water.

• Sound waves can travel through any type of mattersolid, liquid, or gas.

Moving Through Materials 11.111.1The Nature of SoundThe Nature of Sound

• The matter that a wave travels through is called a medium.

• For example--In old western movies, you may see someone put an ear to a railway track to tell if a train is on the way. The sound of the train travels easily through the steel tracks.

Moving Through Materials 11.111.1The Nature of SoundThe Nature of Sound

•Sound can travel only if there is a medium to transmit the compressions and 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 outer space.

How Sound Bends11.111.1The Nature of SoundThe Nature of Sound

•When sound waves go through a doorway, they spread out.

•Even if you are off to the side of the room, you may still hear sound from outside.

•If you are outside the room and not too far from the doorway, you can hear sound coming from inside the room.

The Speed of Sound in Different Materials

11.111.1The Nature of SoundThe Nature of Sound

• The speed of a sound wave through a medium depends on the substance the medium is made of and whether it is solid, liquid, or gas.

The Speed of Sound in Different Materials

11.111.1The Nature of SoundThe Nature of Sound

• In general, sound travels the slowest through gases, faster through liquids, and even faster through solids.

The Speed of Sound in Different Materials

11.111.1The Nature of SoundThe Nature of Sound

• Sound travels faster in liquids and solids than in gases because the individual molecules in a liquid or solid are closer together than the molecules in a gas.

The Speed of Sound in Different Materials

11.111.1The Nature of SoundThe Nature of Sound

• Loud sounds travel through a medium at the same speed as soft sounds.

• However, the speed of sound doesn’t depend on the loudness of the sound.

The Speed of Sound in Different Materials

11.111.1The Nature of SoundThe Nature of Sound

•The speed of sound also depends on the temperature of the medium it travels through.

•Sound travels more slowly at lower temperatures and faster at higher temperatures.

•At room temperature, about 20°C, sound travels at about 342 m/s.

•This is much faster than most jet airplanes travel through the air.

Temperature and the Speed of Sound

11.111.1The Nature of SoundThe Nature of Sound

• As the temperature of a substance increases, its molecules move faster.

• This makes them more likely to collide with each other.

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Elasticity11.111.1The Nature of SoundThe Nature of Sound

•Since sound is a transfer of energy, its speed depends on how well the particles in the medium bounce back after being disturbed.

•If you stretch a rubber band and then let it go, it returns to its original shape.

•However, when you stretch modeling clay and then let it go, it stays stretched.

Elasticity11.111.1The Nature of SoundThe Nature of Sound

•Elasticity is 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.

Elasticity11.111.1The Nature of SoundThe Nature of Sound

•Solid materials are usually more elastic than liquids or gases, so compressions and rarefactions travel very well in solids.

•Most liquids are not very elastic. Sound is not transmitted as well in liquids as it is in solids.

•Gases are generally very inelastic and are the poorest transmitters of sound.

Density11.111.1The Nature of SoundThe Nature of Sound

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

•The density of a medium is how much matter, or mass, there is in a given amount of space, or volume.

Density11.111.1The Nature of SoundThe Nature of Sound

•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 lead or silver, than in iron or steel.

Faster Than Sound11.111.1The Nature of SoundThe Nature of Sound

•The supersonic age began with a bang on October 14, 1947.

•Far above the California desert, Captain Chuck Yeager of the United States Air Force had just “broken the sound barrier.”

•Captain Yeager was at an altitude of 12,000 meters and just about out of fuel.

•He had used much of his fuel to get to a higher altitude because the speed of sound is slower higher up.

Faster Than Sound11.111.1The Nature of SoundThe Nature of Sound

•Thus, when he hit 294 meters per second, he exceeded the speed of sound at that altitude.

•At a lower altitude, the speed of sound is much faster and he would not have had the power or speed to exceed it.

•Yeager’s team chose to go high in part because the temperature there is lower and the speed of sound is slower.

Faster Than Sound11.111.1The Nature of SoundThe Nature of Sound

•Fifty years later, Andy Green stood poised on Nevada’s Black Rock desert.

•He had traveled all the way from Great Britain to go supersonic—on the ground!

•He chose the desert because it is flat, wide open, and cold in the morning.

Faster Than Sound11.111.1The Nature of SoundThe Nature of Sound

•On October 15, 1997, at the coolest time of the day, Green blasted off in his jet-powered car, Thrust.

•A short time later he traveled a measured distance at an average speed of 339 meters per second—7 meters per second faster than the speed of sound at that altitude.

•Andy Green was the first person to break the sound barrier on the ground.

Faster Than Sound11.111.1The Nature of SoundThe Nature of Sound

Faster Than Sound11.111.1The Nature of SoundThe Nature of Sound

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