Sound

SoundSound

What causes What causes sound?sound?

Sound WavesSound Waves Speed of SoundSpeed of Sound Frequency and Frequency and

PitchPitch The Doppler EffectThe Doppler Effect

What Causes Sound?What Causes Sound?

• 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.

Sound WavesSound Waves

Sound waves are compressional waves.

• A compressional wave is made up of two types of regions called compressions and rarefactions.

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

Sound Waves (2)Sound Waves (2)

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

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

Sound Waves (3)Sound Waves (3)

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

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

• This sound wave is what you hear.

Moving through MaterialsMoving through Materials

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

• 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.

• Remember the matter that a wave travels through is called a medium.

• Sound waves cannot travel through empty space.

The Speed of Sound in The Speed of Sound in Different MaterialsDifferent Materials

• 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.

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

The Speed of Sound in The Speed of Sound in Different Materials (2)Different Materials (2)

• 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.

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

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

PitchPitch

If you were to sing a If you were to sing a scale, your voice would scale, your voice would start low and become start low and become higher with each note.higher with each note.

PitchPitch is how high or low is how high or low a sound seems to be.a sound seems to be.

The pitch of a sound is The pitch of a sound is related to the frequency related to the frequency of the sound waves. of the sound waves.

Frequency and PitchFrequency and Pitch

Frequency is a measure of how many Frequency is a measure of how many wavelengths pass a particular point wavelengths pass a particular point each second.each second.

For a compressional wave, such as For a compressional wave, such as sound, the frequency is the number sound, the frequency is the number of compressions or the number of of compressions or the number of rarefactions that pass by each rarefactions that pass by each second. second.

Frequency is measured in hertz (Hz).Frequency is measured in hertz (Hz).

Frequency and Pitch (2)Frequency and Pitch (2)

When a sound wave with high When a sound wave with high frequency hits your ear, many frequency hits your ear, many compressions hit your eardrum each compressions hit your eardrum each second.second.

Your brain interprets these fast Your brain interprets these fast vibrations caused by high-frequency vibrations caused by high-frequency waves as a sound with a high pitch.waves as a sound with a high pitch.

As the frequency of a sound wave As the frequency of a sound wave decreases, the pitch becomes lower.decreases, the pitch becomes lower.

Frequency and Pitch (3)Frequency and Pitch (3)

This figure shows This figure shows different notes and different notes and their frequencies.their frequencies.

A healthy human ear A healthy human ear can hear sound waves can hear sound waves with frequencies from with frequencies from about 20 Hz to 20,000 about 20 Hz to 20,000 Hz.Hz.

The human ear is most The human ear is most sensitive to sounds in sensitive to sounds in the range of 440 Hz to the range of 440 Hz to about 7,000 Hz.about 7,000 Hz.

The Doppler EffectThe Doppler Effect

The change in pitch or wave The change in pitch or wave frequency due to a moving wave frequency due to a moving wave source is called the source is called the Doppler effectDoppler effect..

The Doppler effect occurs when the The Doppler effect occurs when the source of a sound wave is moving source of a sound wave is moving relative to a listener.relative to a listener.

The Doppler Effect (2)The Doppler Effect (2)

As a race car moves, it As a race car moves, it sends out sound waves sends out sound waves in the form of in the form of compressions and compressions and rarefactions. rarefactions.

The race car creates a The race car creates a compression, labeled A. compression, labeled A.

Compression A moves Compression A moves through the air toward through the air toward the flagger standing at the flagger standing at the finish line.the finish line.

The Doppler Effect (3)The Doppler Effect (3)

By the time By the time compression B leaves compression B leaves the race car, the car the race car, the car has moved forward.has moved forward.

Because the car has Because the car has moved since the time moved since the time it created compression it created compression A, compressions A and A, compressions A and B are closer together B are closer together than they would be if than they would be if the car had stayed the car had stayed still.still.

The Doppler Effect (4)The Doppler Effect (4)

As a result, the flagger hears a higher pitch. As a result, the flagger hears a higher pitch. The Doppler effect happens any time the source of The Doppler effect happens any time the source of

a sound is changing position compared with the a sound is changing position compared with the observer.observer.

It occurs no matter whether it is the sound source It occurs no matter whether it is the sound source or the observer that is moving.or the observer that is moving.

The faster the change in position, the greater the The faster the change in position, the greater the change in frequency and pitch.change in frequency and pitch.

Using the Doppler EffectUsing the Doppler Effect

The Doppler effect also occurs for The Doppler effect also occurs for other waves besides sound waves.other waves besides sound waves.

For example, the frequency of For example, the frequency of electromagnetic waves, such as electromagnetic waves, such as radar waves, changes if an observer radar waves, changes if an observer and wave source are moving relative and wave source are moving relative to each other.to each other.

Using the Doppler Effect (2)Using the Doppler Effect (2)

Radar guns use the Radar guns use the Doppler effect to Doppler effect to measure the speed measure the speed of cars.of cars.

Weather radar also Weather radar also uses the Doppler uses the Doppler shift to show the shift to show the movement of movement of winds in storms, winds in storms, such as a tornado.such as a tornado.

The Doppler Effect (5)The Doppler Effect (5)

The animation to the left The animation to the left shows a stationary sound shows a stationary sound source.source.

Sound waves are produced Sound waves are produced at a constant frequency at a constant frequency and the wave fronts move and the wave fronts move away from the source at a away from the source at a constant speed.constant speed.

The distance between The distance between wave fronts is the wave fronts is the wavelength.wavelength.

All observers will hear the All observers will hear the same frequency.same frequency.

The Doppler Effect (6)The Doppler Effect (6) In the animation at the left the In the animation at the left the

same sound source is radiating same sound source is radiating sound waves at a constant sound waves at a constant frequency in the same medium.frequency in the same medium.

However, now the sound source is However, now the sound source is moving to the right.moving to the right.

The wave fronts are produced with The wave fronts are produced with the same frequency as before. the same frequency as before. However, since the source is moving However, since the source is moving the center of each new wave front is the center of each new wave front is now slightly displaced to the right.now slightly displaced to the right.

As a result, the wave fronts begin to As a result, the wave fronts begin to bunch up on the right side (in front bunch up on the right side (in front of) and spread further apart on the of) and spread further apart on the left side (behind) the source.left side (behind) the source.

An observer in front of the source An observer in front of the source will hear a higher frequency and an will hear a higher frequency and an observer behind the source will hear observer behind the source will hear a lower frequency.a lower frequency.

Mach 1 – Breaking the Sound Mach 1 – Breaking the Sound BarrierBarrier

Now the source is moving at the speed Now the source is moving at the speed of sound in the medium (at sea level of sound in the medium (at sea level approximately 750 mph).approximately 750 mph).

The wave fronts in front of the source The wave fronts in front of the source are now all bunched up at the same are now all bunched up at the same point.point.

As a result, an observer in front of the As a result, an observer in front of the source will detect nothing until the source will detect nothing until the source arrives.source arrives.

The pressure front will be quite intense The pressure front will be quite intense (a shock wave), due to all the wave (a shock wave), due to all the wave fronts adding together, and will not be fronts adding together, and will not be perceived as a pitch but as a “thump” of perceived as a pitch but as a “thump” of sound as the pressure wall passes by.sound as the pressure wall passes by.

Mach 1 – Breaking the Sound Mach 1 – Breaking the Sound Barrier (2)Barrier (2)

The figure to the The figure to the left shows a bullet left shows a bullet travelling at Mach travelling at Mach 1.01.1.01.

You can see the You can see the shock wave just shock wave just ahead of the bullet.ahead of the bullet.

Mach 1.4 - SupersonicMach 1.4 - Supersonic

The sound source has now broken The sound source has now broken through the sound speed barrier, and is through the sound speed barrier, and is travelling at 1.4 times the speed of sound.travelling at 1.4 times the speed of sound.

Since the source is moving faster than the Since the source is moving faster than the sound waves it creates, it actually leads sound waves it creates, it actually leads the advancing wave front. The sound the advancing wave front. The sound source will pass by a stationary observer source will pass by a stationary observer before the observer actually hears the before the observer actually hears the sound it creates.sound it creates.

As you watch the animation, notice the As you watch the animation, notice the clear formation of the Mach cone. It is clear formation of the Mach cone. It is this intense pressure front on the Mach this intense pressure front on the Mach cone that causes the shock wave known cone that causes the shock wave known as a sonic boom as a supersonic aircraft as a sonic boom as a supersonic aircraft passes overhead.passes overhead.

Mach 1.4 – Supersonic (2)Mach 1.4 – Supersonic (2)

The figure at the The figure at the left shows a bullet left shows a bullet travelling at Mach travelling at Mach 2.45.2.45.

The Mach cone and The Mach cone and show wave fronts show wave fronts are very are very noticeable.noticeable.

Mach 1.4 – Supersonic (3)Mach 1.4 – Supersonic (3)

The picture shows The picture shows the shock wave the shock wave front created by a front created by a T-38 Talon, a twin-T-38 Talon, a twin-engine, high-engine, high-altitude, Super altitude, Super Sonic jet trainer.Sonic jet trainer.

Mach 1.4 – Supersonic (5)Mach 1.4 – Supersonic (5)

This picture shows This picture shows the sonic boom the sonic boom created by the created by the THRUST SSC team THRUST SSC team car as it broke the car as it broke the land speed record land speed record (and also broke the (and also broke the sound barrier on sound barrier on land).land).