Chapter 26Chapter 26

SoundSound

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Origin of SoundOrigin of Sound All sounds are produced by the All sounds are produced by the

vibrations of material objectsvibrations of material objects For example in a piano, violin, or For example in a piano, violin, or

guitar a sound wave is produced by guitar a sound wave is produced by vibrating stringsvibrating strings

Sound is made when something Sound is made when something vibrates.vibrates.

The vibration disturbs The vibration disturbs the air around it.the air around it.

This makes changes in This makes changes in air pressure. air pressure.

These changes in air These changes in air pressure move through pressure move through the air as sound the air as sound waves.waves.

Origin of SoundOrigin of Sound

Sound WavesSound Waves- Alternating areas of high & low Alternating areas of high & low

pressurepressure in the air in the air- ALL sound is carried through matter as ALL sound is carried through matter as

sound wavessound waves- Sound waves Sound waves move out in ALL move out in ALL

directionsdirections from a vibrating object from a vibrating object

Nature of WavesNature of Waves

Waves Waves (Def.)(Def.) – A wave is a disturbance – A wave is a disturbance that transfers energy.that transfers energy.

Medium – Substance or region through Medium – Substance or region through which a wave is transmitted.which a wave is transmitted.

Speed of Waves – Depends on the Speed of Waves – Depends on the properties of the medium.properties of the medium.

Properties of WavesProperties of Waves

WavelengthWavelength ( () is measured from crest-to-crest) is measured from crest-to-crest

– or trough-to-trough, or upswing to upswing, etc.or trough-to-trough, or upswing to upswing, etc. For traveling waves (sound, light, water), there is a For traveling waves (sound, light, water), there is a speedspeed ( (cc)) FrequencyFrequency ( (ff) refers to how many cycles pass by per second) refers to how many cycles pass by per second

– measured in Hertz, or Hz: cycles per secondmeasured in Hertz, or Hz: cycles per second

– associated with this is period: associated with this is period: T = 1/fT = 1/f These three are closely related:These three are closely related:

ff = = cc

or T

horizontal axis could be:space: representing snapshot in timetime: representing sequence at a par- ticular point in space

pres

sure

Longitudinal vs. Transverse Longitudinal vs. Transverse WavesWaves

Sound is a Sound is a longitudinallongitudinal wave, meaning that wave, meaning that the motion of particles is the motion of particles is alongalong the direction the direction of propagationof propagation

TransverseTransverse waves—water waves, light— waves—water waves, light—have things moving have things moving perpendicularperpendicular to the to the direction of propagationdirection of propagation

Why is Sound Longitudinal?Why is Sound Longitudinal? Waves in air can’t really be transverse, because the atoms/molecules are Waves in air can’t really be transverse, because the atoms/molecules are

not boundnot bound to each other to each other– can’t pull a (momentarily) neighboring molecule sidewayscan’t pull a (momentarily) neighboring molecule sideways– only if a “rubber band” connected the molecules would this workonly if a “rubber band” connected the molecules would this work– fancy way of saying this: gases can’t support shear loadsfancy way of saying this: gases can’t support shear loads

Air molecules can really only bump into one anotherAir molecules can really only bump into one another Imagine people in a crowded train station with hands in pocketsImagine people in a crowded train station with hands in pockets

– pushing into crowd would send a wave of compression into the pushing into crowd would send a wave of compression into the crowd crowd in the direction of pushin the direction of push (longitudinal) (longitudinal)

– jerking people back and forth (sideways, over several meters) jerking people back and forth (sideways, over several meters) would would notnot propagate into the crowd propagate into the crowd

– but if everyone held hands (bonds), this transverse motion but if everyone held hands (bonds), this transverse motion wouldwould propagate into crowdpropagate into crowd

A wave in which the particles of the medium move at right angles to the direction of the wave is a __________.

Transverse wave

High pitch is produced by sounds that have__________.

High frequency

QuestionsQuestions

Speed of SoundSpeed of Sound Sound speed in air is related to the frantic motions of molecules as

they jostle and collide

– since air has a lot of empty space, the communication that a wave is coming through has to be carried by the motion of particles

– for air, this motion is about 500 m/s, but only about 350 m/s directed in any particular direction

Solids have faster sound speeds because atoms are hooked up by “springs” (bonds)

– don’t have to rely on atoms to traverse gap

– spring compression can (and does) travel faster than actual atom motion

Examples of Sound SpeedsExamples of Sound SpeedsMedium sound speed (m/s)

air (20C) 343

water 1497

gold 3240

brick 3650

wood 3800–4600

glass 5100

steel 5790

aluminum 6420

Components of SoundComponents of Sound

Pitch (how high or low)Pitch (how high or low)

Loudness (volume)Loudness (volume)

Timbre (tone color)Timbre (tone color)

VibrationVibration

- Back and forth movement of moleculesBack and forth movement of molecules of of mattermatter

- For example,For example,

Does high density material tend to cause sound to go faster or slower?

Is low temperature cause sound to go faster or slower?

Is highly elastic material cause sound to go faster or slower?

The Physics of SoundThe Physics of Sound

PitchPitch … … is the "highness" or "lowness" of a tone.is the "highness" or "lowness" of a tone.

Pitch corresponds to frequency.Pitch corresponds to frequency.

PitchPitch The vibration patterns of The vibration patterns of

some sounds are repetitive.some sounds are repetitive. Vibration patterns are also Vibration patterns are also

called waveforms. called waveforms. Each repetition of a waveform Each repetition of a waveform

is called a cycle.is called a cycle. We can hear frequencies We can hear frequencies

between 20 hertz or cycles between 20 hertz or cycles (vibrations) per second (low (vibrations) per second (low pitches)to 20 kilohertz, i.e. pitches)to 20 kilohertz, i.e. 20,000 Hz (high pitches).20,000 Hz (high pitches).

Pitch ContinuedPitch Continued InfrasonicInfrasonic

frequencies < 20 Hzfrequencies < 20 HzUltrasonicUltrasonic

frequencies > 20,000 Hzfrequencies > 20,000 HzHuman hearing rangeHuman hearing range

frequencies between 20 Hz and 20,000 frequencies between 20 Hz and 20,000 HzHz

When the frequency of When the frequency of a sound doubles we a sound doubles we say that the pitch goes say that the pitch goes up an octave.up an octave.

We can hear a range of We can hear a range of pitches of about ten pitches of about ten octaves.octaves.

Many animals can Many animals can make sounds and hear make sounds and hear frequencies that are frequencies that are beyond what we can beyond what we can hear.hear.

Could you hear a pitch of 19 Hz?Could you hear a pitch of 19 Hz?

Sound in AirSound in Air ...a ...a longitudinallongitudinal

wave in air caused wave in air caused by a vibrating by a vibrating object.object.

Demo: slinkyDemo: slinky

The EarThe Ear Sound is carriedSound is carried to our ears to our ears through vibrating through vibrating

air molecules.air molecules. Our ears take in sound waves & turn them into Our ears take in sound waves & turn them into

signals that go to our brains.signals that go to our brains. Sound waves move through 3 parts of the ear; Sound waves move through 3 parts of the ear;

outer ear, middle ear, & inner ear.outer ear, middle ear, & inner ear.

Middle Ear

1. The outer ear gathers sound waves, passing them through the ear canal to a tough membrane called the eardrum

2. The vibrating eardrum passes the sound to three tiny bones in the middle ear – the hammer, anvil, and stirrup – which amplify the sound wave

The EarThe Ear

3. The stirrup vibrates and transfers the sound to a membrane in the oval window, then on to the inner ear’s cochlea, a spiral-shaped structure that contains hair cells

4. As the hair cells in the cochlea vibrate, nerve impulses are sent through the auditory nerve to the brain

The EarThe Ear

The sound waves The sound waves cause pressure cause pressure changes against changes against our ear drum our ear drum sending nerve sending nerve impulses to our impulses to our brain.brain.

The EarThe Ear

Speakers: Inverse EardrumsSpeakers: Inverse Eardrums Speakers vibrate and push on the airSpeakers vibrate and push on the air

– pushing out creates compressionpushing out creates compression

– pulling back creates rarefactionpulling back creates rarefaction Speaker must execute complex motion Speaker must execute complex motion

according to desired waveformaccording to desired waveform

QuestionQuestion

What type of wave would moving a rope What type of wave would moving a rope attached to a wall simulate?attached to a wall simulate?

Sound in Air Sound in Air ContinuedContinued

Sound requires a medium.Sound requires a medium.

– solid, liquid or gassolid, liquid or gas

– Demo: Bell in a evacuated Bell JarDemo: Bell in a evacuated Bell Jar

Sound waves have Sound waves have compressioncompression and and rarefactionrarefaction regions. regions.

Noise Measuring EquipmentNoise Measuring Equipment

Sound Level Meters (SLM)Sound Level Meters (SLM)

Continuous on-mobile sources Continuous on-mobile sources

Noise Dosimeters

Mobile/variable

noise sources

QuestionsQuestionsWould sound travel faster on a train track or Would sound travel faster on a train track or

in a pool?in a pool?

CompressionCompression- Where molecules are being pressed together as Where molecules are being pressed together as

the sound waves move through matterthe sound waves move through matter- For example,For example,

- a wave travels through the springs just like sound a wave travels through the springs just like sound waves travel through the airwaves travel through the air

- the places where the springs are close together are the places where the springs are close together are like compressions in the air.like compressions in the air.

RarefactionRarefaction The change in The change in

direction of a direction of a wave as it crossed wave as it crossed the boundary the boundary between two between two media in which media in which the wave travels at the wave travels at different speedsdifferent speeds

Refraction occurs because waves__________.

Move at different speeds in different mediums

In rarefaction, the molecules of the medium are _____.

Spaced apart

Questions

The Physics of SoundThe Physics of Sound

The Physics of SoundThe Physics of Sound

Are the air molecules more spread Are the air molecules more spread out in compression or out in compression or

rarefraction?rarefraction?

Rarefraction

Media that Transmits SoundMedia that Transmits Sound Most sounds you hear are Most sounds you hear are

transmitted through air. transmitted through air. Solids and liquids are Solids and liquids are

generally good conductor generally good conductor of sound— much better of sound— much better than airthan air

Sounds cannot travel Sounds cannot travel through a vacuumthrough a vacuum

Speed of Sound in AirSpeed of Sound in Air Speed of sound = 340 meters/second or Speed of sound = 340 meters/second or

760 miles/hour760 miles/hour Sound travels faster in hot, humid Sound travels faster in hot, humid

climatesclimates Speed of sound depends on elasticitySpeed of sound depends on elasticity

Speed of sound = Speed of sound = (331.5 + 0.6T) m/s(331.5 + 0.6T) m/s

T = temperature in T = temperature in CelsiusCelsius

So at higher temperatures the speed of sound is faster because of the faster moving molecules. At 0 degrees the speed in air is 331.5 m/s. This speed increases with the temperature at about 0.6 m/s per degree Celsius.

Speed of Sound in AirSpeed of Sound in Air

Speed of Sound and RefractionSpeed of Sound and Refraction

Sound travels faster in warm air. Why?

Speed of sound = ((331.5) + 0.6(20))m/sSpeed of sound = ((331.5) + 0.6(20))m/s

Speed of sound = 343.5 m/sSpeed of sound = 343.5 m/s

V = d/t V = d/t

343.5 m/s = d / 5s 343.5 m/s = d / 5s

D = 1717.5 mD = 1717.5 m

Late one summer night ( T = 20 degrees Celsius), lightning is seen from an approaching storm and five seconds later, thunder sound is heard. How far

away is the storm?

Wavelength & FrequencyWavelength & Frequency- Wavelength is the Wavelength is the distance between one distance between one

part of a wavepart of a wave andand the the same part of the next same part of the next wavewave

- Frequency is the Frequency is the number of waves moving number of waves moving past a point in one secondpast a point in one second

Can you predict how far a storm Can you predict how far a storm is if you here thunder 8 seconds is if you here thunder 8 seconds

away?away?

340m/s x 8 s = 2720 m

Sound Intensity and Sound Intensity and LoudnessLoudness

Intensity of SoundIntensity of Sound refers to refers to the amplitude of the the amplitude of the

pressure variations in the pressure variations in the sound wavesound wave

LoudnessLoudness

The physiological sensation The physiological sensation directly related to the sound directly related to the sound intensityintensity

Measured in bels Measured in bels (10 bels = 1 decibels)(10 bels = 1 decibels)

•As the sound spreads out from its source, the As the sound spreads out from its source, the concentration of power becomes less.concentration of power becomes less.

As the distance As the distance from the source from the source increases the increases the amount of power is amount of power is spread over a spread over a greater area.greater area.

The amount of The amount of power per square power per square meter is called the meter is called the intensityintensity of the of the sound.sound.

LoudnessLoudness To create vibrations To create vibrations

energy is used.energy is used. The greater amount of The greater amount of

energy used the louder energy used the louder the sound.the sound.

The strength of the The strength of the changes in air pressure changes in air pressure made by the vibrating made by the vibrating object determines object determines loudness.loudness.

LoudnessLoudness A sound of A sound of 110 0

decibels is or 10decibels is or 1011 or or 1010 times as intense times as intense as 0 decibels.as 0 decibels.

220 decibels is 100 decibels is 1022 or or 100 times the 100 times the intensity 0 decibels.intensity 0 decibels.

Source of SoundSource of Sound Loudness (db)Loudness (db)

Threshold of HearingThreshold of Hearing 00

ConversationConversation 6060

Ear Damage BeginsEar Damage Begins 8585

Amplified MusicAmplified Music 110110

Jet Airplane at 30 metersJet Airplane at 30 meters 140140

Common Sound IntensitiesCommon Sound IntensitiesSource of Sound Intensity (W/m2) Sound Level (db)

Jet 30 m away 102 140

Air-raid siren, nearby 1 120

Disco music, amplified 10-1 110

Riveter 10-3 90

Busy street traffic 10-5 70

Conversation in home 10-6 60

Quiet radio in home 10-8 40

Whisper 10-10 20

Rustle of leaves 10-11 10

Threshold of hearing 10-12 0

Hearing Protection Devices and Their Hearing Protection Devices and Their Noise Reduction RatingsNoise Reduction Ratings

EAR Foam Plugs (NRR = 29 dB)EAR Foam Plugs (NRR = 29 dB)

EAR CARBOFLEX (NRR= 20 dB)

Moldex PURAFIT Foam Plugs

(NRR=30 dB)

The human perception of intensity is known as _____.

Loudness

Sound waves are _______________ waves.

Compressional

The speed of sound depends on?

Temperature Density Elasticity

Reflection of a wave occurs when the wave______.

Strikes a boundary and bounces back

The pitch of a sound depends on its__________.

Frequency

Humans do not perceive sound intensity linearly.Humans do not perceive sound intensity linearly.

For us to perceive a For us to perceive a sound as twice as loud its sound as twice as loud its intensity must be ten intensity must be ten times greater.times greater.

The perceived intensity The perceived intensity level of sound is level of sound is measured in a logarithmic measured in a logarithmic scale using a unit called scale using a unit called

the decibel the decibel (dB)(dB) 22

From the perspective of the logarithmic scale From the perspective of the logarithmic scale the threshold of pain isthe threshold of pain is

1,000,000,000,0001,000,000,000,000times as great as the threshold of times as great as the threshold of

hearing.hearing.

What is the loudest setting What is the loudest setting should set their ipod at to should set their ipod at to maintain a healthy ear?maintain a healthy ear?

TimberTimber

is the specific is the specific property of property of sound that sound that enables us to enables us to determine the determine the difference difference between a between a piano and a piano and a harp.harp.

An extremely broad variety of An extremely broad variety of tone colors exist because most tone colors exist because most

sounds that we perceive as sounds that we perceive as pitch actually contain many pitch actually contain many

frequencies.frequencies.

The predominant pitch is The predominant pitch is called the called the fundamental fundamental

frequency.frequency.

Although we would perceive a string Although we would perceive a string vibrating as a whole,vibrating as a whole,

it actually vibrates in a pattern that at first appears to be it actually vibrates in a pattern that at first appears to be erratic producing many different overtone pitches. erratic producing many different overtone pitches.

What results are particular tone colors or timbres of What results are particular tone colors or timbres of instruments and voices.instruments and voices.

The other frequencies which occur in a The other frequencies which occur in a mathematical series are called the mathematical series are called the harmonicharmonic

or or overtone series.overtone series.

When C1 is the fundamental the following When C1 is the fundamental the following pitches represent its first fifteen successive pitches represent its first fifteen successive

overtones. overtones.

Forced VibrationsForced Vibrations ……the setting up of vibrations in an object the setting up of vibrations in an object

by a vibrating force.by a vibrating force. Examples of Forced Vibration:Examples of Forced Vibration:

– A tuning fork touching a wood surfaceA tuning fork touching a wood surface

– Sounding boards for stringed instrumentsSounding boards for stringed instruments

– Matching tuning fork boxesMatching tuning fork boxes

Natural FrequencyNatural Frequency ……the frequency at which an elastic the frequency at which an elastic

object naturally tends to vibrate. object naturally tends to vibrate. At this frequency, a At this frequency, a minimum energyminimum energy

is required to produce a forced is required to produce a forced vibration.vibration.

The natural frequency of a body The natural frequency of a body depends on its depends on its elasticityelasticity and its and its shapeshape. .

What factor besides shape What factor besides shape affects natural frequency?affects natural frequency?

Natural Frequency Natural Frequency ExamplesExamples

Dropping Aluminum RodsDropping Aluminum Rods Ringing Small and Large BellsRinging Small and Large Bells Xylophone Xylophone Rubbing a Wine GlassRubbing a Wine Glass Mass on a SpringMass on a Spring

38. What is the speed of a wave if the frequency is 300 Hz and the wavelength is 150m?

S = λfS = (150)(300) = 45000 m/s

39. What is the wavelength of a 350 Hz wave traveling at 500m/s?

S = λfλ = 1.4 meters

40. How many cycles per second would be characteristic of a 30 m wave traveling at 300 m/s?

S = λf10 cycles/sec (Hz)

All Shapes of WaveformsAll Shapes of Waveforms Different Instruments have Different Instruments have

different waveformsdifferent waveforms

– a: glockenspiela: glockenspiel

– b: soft pianob: soft piano

– c: loud pianoc: loud piano

– d: trumpetd: trumpet Our ears are sensitive to the Our ears are sensitive to the

detailed shape of waveforms!detailed shape of waveforms! More waveforms:More waveforms:

– e: french horne: french horn

– f: clarinetf: clarinet

– g: violing: violin

Section 3 – Music

A.Music – sounds that are deliberately used in a regular pattern

1. Natural frequency – frequency at which the material vibrates

2. Resonance – the ability of a medium to vibrate by absorbing energy at its own natural frequency

ResonanceResonance ……is the result of is the result of forced vibrationsforced vibrations in a in a

body when the applied frequency body when the applied frequency matches the matches the natural frequencynatural frequency of the of the body.body.

The resulting vibration has a The resulting vibration has a high high amplitudeamplitude and can destroy the body and can destroy the body that is vibrating.that is vibrating.

Examples of ResonanceExamples of Resonance breaking a wine glass using soundbreaking a wine glass using sound mass on a spring at resonancemass on a spring at resonance a singing rod caused by forced vibration a singing rod caused by forced vibration a tuning fork exciting a guitar stringa tuning fork exciting a guitar string In 1940, the Tacoma Narrows Bridge was In 1940, the Tacoma Narrows Bridge was

destroyed by wind-generated resonance.destroyed by wind-generated resonance.

1. Frequency times wavelength equals_________.

2. What is the number of complete wave cycles per unit time?

3. A ____________ is a region in the medium in which molecules are crowded together.

4. The ability of an object to vibrate by absorbing energy at its natural frequency is called _________.

Sound InterferenceSound Interference Overlapping crests of a wave will Overlapping crests of a wave will

result in an increased amplitude.result in an increased amplitude.

Overlapping a crest and a trough Overlapping a crest and a trough results in a decrease in amplitude.results in a decrease in amplitude.

BeatsBeats - the periodic variation in - the periodic variation in loudness of two sounds played togetherloudness of two sounds played together

The The beat frequencybeat frequency is equal to the is equal to the difference in the frequency of the two difference in the frequency of the two sounds.sounds.

BeatsBeats

Acoustics...Acoustics...

...the study of sound properties....the study of sound properties.

When a sound wave strikes a surface it can be.…When a sound wave strikes a surface it can be.…(a) reflected.(a) reflected.(b) transmitted.(b) transmitted.(c) absorbed.(c) absorbed.(d) all of these.(d) all of these.

Reflection of SoundReflection of Sound e.g. an e.g. an echoecho

ReverberationReverberation - re-echoed sound, multiple - re-echoed sound, multiple reflections of sound waves from wallsreflections of sound waves from walls

Compare reflections from a hard wall with Compare reflections from a hard wall with that from a carpet wall.that from a carpet wall.

Demo: Whip

Refraction of SoundRefraction of Sound Refraction - the bending of a waveRefraction - the bending of a wave

Sound waves bend toward cooler air.Sound waves bend toward cooler air.

Desert and Lake Example

Radio BroadcastsRadio Broadcasts AM - Amplitude ModulationAM - Amplitude Modulation

» 535 kHz to 1605 kHz535 kHz to 1605 kHz

FM - Frequency ModulationFM - Frequency Modulation» 88 MHz to 108 MHz88 MHz to 108 MHz

ModulationModulation - an impression of the sound - an impression of the sound wave on a higher frequency radio waveswave on a higher frequency radio waves

Modulating Radio WavesModulating Radio Waves

Modulation - variation of amplitude or Modulation - variation of amplitude or frequency when waves are broadcastfrequency when waves are broadcast– AM – amplitude modulation AM – amplitude modulation

» Carries audio for T.V. BroadcastsCarries audio for T.V. Broadcasts

» Longer wavelength so can bend around hillsLonger wavelength so can bend around hills

– FM – frequency modulation FM – frequency modulation » Carries video for T.V. BroadcastsCarries video for T.V. Broadcasts

© 2000 Microsoft Clip Gallery

SonarSonar

- An instrument that uses An instrument that uses reflected sound reflected sound waveswaves to find underwater objects to find underwater objects

- For example,For example,

Animals use sonar or echo location to find their prey; these sounds have such a high pitch or frequency that the human ear cannot hear

Humans use sonar to locate or

map objects

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