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    Sound

    Sound is a longitudinal mechanicalwave.

    Compressions(High Pressure)

    Rarefactions(Low Pressure)

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    Speed of Sound

    Depends on the medium.

    The more elastic the medium thefaster sound will travel through it.

    Speed in metals>speed inwater>speed in air

    Sound cant travel throughvacuum.

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    Speed of Sound in Air

    v = 331 + 0.6 T ( in meters/sec)

    T is the temperature in 0C.In higher humidity, sound will travelfaster.

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    Properties of Sound

    Reflection (Echo)Refraction

    InterferenceDiffraction

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    Pitch: the frequency of asound wave.

    Musical notes have a given pitch.

    The note C has a frequency of327Hz.

    When two notes differ by a ratio of2:1 they are one octave apart.

    What would be the next higher C?

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    Bow (Shock) Waves

    When the speed of a moving soundsource is greater than the speed of thewave, a pressure ridge builds similar

    to the wave created by the bow of aship.

    physlet animation

    http://online.cctt.org/physicslab/content/applets/doppler/Doppler.htmlhttp://online.cctt.org/physicslab/content/applets/doppler/Doppler.htmlhttp://online.cctt.org/physicslab/content/applets/doppler/Doppler.html

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    Sonic Boom

    When the pressure ridge of a bowwave of a jet passes over an observeron the ground, the observerexperiences a sonic boom.

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    Doppler Effect

    The change in a wave's perceivedfrequency due to the motion of either thesound source or the observer.

    It is applicable to any type of wave.

    Austrian physicist Christian Doppler (1803-1853).train sound clip

    Simulationsphyslet animationhttp://www.walter-fendt.de/ph14e/dopplereff.htm

    http://online.cctt.org/physicslab/content/Phy1/lessonnotes/waves/TrainDoppler.auhttp://online.cctt.org/physicslab/content/applets/doppler/Doppler.htmlhttp://www.walter-fendt.de/ph14e/dopplereff.htmhttp://www.walter-fendt.de/ph14e/dopplereff.htmhttp://www.walter-fendt.de/ph14e/dopplereff.htmhttp://www.walter-fendt.de/ph14e/dopplereff.htmhttp://online.cctt.org/physicslab/content/applets/doppler/Doppler.htmlhttp://online.cctt.org/physicslab/content/applets/doppler/Doppler.htmlhttp://online.cctt.org/physicslab/content/Phy1/lessonnotes/waves/TrainDoppler.au

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    The Doppler Effect

    http://www.physicsclassroom.com/Class/sound/u11l3b.cfm

    http://www.physicsclassroom.com/Class/sound/u11l3b.cfmhttp://www.physicsclassroom.com/Class/sound/u11l3b.cfm

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    Doppler EffectDetected Frequency, (f D)

    f D = f ( v vo )vvsf : frequency of source

    v : speed of sound

    vo : speed of observer(+)when observer moving towards

    source

    vs

    : speed of source(-)when source is moving towards

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    Example 1

    An ambulance is approaching astationary observer.

    The siren of the ambulance emits a

    frequency of 480Hz and the speedof the ambulance is50km/h(=13.88m/s).

    What frequency will the stationaryobserver hear when the ambulanceis approaching? Assume T=20 oC.

    Answer: f d = 500Hz

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    Sound Intensity (I)

    I = (Power transmitted by the Wave)/Area

    Threshold of human hearing:

    Io =1 x 10 -12 Watts/m 2

    Threshold of Pain:IP = 1 Watts/m 2

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    Sound (Intensity) Level

    A measure of our perception of theloudness of the sound.

    Unit: decibel(dB)

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    Sound (Intensity) Level,(decibel)

    b= 1 log I , unit for b is dB ( decibel )I0

    The decibel compares the sound intensity (I),to I o (1 x 10 -12 Watts/m 2) ,

    the threshold of human hearing.

    Remember: log(10 x )= x

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    Examples of Sound IntensityLevels

    jet plane taking off 140 dBair raid siren 125 dBthreshold of pain 120 dBloud rock music 115 dBear damage starts 85 dBbusy traffic 70 dBnormal conversation 60 dBquiet library 40 dBsoft whisper 20 dBthreshold of human hearing 0 dB

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    Example 2

    What is the sound level (decibel) ofa sound of 10 -4 W/m 2 intensity ?

    Answer: 80dB

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    Example 3

    How many times louder is a quiet librarysound (40 dB) compared to a soft whisper(20 dB)?

    Hint 1: Property of logs:log(A/B) = logA logB

    Hint 2: Take the difference in dB and thenfind

    (I2 /I1)


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