What is sound?
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Transcript of What is sound?
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What is sound?Sound is a longitudinal wave produced by a
vibrating sourceExamples of sources: tuning fork, vocal
cords, lips or reed on a musical instrumentThe vibration produces COMPRESSIONS and
RAREFACTIONS in the airMust have a medium to travel through, will
not travel through a vacuum http://dev.physicslab.org/Document.aspx?doctype=3&filename=WavesSound_IntroductionWaves.xml
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Sound waves propagate in three dimensions
Compressions and Rarefactions produce wave fronts that become a spherical wave
• Energy radiates outward from sound source in a direction perpendicular to the wave front.
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What determines pitch?Pitch is the audible sound our ears
perceive, and it depends upon FREQUENCYHigher frequency = higher pitch If sound source is moving, the perceived
frequency of the hearer will change, so the pitch will change
This is called the DOPPLER EFFECT
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Doppler Effect Misconceptionthe frequency does not change as the
moving source approaches or recedesthe frequency only changes as the
moving source passes you ( the perceived frequency)
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ResonanceResonance occurs when the frequency of a
force applied to a system matches the natural frequency of the system
Result: large amplitudes of vibrationExample: breaking glass with your voicehttp://www.youtube.com/watch?v=ofESdVdX-fY
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OR a bridge collapsing
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Beats
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Beats
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Beats
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BeatsThe beat frequency (how often the
beat is heard) is determined by the difference in the source frequencies
Example: A tuning fork at 440 Hz and 448 Hz are struck at the same time. What is the beat frequency produced by the two sound waves?
As the sound source frequencies are farther and farther apart….
Beat frequency is created by superposition/interference- there is constructive & destructive interference that creates the beat
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What determines volume?Energy of a wave is determined by
amplitudeVolume of sound is also determined by
amplitudeHow do we measure volume?On the Decibel (dB) scale
Measure of sound intensity
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rate at which energy flows through a unit area perpendicular to the direction of wave motion
intensity = ΔE / (Δt • area) = P / areaSince the area of a sphere is equal to
4πr2 , Intensity = P / 4πr2 (units: W/m2)
E: energy (J)t: time (s)P: power (W)r: radius from source (m)
Intensity
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Decibel (dB)describes the ratio of two intensities of
sound; the threshold of hearing is commonly used as the reference intensity
Ratio = Intensity of source/threshold of hearing
threshold of hearing = 1.0 x 10-12 W/m2
logarithmic scaleMeans that an increase of 10 dB is power
of ten increase (ten fold increase) in intensity
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Intensity (W/m2)
Decibel level (dB) Examples
1.0 x 10-12 0 threshold of hearing1.0 x 10-10 20 quiet whisper1.0 x 10-8 40 mosquito buzzing
1.0 x 10-6 60 air conditioning at 6m
1.0 x 10-4 80 busy traffic, alarm clock
1.0 x 10-2 100 subway, power motor
1.0 x 100 120 threshold of pain1.0 x 103 150 nearby jet airplane
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Human hearing
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Audiogram