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SOUNDTiffany Rhodes
Physics
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Topics Covered
• Source • Wave• Medium• Frequency• Amplitude• Speed• Mach Number
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If a tree falls in the forest and nobody is around to hear it, does it make a sound?
•
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I. EVERY SOUND IS CREATED BY A VIBRATION
• Guitar string vibrates• Reed of clarinet vibrates• Vocal cords vibrate• Table top vibrates when hit with a
hammer• Brain translates vibration into sound
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II. SOUND IS A LONGITUDINAL WAVE
• Compressions and rarefactions
• AIR DENSITY CHANGES AS SOUND ENERGY TRAVELS THROUGH
• Only energy is transported!!!!
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III. SOUND NEEDS A MEDIUM
• Sound can travel through air, liquids, and solids
• Sound travels faster thru solids!
• Sound can NOT travel thru a vacuum
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Can sound travel through space?
Why?
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IV. Frequency of Sound = PITCH
• LOW PITCH = LOW FREQUENCY
• HIGH PITCH = HIGH FREQUENCY
•1 Hertz = 1 vibration/second
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V. HUMAN HEARING• Range 20 – 20,000hz
• Most sensitive at 200 to 4,000 Hz.
• Normal voice range is about 500 Hz to 2000 hz Low frequencies are vowels and bass. High frequencies are consonants
• Lose hearing as we age, especially at the high frequency end.
• Girls usually hear higher frequencies better than boys
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Range of frequencies
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FREQUENCIES
• Below 20 HZ = INFRASONIC
• Above 20,000 HZ = ULTRASONIC
• We can NOT hear either spectrum
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Range of frequencies
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VI. INTENSITY DEPENDS ON AMPLITUDE
• Intensity can be measured by an oscilloscope
• More Intense = Bigger amplitude
• Less intense = smaller amplitude
• Loudness is perceived intensity. Subjective
http://www.youtube.com/watch?v=ydxoTPwFwjs
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Intensity• Unit of Intensity = Decibels (dB)
• Named after Alexander Graham Bell
• Zero Decibels = threshold of hearing
• Increase to 10 decibels (intensity increased by a factor of 10)
• Thus it is 10 times more intense
• 60 dB is 100 times more intense than 40 dB http://www.youtube.com/watch?v=RyIphO4Ypoo http://www.youtube.com/watch?v=oPWucNgN8TQ
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Source of Sound Level in Decibels
Hearing threshold 0
Normal Breathing 10
Close Whisper 20
Library 40
Normal Speech 60
Busy Street Traffic 70
Average Factory 90
Old Subway train 100
Loud rock Music 115
Threshold of Pain 120
Jet Engine at 30 m 140
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VI. SPEED OF SOUND
• Medium Affects Speed
• 2 Properties of a medium affect speed–1. Elastic properties–2. Inertial property
Temperature of medium also affects it.–Generally, Hotter = faster Cooler = slower
–See Hand out for more information
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Speed of sound in Various Mediums
• Carbon Dioxide 258
• Air 332• Cork 500• Alcohol 1241• Fresh water 1500• Marble 3810• Steel 5050• Aluminum 5104• Earth 7000-13000
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Which medium would sound travel fastest
through?
•Air•Aluminum•Ocean water•Helium
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SPEED EQUATION
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What is the Speed of Sound in air at 30 C
and –30 C?
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At 30 Celsius:
V= (332 + 0.6 * 30)V = 350 m/s
At –30 Celsius:
V = (332 + 0.6 * -30)V = 314 m/s
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Mach Number
• What do you already know about Mach Number?
Related to air planes??
Related to sound??
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Mach Number• Relates speed of object with speed of sound• Usually used with aircrafts since they are our
fastest means of travel
• Mach Number = Speed of Object/ Speed of sound
• Speed of object and sound have to have same units. They cancel out…
• Mach Number has No units!
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What do you know about an airplane that travels at mach 1?
• It is traveling at the same speed of sound
What do you know about an airplane that travels at mach 2?
It is traveling double the speed of sound
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II. Wave Equation
• Sound is a wave so therefore the wave equation still applies.
• Speed of the wave = wavelength * frequency
• V = * f
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Questions
• Mach Number problems?
• Wave Equation problems?
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Warm Up
A rocket is traveling at 8643.6 km/hr.
If the air temperature is 20ºC, what is the rocket’s Mach
Number?
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Answer• Speed of object = 8643.6 km/h
• Speed of sound = 332 + 0.6* Temp
= 332 + 0.6 (20)
= 344 m/s
Convert to km/h
344m 1 km 3600sec
1 sec 1000m 1h
= 1238.4 km/h
Mach # = Speed of Object/Speed of sound
= 8643.6/ 1238.4 = 7