phys16 – lecture 36 & 37
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PHYS16 – Lecture 36 & 37. Sound December 5 & 7, 2010. Outline for Sound. Sound Longitudinal Pressure Waves Sound Velocity and Reflection Resonance and Standing waves Intensity and Sound Level Interference and Beats Doppler. Resonance and Standing Waves. Guitar Demonstration. - PowerPoint PPT PresentationTRANSCRIPT
PHYS16 – Lecture 36 & 37
Sound December 5 & 7, 2010
Outline for Sound
• Sound– Longitudinal Pressure Waves– Sound Velocity and Reflection– Resonance and Standing waves– Intensity and Sound Level– Interference and Beats– Doppler
Resonance and Standing Waves
Guitar Demonstration
Example Question
• A guitar string with a greater mass density has a/an:A) Greater velocityB) Higher frequencyC) Smaller wavelengthD) Unchanged wavelength
Example Question
• So what does the hole do?
Standing Waves with Sound
• Need two “ends” where sound reflects• Reflections constructively and destructively
interfere to setup up standing waves
closedor open both 1,2,3...n ,2
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closed oneopen one 1,2,3...n ,12
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Ruben’s Tube Demo
http://en.wikipedia.org/wiki/Rubens'_tube
Standing Waves with Sound
• Helmholtz Resonator
AVL
lfundamenta 2
A
As A increases, Wavelength decreases and Frequency increases!
http://www.diracdelta.co.uk/science/source/h/e/helmholtz%20resonator/image001.gif
Bottle Demo
http://www.physicsclassroom.com/class/sound/u11l4a3.gif
Intensity and Sound Level
Wave Intensity
• Intensity is the power per unit area in a wave
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21 , AvA
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API
http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/imgaco/isc2.gif
Sound Level
• Sound Level (β) - a relative intensity in decibels (dB)– 0 dB = smallest sound that can be heard– 50 dB = background noise– 70 dB = traffic– 110 dB = rock concert
2120
0
W/m10 ),log(10 III
Example Question
• If a Rock Concert is 110 dB, what is the intensity level in W/m2?
A) 0.1 W/m2
B) 10 W/m2
C) 1011 W /m2
D) 1098 W/m2
Example Question
• A bell is rung in a room and can be assumed to be a point source. If you sit three times the distance your friend is sitting from the bell, what is the decibel drop?
A) 0.1 dBB) 1 dBC) 10 dBD) 100 dB
Interference and Beats
Sound Interference
• Sound waves can interfere destructively or constructively
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Dead Zones
• Places of destructive interference are called Dead Zones
• For Sound we usually don’t get dead zones because sounds are composed of many different frequencies
• How does having many frequencies in a sound eliminate dead zones?
Beats
• Beats occur when two sine waves (two notes) with very similar frequency interfere
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Beat
avg
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Example Question
• What is the beat frequency between 440 Hz and 442 Hz?
A) 441 HzB) 2 HzC) -2HzD) 1 Hz
Doppler Effect
Doppler Effect
• Frequency of sound changes when relative velocity changes– Frequencies on approach sound higher– Frequencies that recede sound lower
actualsoundobject
recede
actualsoundobject
approach
fvv
f
fvv
f
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Example Question
• The frequency of an ambulance siren is 1200 Hz. The ambulance is going 60 mph (26.8 m/s). As the ambulance approaches your stopped car, you hear what frequency?
fapproach = 1300 Hz
Sound + Light = Optical Communication
http://www.scientificamerican.com/media/inline/F8CDD0E6-DFB3-6D63-DEE9270DAAFA852D_1.jpg
Main Points - Sound• Sound– a longitudinal pressure wave that
moves through a medium
• Velocity – 343 m/s in air• Reflection – occurs when velocity changes
• Resonance and Standing Waves – depends on open/closed ends
3Din except ,Tv
closedor open both 1,2,3...n ,2
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Main Points - Sound• Intensity and Sound Level
• Interference and Beats
• Doppler Effect
2120
0
W/m10 ),log(10 III
21 fffBeat
actualsoundobject
approach fvv
f
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