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Adv Physics

Adv Physics. Chapter 14 Sections 3 and 5. Doppler Effect. How does the speed of the source effect your results? http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Flash/ClassMechanics/DopplerWaveFronts/DopplerWaveFronts.swf. Doppler Effect.

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Adv Physics

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  1. Adv Physics Chapter 14 Sections 3 and 5

  2. Doppler Effect • How does the speed of the source effect your results? • http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Flash/ClassMechanics/DopplerWaveFronts/DopplerWaveFronts.swf

  3. Doppler Effect • What does the wave pattern look like when you are traveling just below the speed of the wave? • The waves compress in front of you

  4. Doppler Effect • What does the wave pattern look like when you are traveling at the speed of the wave? • The waves in front of you compress into a single wave front • In terms of sound you have created a region with a dramatic pressure change - called the sound barrier

  5. Doppler Effect • What does the wave pattern look like if you are traveling faster than the speed of the wave? • You “out run” the waves and a cone-shaped pattern is formed -called a shock wave

  6. Shock Wave • commons.wikimedia.org/wiki/File:Sonic_boom.svg • Along the edges of the cone a large pressure ridge is formed • When the edge of the cone reaches the ear of a listener a loud “boom” is heard -called a sonic boom

  7. Sonic Boom • Common misconception that sonic boom is heard when a plane breaks the sound barrier • Sonic boom is heard whenever a listener comes in contact with the edge of the shock wave

  8. Shock Wave • Is possible for a shock wave to break windows and cause damage to structures • Pilots are instructed to fly supersonically at high altitudes and away from populated areas

  9. Intensity • How a wave spreads its power over an area -power divided by the cross-sectional area perpendicular to the direction of wave motion I = P/A where I – intensity P – power A – cross sectional area [I] = W/m2

  10. Sample Problem Suppose 12 x 10-5 W of power pass perpendicularly through 2 surfaces having areas of 4 m2 and 12 m2. Find the sound intensity at each surface.

  11. Range of Intensities • Threshold of hearing – smallest intensity the human ear can detect I0 = 12 x 10-5 W/m2 at 1000 Hz • Threshold of pain – intensity that is so large that it causes the human ear actually pain, 1 W/m2

  12. Sample Problem During a fireworks display a rocket explodes high in the air. Sound spreads out uniformly. When the sound reaches a listener 640 m away the sound has an intensity of 0.1 W/m2. What is the intensity detected by a listener 160 m away?

  13. Intensity Level • Measure of the intensity of sound relative to a reference intensity using the logarithmic scale β = 10 log (I /I0) where β – intensity level I – intensity I0 – reference intensity (usually threshold of hearing) [β] = decibels, dB

  14. Notes about Decibels • Threshold of hearing is 0 dB • Threshold of pain is 120 dB • 0 dB doesn’t mean no sound – it means the sound has the same intensity as the threshold of hearing • Doubling the intensity doesn’t double the loudness (takes 10 dB increase to do that) • 1 dB change is the smallest change in loudness detectable

  15. Decibels http://www.nidcd.nih.gov/staticresources/health/education/decibel/decible4.swf • Sound levels of 90 dB and above will damage receptor nerves in the ear resulting in a loss of hearing

  16. Sample Problem Two people talk simultaneously. If the intensity level is 60 dB when either one speaks alone, what is the intensity level when both speak at once?

  17. Sample Problem When a 100 dB sound wave comes through an open window of area 0.5 m2, how much acoustic energy passes through the window in a 10 min interval?

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