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Physics 123

Physics 123. 12. Sound. 12.1 Characteristics of Sound 12.2 Intensity of Sound - Decibels 12.5 Vibrating Strings and Air Columns 12.7 Interference of Sound Waves - Beats 12.8 Doppler Effect. Characteristics of Sound. Sound waves are longitudinal

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Physics 123

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  1. Physics 123

  2. 12.Sound 12.1 Characteristics of Sound 12.2 Intensity of Sound - Decibels 12.5 Vibrating Strings and Air Columns 12.7 Interference of Sound Waves - Beats 12.8 Doppler Effect

  3. Characteristics of Sound • Sound waves are longitudinal • Speed of sound is different in different media • Speed of sound in air is 331 m/s at 00C and 343 m/s at 200C • An echo is a reflected sound wave • Loudness is related to intensity (W/m2) • Pitch is frequency of sound • Humans can hear 20 Hz to 20,000 Hz • Dogs and bats can hear the ultrasonic range

  4. Intensity of Sound - Decibels The intensity of sound decreases with distance from the source as the inverse square of the distance. Double the distance and the intensity becomes a quarter. I / Io = ( ro / r )2 Note: Humans can hear from 1 W/m2 to 10 -12 W /m2

  5. Decibels dB = 10 log I / Io

  6. Example 12.1 - Decibels The sound intensity of soft radio music is 10 -8 W /m2 and Io= 10 -12 W /m2. How loud is the radio in dB?

  7. Solution 12.1 - Decibels dB = 10 log I / Io dB = 10 log (10 -8 / 10 -12 ) dB = 10 log 104 dB = 10 x 4 40 dB

  8. Standing Waves on a String

  9. Tube Closed at one end

  10. Tube open at both ends

  11. Standing Waves - Chart String open tube closed tube L =  / 2 L =  / 2 L =  / 4 L =  L =  L = 3 / 4 L = 3 / 2 L = 3 / 2 L = 5 / 4 L = 2 L = 2 L = 7 / 4 L = 5 / 2 L = 5 / 2 L = 9 / 4 Odd and Even HarmonicsOdd Harmonics

  12. Example 12.2 - Organ Pipe An organ pipe open at both ends vibrates in its third harmonic at a frequency of 1000 Hz. If the speed of sound is 343 m/s, the length of the pipe is most nearly A. 0.3 m B. 0.5 m C. 0.7 m D. 0.9 m

  13. Solution 12.2 - Organ Pipe L = 3 / 2 and v =  f So v = (2L/3)(f) 343 = (2L/3)(1000) L = 0.5 m

  14. Interference Constructive Interference: When two waves run into each other in step (in phase). The outcome is increased amplitude Destructive Interference: When two waves run into each other out of step (out of phase). The outcome is decreased amplitude

  15. Spatial Interference Same frequency In phase out of phase

  16. Spatial Interference Path difference In phase if path difference is an even multiple of  Out of phase if path difference is an odd multiple of 

  17. Temporal Interference - Beats f1 f2 In phase out of phase

  18. Problem 12.3 . . . Blinking Lights One car’s turn signal blinks 10 times in 10 seconds and another car’s blinks 15 times in 10 seconds. How often are they going to be in phase (sync)?

  19. Solution 12.3 . . . Blinking Lights Beat frequency = f 1 - f 2 The blinking lights will be in sync 5 times in 10 seconds or once every 2 seconds!

  20. Doppler Effect The change in the frequency of a moving source of sound

  21. f1 f2 Doppler Effect This may sound like this!

  22. Doppler Effect Observer approaching f ' = f ( 1 + VO / V) Observer receding f ' = f ( 1 - VO / V) Source approaching f ' = f / ( 1 - VS / V) Source receding f ' = f / ( 1 + VS / V)

  23. That’s all folks!

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