Sound waves

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Ch 15 Notes

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Sound waves

  1. 1. Ch 15Sound Waves
  2. 2. Sound Wave Properties Sound waves are longitudinal waves consisting of alternate areas of high pressure compressions and low pressure rarefactions The frequency of a sound wave is the # of oscillations in pressure each second The wavelength of a sound wave is the distance between adjacent regions of maximum pressure
  3. 3. Velocity of a Sound Wave The velocity of a sound wave is 343 m/s at 20 degrees Celsius. The velocity depends on the temperature and increases 0.6 m/s for each Celsius degree increase in temperature. Sound waves generally travel faster in solids and liquids
  4. 4. How fast will sound travel at 30 degrees Celsius? Answer : 30 – 20 = 10 10 x 0.6 m/s = 6 m/s So, 343 + 6 = 349 m/s
  5. 5. How fast will sound travel at -5 degrees Celsius? Answer : -5 – 20 = -25 -25 x 0.6 = -15 So, 343 -15 = 328 m/s
  6. 6. Why does the siren sound different when it iscoming toward you than when it is going away?  This is called the doppler shift – the apparent change in frequency of a sound wave caused by the motion of either the source or the detector
  7. 7. Doppler Equation fd = fs (v + vd) (v - vs) fd = detector frequency fs = source frequency v = wave velocity vd = detector velocity vs = source velocity vd and vs are positive if they are moving towards one another and negative if they are moving away from one another
  8. 8. A police car traveling at 30 m/s towards us soundsthe siren at 300 Hz. What frequency do we hear if we are standing still?Answer:300 (343 + 0)/(343 – 30) =300 (343/313) =328.75 Hz
  9. 9. A police car traveling at 30 m/s towards us soundsthe siren at 300 Hz. What frequency do we hear if we are running away at 5 m/s?Answer:300 (343 + -5)/(343 – 30) =300 (338/313) =323.96 Hz
  10. 10. A police car traveling at 30 m/s towards us soundsthe siren at 300 Hz. What frequency do we hear if we are running toward the car at 5 m/s?Answer:300 (343 + 5)/(343 – 30) =300 (348/313) =333.55 Hz
  11. 11. A police car traveling at 30 m/s away from us sounds the siren at 300 Hz. What frequency dowe hear if we are running toward the car at 5 m/s?Answer:300 (343 + 5)/(343 – - 30) =300 (348/373) =279.89 Hz
  12. 12. More Sound Properties Sound is produced by vibrating objects The pitch is the frequency of the wave Loudness depends on amplitude An octave is two notes whose frequency differ by a ratio of 2:1 Sound level is measured in decibels (dB) For every 20 dB increase the sound is ten times louder.
  13. 13. How much louder is 100 db than 60 db? Answer 100 – 60 = 40 40/20 = 2 10^2 = 100 x louder
  14. 14. Sound Definitions Fundamental – lowest resonating frequency (1st harmonic) Harmonics – whole # multiples of the fundamental frequency
  15. 15. Open-pipe vs closed-pipe resonators
  16. 16. Closed-pipe Resonators Closed at one end and open at the other The open end is a pressure node and the closed end is a pressure antinode The first resonating frequency is at one-quarter wavelength It resonates at odd-quarter wavelengths and picks up only the odd harmonics Spacing between resonances is one-half a wavelength
  17. 17. Open-pipe Resonators Open at both ends The ends are pressure nodes The first resonating frequency is at one- half wavelength It resonates at all half wavelengths and picks up all the harmonics Spacing between resonances is one-half a wavelength
  18. 18. Beat The beat is the oscillation of the wave amplitude produced by two waves of slightly different frequencies The beat frequency is the difference between the two frequencies
  19. 19. A 256 Hz tuning fork and a 262 Hz tuning fork are struck simultaneously. What do you hear? Answer: 262 – 256 = 6 Hz

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