CDO 463 2 Basic Acoustics

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CDO 463 2 Basic Acoustics
2012-10-01 19:37:48
Absorption Reflection

Absorption and Reflection
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  1. tuning forks amplitude is determined by
    the strength of the initial blow
  2. a tuning forks energy is used up
    because of damping - friction within the fork itself and friction against the air molecules being moved imparts energy into the medium
  3. inverse square law
    4 part
    • intensity of sound is expressd as force/area
    • as the wave moes away from the source it covers a greater and greater area
    • however the wave has a finite amount of force
    • the intensity varies inversely with the square of the distance from the source
  4. What 4 things can happen when a sound wave strikes an object
    • reflection - a bouncing back of a wave in the opposite direction of the original or incident wave
    • absorption - reduction in the amount of sound energy due to phyical characteristics of the struck object
    • transmission - movement of the wave through the struck object
    • diffraction - spreading apart of a wave as it encounters obstacles
  5. reflection
    • portion of a wave bounces back when it hits an object
    • echo or reverberation
    • will be of a smaller amplitude than the original wave
  6. absorption
    • reduction in the amount of sound energy
    • similar to damping
    • acoustic energy dissipated as heat
    • amount varies depending on the object struck
  7. poor absorbers v good absorbers
    • poor - hard, smooth surface
    • good - soft, rough surface
  8. standing wave
    interference between an original and a reflected wave
  9. interference is the result of
    the addition of two or more waves
  10. constructive interference
    waves reinforce each other increasing amplitude (pressure)
  11. destructive interference
    waves cancel each other, decreasing amplitude (pressure)
  12. node
    that point in the standing wave where the pressure of the original wave is always equal and opposite the pressure of the reflected wave
  13. antinode
    the point in the standing wave that undergoes maximum variation in pressure (mx negative to max positive)
  14. The complete wavelength of a standing wave is composed of
    three nodes and two antinodes
  15. Guitar strings:
    increase mass
    more tension
    increase length
    • decrease frequency
    • higher frequency
    • decrease frequency
  16. Vocal folds
    increase mass
    more tension
    increase length
    • decrease frequency
    • high frequency
    • increase frequency (because you lower the mass)
  17. guitar strings
    wavelength =
    • wavelength = 2L
    • (L = length)
  18. guitar strings
    frequency =
    f = c/wavelength

    • f = square root of (t/m) / 2L
    • (t = tension) (m = mass)


    f = 1/2L * square root (t/m)
  19. Higher mode of vibration give rise to
    overtones, or harmonics, that are at even and odd integer multiples of the fundamental frequency
  20. harmonic structure is the combination of
    fundamental frequency and harmonics, and their amplitudes
  21. the vocal tract is essentially a
    column of air in a tube that is open at one end (the lips) and closed at the other end (the larynx)
  22. air columns
    f =
    • f = 1/4L * c
    • c = square root (E/p)
  23. Harmonic structure of the sound produced by the air column is
    • a fundamental frequency and harmonics that are at the
    • odd integer multiples of the f0.
  24. Harmonic structure
    the combination of a sound's fundamental frequency and harmonics and their amplitudes
  25. showing of harmonic structure
    spectrum - desription of the amp and frequency characteristics of a sound wave
  26. sound quality is determined by
    the harmonic structure of the sound