Acoustics Ch. 2

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elz125
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104460
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Acoustics Ch. 2
Updated:
2011-09-26 17:14:37
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Acoustics
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Intro to sound and acoustics
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  1. What is simple harmonic motion?
    • Equilibrium
    • Maximum displacement in one direction
    • Equilibrium
    • Maximum displacement in the opposite direction
    • Equilibrium
  2. What is Hooke's law?
    • Magnitude of restoring force is proportional to distance displaced
    • Fr is directly proportional to X
  3. Does the magnitude of restoring force change over time?
    Yes. Kinetic energy changes to thermal energy because of friction. If X changes, Fr must change
  4. What is friction?
    An opposition to motion
  5. How many frequencies is a sine wave?
    • one
    • speech is many frequencies
  6. What is on the x and y axis of a waveform?
    • time is on the x axis
    • amplitude is on the y axis
    • amplitude can be anything (displacement, velocity, acceleration, force, pressure, momentum, etc)
  7. What is a waveform?
    • a plot of change in amplitude (loudness) of displacement over time
    • amplitude vs. time
  8. What is uniform circular motion?
    • a point moves about the circumference of a circle at a constant number of degrees of rotation per second
    • the point engages in SHM
  9. One cycle of uniform circular motion is equal to how many degrees?
    360 degrees
  10. Is the motion of a spring-mass system circular?
    No, it's rectilinear
  11. Which direction does uniform circular motion move?
    • counterclockwise
    • 90 degrees = max displacement
    • 270 degrees = max displacement
    • 0, 180, and 360 degrees = equilibrium
  12. What is sinusoidal motion?
    • same as simple harmonic motion
    • projection of sinusoidal motion is called a sine wave, or sinusoidal wave
  13. What are the 5 dimensions of sine waves?
    • amplitude
    • frequency
    • period
    • phase
    • wavelength
  14. What is a phase?
    where in time the wave begins (0, 90, 180, 270, or 360 degrees)
  15. What is amplitude?
    • sound pressure
    • directly proportional to force
  16. What is the starting phase of a sine wave?
    it can be anything
  17. What are the phasic relations?
    • displacement
    • velocity
    • acceleration
    • pressure
    • these relationships are always the same
  18. What would two waves starting at 0 degrees be considered?
    in phase
  19. What is the relationship between particle velocity starting at 90 degrees and particle displacement starting at 0 degrees?
    particle velocity leads particle displacement by 90 degrees
  20. Particle velocity leads particle displacement by 90 degrees. Why?
    velocity is maximal at equilibrium where X is at zero; velocity is zero at Xmax where motion is momentarily halted
  21. How much does particle acceleration lead particle displacement by?
    180 degrees
  22. What happens if two waves are 180 degrees out of phase?
    they cancel each other out
  23. What are the starting phases of displacement, velocity, acceleration and sound pressure?
    • displacement = 0 degrees
    • velocity = 90 degrees
    • acceleration = 180 degrees
    • sound pressure = 90 degrees
  24. What is instantaneous amplitude?
    • amplitude at a moment of time
    • infinite amounts on a sine wave
  25. What is maximum amplitude?
    amplitude from equilibrium to the highest possible peak
  26. What is peak-to-peak amplitude?
    • amplitude from the highest point to the lowest point
    • peak-to-valley
    • have to take absolute value (2 and -2 = 4)
  27. What is amplitude measured in?
    • voltage
    • pascals
    • decibels (dB)
  28. What is frequency?
    • cycles per second (cps)
    • measured in Hertz (Hz)
    • inverse of period
  29. What is period?
    • seconds per cycle (spc)
    • the time required to complete one cycle
    • inverse of frequency
    • measured in seconds (s)
  30. How do you get from ms to s?
    divide by 1000
  31. How do you get from s to ms?
    multiply by 1000
  32. How do you get from Hz to kHz?
    divide by 1000
  33. How do you get from kHz to Hz?
    multiply by 1000
  34. What does frequency depend on?
    • properties of the source of sound
    • spring-mass system: mass and stiffness of system
  35. What does more mass mean?
    lower frequency (male voice)
  36. What does stiffer mean?
    • higher frequency
    • vocal folds stretched apart (norah jones- stiff vs. rihanna- not stiff)
  37. What is natural frequency?
    • the frequency with which a system oscillates freely (fnat)
    • fnat = square root of s/m
  38. What are the proportional relations of fnat with s and m?
    • fnat is directly proportional to square root of s- frequency increases as stiffness increases
    • fnat is directly proportional to 1/square root of m- frequency decreases as mass increases

    *frequency review page 26
  39. *Phase page 27
  40. What is wavelength?
    the space occupied by a complete cycle
  41. Do men or women have greater wavelength?
    men
  42. What quantities are measured in wavelength with respect to time?
    • frequency
    • speed of sound

    *page 28 slide 56
  43. What are the proportional relations of wavelength with speed of sound and frenquency?
    • wavelength is directly proportional to speed of sound
    • wavelength is directly proportional to 1/f
  44. *Phase review page 28
  45. What is damping?
    an opposition to motion (friction or frictional resistance)
  46. What does friction limit?
    velocity
  47. What is the relationship between velocity and damping?
    directly proportional
  48. How much does momentum lead elasticity by?
    90 degrees
  49. What is the relationship between displacement and elasticity?
    in phase
  50. By how much does acceleration lead velocity and displacement?
    • velocity- 90 degrees
    • displacement- 180 degrees
  51. *Wavelength review page 29
  52. What causes damping?
    as velocity increases, kinetic energy is transformed to thermal energy (friction)
  53. What is damping in phase with?
    • velocity
    • it varies over time
  54. What does magnitude of displacement depend on?
    force applied
  55. What are the 3 types of damping systems?
    • a lossless system
    • low-damped system (vocal folds)
    • high-damped system (ipod)

    *page 30 slide 67
  56. What would happen if shock absorbers and VU meters where nearly undamped?
    the sound would go on forever (excessive oscillation/vibration)
  57. What is impedance?
    an opposition to motion
  58. What is the relationship between mass and frequency?
    as mass goes up, frequency goes down
  59. What are the two components of total impedance?
    resistance (R) and reactance (X)
  60. What is resistance?
    • friction
    • use up energy (kinetic energy is transformed to thermal energy)
  61. What is reactance?
    • storage of energy as potential energy
    • the amount of energy stored depends on the frequency
  62. What are the two types of reactiance?
    • mass reactance (Xm)
    • compliant reactance (Xc)
  63. Are resistance and reactance independent or dependent of frequency?
    • resistance is independent of frequency (it doesn't matter what the freq. is)
    • reactance is frequency dependent
  64. What is resistance measured in?
    ohms
  65. *Reactance and phasic relations page 31
  66. What is the relationship between mass reactance and compliant reactance?
    • when one reactance component stores energy, the other gives up energy
    • they are 180 degrees out of phase
  67. What is mass reactance measured in and directly proportional to?
    • ohms
    • frequency
  68. What is the difference between mass reactance at low frequencies and high frequencies?
    • low = negligible (small), larger amplitude of vibration
    • high = large, smaller amplitude of vibration
  69. What is compliant reactance measured in and inversely proportional to?
    • ohms
    • frequency
  70. What is the difference between compliant reactance at low frequencies and high frequencies?
    • low = large, smaller amplitude of vibration
    • high = negligible, larger amplitude of vibration
  71. If frequency is less than natural frequency...
    • impedance increases
    • amplitude of vibration decreases
    • compliance dominant
  72. If frequency is greater than natural frequency...
    • impedance increases
    • amplitude of vibration decreases
    • mass dominant
  73. When is the amplitude of a wave the greatest?
    • at natural frequency
    • mass and compliant reactance cancel each other out (18- degrees out of phase)
  74. What are the phases of resistance and reactance?
    • mass reactance leads resistance by 90 degrees
    • compliant reactance leads resistance by 90 degrees
    • mass reactance leads compliant reactance by 180 degrees

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