Physics Ch. 6

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rbeacr
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141805
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Physics Ch. 6
Updated:
2012-03-15 15:29:33
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Interatction Sound Media
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Interaction of Sound and Media
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  1. Attenuates
    sound weakens as it travels in the body
  2. Amplified
    stregnthening of soundwave that is converted to electricity
  3. decibel notation
    standard mesaurement tool used to report changes in sound
  4. logarithm of 1000
    3

    basically how many zeros for even powers of ten
  5. decibels report what kind of changes
    relative not absolute
  6. decibels need how many intensities
    2 compared (ratio)


    actual level/starting level
  7. decibels

    1/2
    1/10

    double
    ten times
    • -3
    • -10


    • 3
    • 10
  8. attenuation
    weakening of intensity power and amplitude
  9. two factors for attenuation
    depth

    frequency


    Directly Related
  10. how is attenuation measured
    decibels

    omit negative sign
  11. 3 attenuation processes
    • reflection (specular diffuse)
    • scattering (rayleigh)
    • absorption
  12. absorption


    relaxation time
    • primary component of attenuation
    • sound energy turns to heat (vibration of particles/friction)
    • bone has a lot of absorption


    relaxation time- time for particles to go back to equalibrium


    RT and absorption directly related
  13. reflection
    • redirecting wave back to sound source
    • wave is weakened
  14. two typed of reflection
    specular and diffuse
  15. reflection boundary
    greater than a few wavelegnths in size
  16. specular
    boundary
    pattern
    angle of incidence =
    off axis beam
    color on US
    eg.
    • boundary-greater than a wavelegnth
    • large and smooth
    • pattern- organized reflects in one direction
    • angle of incidence = angle of reflection
    • off axis beam will not come back to transducer
    • color is white- strong reflection
    • eg. blood vessle wall
  17. Diffuse (nonspecular, backscatter)
    boundary
    pattern
    off axis beam
    color
    eg.
    • boundary- greated than a wavelegnth
    • Small and Rough
    • Disorganized
    • pattern- unpredictable come back in gen direction
    • off axis beam- not reflected (may come back to transducer)
    • color med gray
    • eg. hepatocyte
  18. Acoustic Impedance
    definition and symbol
    units
    determined by s or m
    • resistance to sound travel
    • z
    • rayls
    • medium
  19. impedance and reflection
    must have different I for reflection to occur

    bigger difference in I more reflection
  20. Impedance of tissue
    How WELL it travels. not how fast
    • air .0004
    • fat
    • water
    • blood
    • kidney
    • soft tissue 1.63
    • liver
    • muscle
    • bone 7.8 (fast but not well)
  21. impedence formula
    z= p x c

    Impedence = density x prop speed
  22. Scattering
    definition
    • random redirection of sound in many directions
    • toward and away from transducer
  23. scattering
    boundary
    = or less than a wavelegnth in size
  24. scattering relationship to frequency
    • directly related
    • increase frequency increase scattering
  25. Rayleigh Scattering
    Boundary
    • very small
    • much smaller than a wavelegnth
  26. Rayleigh Scattering
    organized or disorganized
    pattern
    eg
    look on US
    • organized
    • equal in all directions
    • redblood cell
    • weak reflectors- look black
  27. Rayleigh Scattering is dramatically effected by
    • Frequency
    • mathmatical relationship of F4
  28. Attenuation of media low to high

    (women fight fat some men buy lunch alone)
    • water low
    • fluid
    • fat
    • soft tissue
    • muscle
    • bone
    • lung
    • air high
  29. sound back to the transducer
    • organized- specular (does not return to transducer if off axis)
    • disorganized- diffuse (backscatter- can return to transducer)
  30. sound in all directions
    • rayleigh scattering- organized
    • scattering -disorganized
  31. attenuation coefficient
    definition
    units
    • amount of attenuation the sound beam gives up for each cm it travels through tissue
    • decibels/cm
  32. does atten change through tissue
    no stays same
  33. Medium
    • Prop Speed
    • Impedance
    • Atten Co
  34. Total Attenuation formula
    Total Attenuation = Atten. Coefe. x distance
  35. attenuation coef formula
    atten. coef. = frequency/ 2

    Soft tissue = .5-.7 d/cm MHz
  36. Half layer thickness
    synonyms
    • half intensity depth
    • depth of penetration
  37. half layer thickness
    definition
    units
    • depth in tissue sound travels that results in 3db of intensity attenuation
    • cm
  38. half layer thickness formula
    = 3/Attenuation Coef.
  39. half layer thickness depends on
    frequency and tissue/medium
  40. half layer thickness in soft tissue
    =6/f
  41. Angles of Incidence
    normal names
    • perpendicular
    • othogonal
    • right
    • ninety
  42. angles of incidence not normal
    oblique
  43. sound wave is either
    transmitted or reflected
  44. 3 intensities at a boundary
    • incident intensity- right before
    • reflected intensity- redirected
    • transmitted intensity- continues in same direction
  45. incident intensity =
    • refected intensity + transmitted intensity
    • conservatin of energy
  46. inorder for reflection must have different
    I - Impedance (z)
  47. Intensity Coeffiencient
    units
    2 coefficients
    • %
    • IRC and ITC (reflected and transmitted)
  48. intensity coefficient formula
    100% = IRC + ITC
  49. % of reflection and echo stregnth of
    soft tissue
    bone
    air
    • st- 1% or less
    • b- 50% (think of fetal skull we see through)
    • a- 100%- strong echo
  50. oblique incidence reflection
    cannot predict if sound will reflect or transmit
  51. formula for reflection with normal incidence with 2 different impedences of medium
    IRC% = [z2 - z1 / z2 + z1]2 x 100

    • minus / plus
    • z2 first
  52. Intensity transmission Coefficient formula
    • ITC %= transmitted int/ incident int x 100
    • or = 1- IRC
  53. 2 principles apply with oblique incidence
    • conservation of energy
    • reflection angle = incident angle (Luez's law)
  54. Refraction
    definition
    two conditions
    bending of transmitted wave

    must have oblique incidence and differenct prop speeds
  55. snells law
    transmission angel/ incident angel= speed of med 2/ speed of med 1

    whichever med has a greated trans angle has a greated speed
  56. Reflection- difference in
    Z or impedance
  57. refraction- deffence in
    c or prop speed
  58. conversion of sound to heat
    absorption
  59. db one hundred fold increase
    20db
  60. db quadrupled
    6db
  61. organized in one direction
    specular
  62. disorganized and in general direction of source
    diffuse
  63. chaotic and in all directions toward and away
    scatterer
  64. organized in all directions
    rayleigh scatterer
  65. formula for calculating db
    current intensity/ original intensity
  66. smooth and larger than a few wavelegnths
    specular
  67. rough and larger than a few wavelegnths
    diffuse backscatter nonspect
  68. equal or less than a wavelegnth
    scatterer
  69. much smaller than a wavelegnth
    rayleigh scatterer
  70. contributers to attenuation
    • refletion
    • scattering
    • absorption
  71. reflection of diffuse reflecter effected by off axis beam t or f
    f

    may or may not return to the transducer regardless of angle
  72. the greater the impedance the stronger the
    reflection

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