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WAVES

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WAVES

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    1. WAVES

    2. TOPICS Waves ELECTROMAGNETIC MECHANICAL Transverse Longitudinal Acoustic variables Wave characteristics Wave Properties

    3. The Nature of a Wave We encounter waves daily, such as sound waves, visible light waves, radio waves, microwaves, water waves, flag waves & stadium waves.

    4. A wave is a disturbance

    5. A wave has a crest & a trough. It travels from one location to another. One crest is followed by another crest. A trough separates each crest to create an alternating pattern of crests and troughs.

    6. Circular waves originate from the point where a disturbance occurred - it travels across the surface in all directions.

    7. Microwaves Light waves We dont see them, yet know they exist. We witness how they carry energy from one location to another. The same is true for radio waves and sound waves. We tune into those wave signals through our ears or a radio.

    8. Waves carry energy from one location to another Wave - a disturbance in a medium where the individual particles of the medium cycle back-and-forth (or up-and-down) and energy is carried from one place to another.

    9. Medium a substance or material that carries the wave The medium transports the wave from its source to other locations.

    10. In a water wave, the medium the wave travels through is water. In this room, sound waves travel through the medium of air.

    11. What is the medium for this wave?

    12. TYPES OF WAVES ELECTROMAGNETIC MECHANICAL Transverse Longitudinal

    13. Mechanical Wave - a wave that requires a medium to physically interact with A vacuum is the absence of particles. A mechanical wave has no medium to interact with in the vacuum and thus cannot propagate through a vacuum. Sound is a mechanical wave

    14. Transverse Wave - a wave in which the mediums particles move in a direction perpendicular to the direction that the wave moves

    15. Transverse Waves If a rope is stretched out & the end is vibrated up and down, the resulting transverse wave would look like this:

    16. Transverse Wave Terminology

    17. Equilibrium represented by the dashed line, is the position the rope is at rest when there is no disturbance moving through it. Once a disturbance is introduced, the particles of the rope begin to vibrate upwards and downwards. At any point in time, a particle on the medium could be above, below or at the rest position.

    18. Crest - point of maximum amount of positive or upwards displacement from the rest position. Points A & C are the crests of this wave

    19. Trough - point of maximum amount of negative or downwards displacement from the rest position. Points B & D are the troughs of this wave

    20. Longitudinal Wave Terminology

    21. Longitudinal Wave - wave whose mediums particles move in a direction parallel to the direction that the wave moves

    22. Compression - point of maximum density in a medium through which a longitudinal wave is traveling

    23. Rarefaction - point of minimum density in a medium through which a longitudinal wave is traveling

    24. Points A, C, & E represent compressions

    26. A slinky can be used to demonstrate both transverse wave motion and longitudinal wave motion

    28. Sound is a mechanical, longitudinal wave

    29. 3 Acoustic Variables Variations that occur to a medium as a sound wave travels through it 1. Pressure - force applied by the wave 2. Distance - mediums particles move temporarily away from original position Density (mass/volume) - medium experiences a temporary change in density as the wave goes through it Temperature mediums particles absorb the heat from the wave as it passes through

    30. Wave Characteristics

    31. Amplitude (A) maximum amount of displacement of a particle from its rest position distance from rest to crest The amplitude can also be measured from the rest position to the trough position.

    32. Amplitude relates to loudness in sound & brightness in light. The amplitude of the wave is directly related to the amount of energy carried by the wave.

    33. A high energy wave is characterized by a high amplitude A low energy wave is characterized by a low amplitude

    35. Wavelength (?) the distance one complete cycle occupies a complete cycle is a crest & a trough

    36. Wavelength

    37. Wavelength OR from one compression to the next compression OR from one rarefaction to the next rarefaction

    39. Frequency (?) - the # of complete cycles per second - Cycle per second is called Hertz

    40. Wavelength Higher frequencies = shorter wavelengths

    41. Wavelength Lower frequencies = longer wavelengths

    42. Frequency and Wavelength

    43. Period (T) - Time for 1 complete cycle to occur Units: seconds Frequency is the reciprocal of period ? = 1/T or T = 1/?

    44. Frequency vs. Period Frequency refers to how often something happens; it is a rate quantity Frequency is cycle/second Period refers to the time it takes for something to happen; it is a time quantity Period is the second/cycle

    45. Period

    47. Sound has a spectrum broken down into ranges Infrasound: Waves that have a frequency < 20 Hz Audible: Waves that have a frequency between 20 Hz 20,000 Hz Ultrasound: Waves that have a frequency > 20,000 Hz

    51. Dont confuse frequency with speed Speed - distance traveled per time; how fast an object is moving The speed of a wave is the distance traveled by a given point on the wave in a given period of time. Frequency - # of cycles per second Speed - meters traveled per second

    52. Speed of a Wave Speed - how fast an object is moving; distance traveled per time In a wave, the speed is distance traveled by a given point on the wave (such as a crest) in a given interval of time Speed = Distance/Time

    53. Speed of a Wave In the time of one period, a wave moved the distance of one wavelength Substituting this information in the equation for speed it can then be said that the speed of a wave is the wavelength/period. Speed = Wavelength/Period

    54. Speed of a Wave RECALL: Period is the reciprocal of the frequency, 1/? can be substituted for the period in the equation Speed = Wavelength * Frequency This is known as the wave equation

    55. Wave Equation - states the relationship between the waves: speed (c) wavelength (?) frequency (?) c = ? * ?

    56. In general, sound waves travel: slowest in gases (air, lungs) faster in liquids (water, blood) fastest in solids (tissue, metal, bone)

    57. Ave. speed of US in ST - 1540 m/s Air 330 m/sec Lung 500 m/sec Fat 1450 m/sec Water 1480 m/sec Brain 1520 m/sec Liver 1550 m/sec Kidney 1560 m/sec Blood 1570 m/sec Muscle 1580 m/sec Bone 4000 m/sec

    58. What affect the speed at which a wave travels through a medium? Wave speed depends on the medium through which the wave is moving Only an alteration in the mediums properties will cause a change in the speed.

    59. Stiffness Density Properties of the medium that affect the speed of the wave:

    60. Stiffness (N/m2) - mediums resistance to be compressed Stiffness - opposite of compressibility and elasticity stiffness & speed are directly proportional to each other

    61. Density (?) - concentration of an objects mass per unit volume (mass/volume ) density & speed are inversely proportional to each other In general, higher density media are extremely stiff. The increased speed is due to the stiffness and not to the density.

    62. Edelmans Rule Stiffness and Speed Both begin with Ss Stiffness increases Speed Density and Speed Begin with opposite letters (D and S) Density decreases Speed

    63. So how does this relate to US?? Well . . . Varying tissue changes the speed, if you dont change the frequency then the wavelength changes. The wavelength affects reflection, resolution, and harmonics.

    64. Ultrasound Waves Properties Parameters - illustrate size/strength of a wave The operator can change these by adjusting the output power on the ultrasound unit. Amplitude Pressure Power Intensity

    65. Amplitude

    66. Amplitude (A) Strength of a sound wave - peak pressure, strength, or loudness Units depend on acoustic variable being measured: Pressure: lbs/in2; Pascals (Pa) Density: grams/cc Temperature: degrees Distance: cm, mm Wave amplitude with propagation through tissue

    67. Interference Combination of 2 waves that overlap at the same time & location Results in a new single wave that is the sum of the 2 original waves Interference of 2 waves can result in increased or decreased amplitude

    68. Constructive Interference Occurs when both waves intersect in-phase. Peaks and troughs occur at the same time Wave amplitudes reinforce each other, building a wave of even greater amplitude Increases the sound beams intensity

    69. Destructive Interference Occurs when both waves intersect out-of-phase Peak of one wave lines up with the trough of the other Results in a decrease in amplitude or a canceling out of the wave altogether Contributes to ultrasound attenuation

    70. Interference

    71. POWER

    72. Power Strength of the sound beam Amount of work/time Rate at which work is done (rate energy is transmitted by the transducer into the body) Units - Watts (W) or milliwatts (mW) W = J /s

    73. Power - determined by the sound source (US unit), can be changed by the operator by adjusting the output power control Power ? as US travels through the body Power & amplitude are directly related Power (Watts) = Amplitude2

    74. Power Power - proportional to the waves amplitude squared (A2) EXAMPLE: a waves amplitude of 1 is doubled, its power is increased 4X Power ? A2 4 ? 22

    75. In other words . . . In this example, increasing the POWER by 4X only doubled the waves amplitude Think how much you would have to increase the POWER if the amplitude was: 5 10

    76. So is turning the POWER Up Another notch very helpful to you or the patient??

    77. Intensity

    78. Intensity (I) Strength of a sound beam Amount of POWER/area Power (W) of a beam divided by its cross-sectional area (cm2) Units - watts/centimeter squared (W/cm2) milliwatts/centimeter squared (mW/cm2) Audible sound range - intensity is loudness

    79. Intensity (I)

    80. Intensity (I) Can be changed by the operator using the output power control to change the amplitude of the wave Can cause bioeffects

    81. Intensity (I) Intensity -directly related to power; if power is 2X, the intensity is 2X Intensity is proportional to amplitude of the wave squared (I ? A2) Example: If you 3X the amplitude; intensity increases by a factor of 9 I ? A2 9 ? 32

    82. So how does this relate to US?? Well . . . Intensity can cause bioeffects Is there a way to compensate (or decrease) intensity??

    83. All of the facts we just went over are true for continuous sound waves in a perfect world In another class we will use the very same principles but apply them to pulsed sound waves the type most commonly used in US

    84. Logarithm a way to rank numbers log = # of 10s multiplied together to get that # Example: The log of 100,000 = 5 (hint: # of 0s) What is the log of 1,000? log 100,000,000 = ?

    85. Decibels (dB) A unit that compares the ratio of intensities or amplitudes of 2 sound waves Uses a logarithmic scale takes a wide range of values & reduces the values to a smaller range typically used to express a large change Does not represent absolute values only gives the relationship between 2 values

    86. Decibels +dB = ? in value Final intensity > original intensity Signal is strengthened Example: ? US gain = + dB = stronger echoes

    87. Decibels -dB = ? in value Final intensity < original intensity The signal is weakened. Attenuation - weakening of the sound beams intensity & amplitude as it passes through tissue Attenuation is -dB

    89. Numeric Values of dB

    90. Example Your employer informs you that you will receive a +6 dB change in your salary, what would that mean? Answer: Your pay would be increased four (4) times or quadrupled! Do you see that a small db number can make a huge change?

    91. So how does this relate to US?? Output power Overall gain TGC (DGC) Smarter sonographer Makes great Registry questions

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