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99 Cards in this Set
- Front
- Back
The decrease in intensity, power, and amplitude as sound travels |
Attenuation
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After a sound wave is received by a transducer, converted into electricity and returned to the system, it is strengthened or ___
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Amplified
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A method of rating numbers. Specifically this represents the number of 10's that are multiplied to create the original number
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Logarithm
100 = 10x10 = Log is 2 1000 = 10x10x10 = Log is 3 10000 = 10x10x10x10 = Log is 4 |
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Changes to a returning sound wave are reported as a ratio: the returning beam's intensity level divided by its starting intensity level
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Decibel (+/- dB)
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Decibel notation measures ___ changes, not absolute numbers
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Relative
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This kind of decibel indicates an increased signal strength
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+ Decibel
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This kind of decibel indicates a decreased signal strength
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- Decibel
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If an intensity doubles express this in decibels
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+3 dB
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If an intensity is reduced by half express this in decibels
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-3 dB
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If an intensity is 10x larger express this in decibels
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+10 dB
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If an intensity is 1/10 as large express this in decibels
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-10 dB
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Attenuation is determined by what two factors?
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Distance & Frequency
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How are distance and attenuation related?
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Directly Related
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How are frequency and attenuation related?
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Directly Related
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Which has less attenuation: shorter distances & lower frequencies? longer distances & higher frequencies?
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Shorter distances & lower frequencies
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How is attenuation measured?
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Decibels
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Is attenuation reported as a relative change or an absolute change?
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Relative change
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Name the three processes that contribute to attenuation
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Reflection
Scattering Absorption |
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This happens when sound strikes a boundary and a portion of the wave's energy is redirected back to the sound source
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Reflection
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Soft tissue creates what two types of reflection?
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Specular & Diffuse
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This happens when a boundary is smooth and a portion of the sound wave's energy is reflected in one direction and in an organized manner
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Specular Reflection
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Explain a limitation of Specular Reflection
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If the wave is slightly off-axis, the reflection does not return to the transducer
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This happens when a sound wave reflects off an irregular surface and it radiates in more than one direction
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Diffuse Reflection (Backscatter)
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Explain one advantage of Diffuse Reflection
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Even if the wave is off-axis or at suboptimal angle, some reflections will still return to the transducer
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Explain one disadvantage of Diffuse Reflection
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Backscattered signals have a lower strength
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Random redirection of sound waves in many directions
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Scattering
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How is frequency and scattering related?
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Directly Related
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Sound scatters when the tissue interface is ___ (equal to or less than the ___ of the sound beam)
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Small / Wavelength
(Lung tissue scatters sound because the alveoli are filled with air) |
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A reduction in the intensity of a sound beam to 1/2 its original value is ___ dB
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-3 dB
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A reduction in the intensity of a sound beam to 1/4 of its original value is ___ db
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-6 db
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-10 db means that the intensity is reduced to ___ of its original value
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1/10
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dB is a mathematical representation with a ___ scale:
A. logarithmic and relative B. division and relative C longitudinal and relative D. logarithmic and absolute |
A
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T/F: we need one intensity to calculate dB
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F
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Every 3 dB change means that the intensity will ___
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Increase 2x (or double)
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Every 10 dB change means that the intensity will ___
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Increase 10x
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A wave's intensity is 2 mW/cm(2). There is a change of 9 dB. What is the final intensity?
A. 6 mW/cm(3) B. 2 mW/cm(2) C. 16 mW/cm(2) D. 16 microW/cm(2) |
C
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This is a special form of scattering that occurs when the structure's dimensions are much smaller than the beam's wavelength (frequently with RBCs)
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Rayleigh Scattering
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This occurs when ultrasonic energy is converted into another form of energy such as heat
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Absorption
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What is the relationship between absorption and frequency?
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Directly Related
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This is the number of decibels of attenuation that occurs when sound travels one centimeter
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Attenuation Coefficient
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Attenuation Coefficient: Units?
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dB/cm
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In soft tissue, what is the relationship between the attenuation coefficient and frequency?
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Directly Related
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Which has the highest attenuation:
A. air, bone, lung B. water, blood, urine |
A
(B is the least) |
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The distance that sound travels in a tissue that reduces the intensity of sound to 1/2 its original value
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Half-Value Layer Thickness
Penetration Depth Half-Boundry Layer |
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Half-Value Layer Thickness: Units?
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cm
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What effect would the following have on the half-value layer thickness?
- high frequency sound - media with high attenuation rate |
Thin Half-Value Layer
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What effect would the following have on the half-value layer thickness?
- low frequency sound - media with low attenuation rate |
Thick Half-Value Layer
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The half-value layer thickness depends on what two factors?
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1. Medium
2. Frequency |
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The acoustic resistance to sound traveling in a medium
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Impedance
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Reflection of an ultrasound wave depends upon the difference in ___ of the two media at a boundary
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Impedance
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Is impedance calculated or measured?
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Calculated
(density x propagation speed) |
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Impedance: Units?
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Rayls (Z)
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As the path length increases, the attenuation of ultrasound in soft tissue ___
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Increases
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Attenuation in lung tissue is <, >, = attenuation in soft tissue
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>
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Attenuation in bone is <, >, = attenuation in soft tissue
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>
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Attenuation in air is <, >, = attenuation in soft tissue
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>
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T/F: in a given medium, attenuation is unrelated to speed of sound
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T
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What is the numeric relationship between frequency and attenuation coefficient in soft tissue?
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Attenuation Coefficient (dB/cm) = Half the Frequency
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As frequency decreases, depth of penetration ___
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Increases
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As path length increases, the half boundary layer ___
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Remains the same
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Impedance is a characteristic of what?
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Medium
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As the path length increases, the attenuation coefficient of ultrasound in soft tissue ___
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Remains the same
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Medium A and B have the same densities. The speed of sound in medium A is 10% higher than in B. Which medium has the higher acoustic impedance?
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A
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An angle less than 90 degrees is called a/an ___ angle
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Acute
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An angle exactly 90 degrees is called a/an ___ angle
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Right
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An angle more than 90 degrees is called a/an ___ angle
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Obtuse
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When the sound beam strikes a boundary at exactly 90 degrees, this is called what?
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Normal Incidence
Perpendicular |
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When the sound beam strikes a boundary at any angle other than 90 degrees, this is called what?
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Oblique Incidence
Non-Perpendicular |
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An angle that is not 90 degrees is called a/an ___ angle
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Oblique
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What type of incidence should be avoided when using doppler?
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Oblique Incidence
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When a beam strikes a soft tissue boundary, one part of it is ___ and the other moves ___
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Reflected (backwards)
Forward |
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The intensity of the beam immediately before it strikes a boundary is called what?
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Incident Intensity
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The intensity of the portion of the beam that returns back in the direction from which it came, is called what?
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Reflected Intensity
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The intensity of the portion of the beam that continues forward, is called what?
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Transmitted Intensity
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The percentage of the intensity that bounces back when a beam strikes a boundary is called what?
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Intensity Reflection Coefficient (IRC, <1%)
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The percentage of the intensity that continues forward after the beam strikes a boundary is called what?
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Intensity Transmission Coefficient (ITC, >99%)
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At the boundary between two media, what principle applies?
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Conservation of Energy
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At the boundary between two media, if the reflected and transmitted intensities are added, what is the result?
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Incident Intensity
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At the boundary between two media, if the IRC and ITC are added, what is the result?
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100%
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A sound wave with intensity of 50 W/cm(2) strikes a boundary and is totally reflected. What is the intensity reflection coefficient (IRC)?
A. 50 W/cm(2) B. 25 W/cm(2) C. 0 W/cm(2) D. 100% E. 0 |
D
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A sound wave with intensity of 50 W/cm(2) strikes a boundary and is totally reflected. What is the reflected intensity?
A. 50 W/cm(2) B. 25 W/cm(2) C. 0 W/cm(2) D. 100% E. 0 |
A
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With normal incidence, what is the relationship between the size of the reflection and the impedance difference?
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Directly Related
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With normal incidence, ___ depends upon a/an ___ difference between the two ___ on either side of a ___
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Reflection
Impedance Media Boundary |
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With oblique incidence, what two laws apply?
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1. Conservation of Energy
2. Reflection Angle = Incident Angle |
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The sum of reflected and transmitted coefficients must always equal 100%. The sum of reflected and transmitted intensities must always equal the incident intensity. This is called what?
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Conservation of Energy
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With oblique incidence, in what direction are reflection sent?
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Away from the transducer (equal, but opposite to the direction of the incident beam)
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With oblique incidence, what may occur to the transmitted beam that continues forward?
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Refraction (it may bend, or change direction, slightly)
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A change in direction of wave propagation when traveling from one medium to another is called what?
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Refraction
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What two conditions must be present for refraction to occur?
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1. oblique incidence
2. two media with different propagation speeds |
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What law defines the physics of refraction?
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Snell's Law
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Under what conditions will the transmission angle equal the incident angle (no refraction)?
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Speed 1 = Speed 2
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Under what conditions will the transmission angle be greater than the incident angle (refraction)?
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Speed 1 < Speed 2
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Under what conditions will the transmission angle be less than the incident angle (refraction)?
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Speed 1 > Speed 2
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A pulse of ultrasound propagates in soft tissue, such as liver. The pulse strikes a soft tissue-soft tissue interface with oblique incidence. Some of the sound energy is transmitted. To what extent is the transmitted beam refracted?
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Little to no refraction
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A sound pulse travels in medium 1 and strikes an interface with medium 2 at 30 degrees. The angle of transmission is 10 degrees. From these facts alone, what can be said about the difference between the speeds of media 1 and 2?
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Large Difference
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A sound pulse travels in medium 1 and strikes an interface with medium 2 at 30 degrees. The angle of transmission is 10 degrees. In which medium does sound travel the slowest?
A. medium 1 B. medium 2 C. cannot be determined |
B
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A sound pulse travels in medium 1 and strikes an interface with medium 2 at 30 degrees. The angle of transmission is 10 degrees. In which medium is the impedance higher?
A. medium 1 B. medium 2 C. cannot be determined |
C
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Sound travels in a medium and orthogonally strikes a boundary with a different medium. Although sound waves traveling in the media have vastly different speeds, there is no refraction. How can this be?
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Refraction cannot occur with normal incidence
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A sound wave strikes a boundary at normal incidence. The impedances of the two media are identical. What percentage of the sound wave is refracted?
A. 100% B. 75% C. 0% D. 25% E. 90% |
C
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