Attenuation, Interaction of Sound and Media

Question 1

How can you determine the log of an even number that is a power of 10?

Answer 1

For even powers of 10 (100, 10,000) you can count the zeroes

Question 2

Do decibels measure absolute or relative changes?

Answer 2

Relative changes

Question 3

What does 3dB indicate?

Answer 3

Intensity is doubled

Question 4

What does 10 dB indicate?

Answer 4

Intensity is increased ten-fold

Question 5

What does -3 dB indicate?

Answer 5

Intensity is halved

Question 6

What does -10 dB indicate?

Answer 6

Intensity is reduced to 1/10 the original value

Question 7

What is attenuation?

Answer 7

Sound waves weakening as they propagate through a medium

Question 8

What units are used to measure attenuation?

Answer 8

Decibels

Question 9

Will attenuation be negative or positive?

Answer 9

Negative

Question 10

How is distance related to attenuation?

Answer 10

Directly

The farther sound travels, the greater the attenuation

Question 11

How is frequency related to attenuation?

Answer 11

Directly

The higher the frequency, the greater the attenuation

Question 12

What are the three processes that contribute to attenuation?

Answer 12

Reflection
Scattering
Absorption

Question 13

What is reflection?

Answer 13

When a portion of a sound wave is reflected back to the sound source after striking a boundary

Question 14

What is specular reflection?

Answer 14

When sound is reflected off a smooth surface in only one direction in an organized manner

Question 15

What is diffuse reflection?

Answer 15

When a wave reflects off an irregular surface in more than one direction

Question 16

What is another term used to describe diffuse reflection?

Answer 16

Backscatter

Question 17

What is the advantage of diffuse reflections?

Answer 17

The reflections at suboptimal angles will still produce reflections that come back to the transducer

Question 18

What is the disadvantage of diffuse reflections?

Answer 18

The backscattered signals have a lower strength than specular

Question 19

What is scattering?

Answer 19

The redirection of sound in many directions

Question 20

What is a tissue that creates scattering?

Answer 20

Lung tissue

Question 21

What is Rayleigh scattering?

Answer 21

Scattering that occurs when the dimensions of the structure are smaller than that of the wavelength

Question 22

What is a structure that produces Rayleigh scattering?

Answer 22

Red blood cells

Question 23

How is scattering related to frequency?

Answer 23

Scattering is proportional to frequency raised to the 4th power

Question 24

What is absorption?

Answer 24

When ultrasonic energy is converted to a different type of energy, especially heat

Question 25

How is absorption related to frequency?

Answer 25

Directly

Question 26

What is the conflict between image accuracy and attenuation with regard to frequency?

Answer 26

Higher frequency sound creates accurate images, but has greater attenuation as it travels

Question 27

What is the attenuation coefficient?

Answer 27

Number of decibels of attenuation that occurs when sound travels one cm Unit is dB

Question 28

What is the formula for calculating attenuation?

Answer 28

Total Atten (dB) = Atten Coefficient (dB/cm) x distance (cm)

Question 29

What is the formula used to calculate the attenuation coefficient in soft tissue?

Answer 29

Atten Coefficient = frequency (MHz) / 2

Question 30

Rank in order from highest to lowest attenuation | muscle, soft tissue, blood, air, bone, fat, water

Answer 30

Water, blood, fat, soft tissue, muscle, bone, air

Question 31

What is the half-value layer thickness?

Answer 31

The distance sound travels in a tissue that reduces the intensity of sound to half its original value

Question 32

What is impedance?

Answer 32

Acoustic resistance to sound traveling in a medium

Question 33

How is impedance calculated?

Answer 33

impedance (rayls) = density (kg/m3) x prop. speed (m/s)

Question 34

What is normal incidence?

Answer 34

When the incident sound beam strikes the sound beam at 90 degrees

Question 35

What are other terms used to describe normal incidence?

Answer 35

Perpendicular, orthogonal, right angle, 90 degrees

Question 36

What is oblique incidence?

Answer 36

When the incident sound beam strikes the boundary at any angle other than 90 degrees

Question 37

What is incident intensity?

Answer 37

The sound wave intensity before it strikes a boundary

Question 38

What is reflected intensity?

Answer 38

The sound wave intensity as it returns back from striking a boundary

Question 39

What is transmitted intensity?

Answer 39

Intensity of the portion of the wave after it strikes a boundary and continues forward in the same direction

Question 40

What are units used to describe intensity?

Answer 40

W/cm^2

Question 41

What is the intensity reflection coefficient?

Answer 41

The percentage of intensity that bounces back when a sound beam strikes a boundary

Question 42

What percentage of a wave is typically reflected at a soft tissue boundary?

Answer 42

Less than 1%

Question 43

What is the intensity transmission coefficient?

Answer 43

Percentage of intensity that passes in the forward direction after striking an interface

Question 44

How are IRC and ITC reported?

Answer 44

As percentages

Question 45

IRC + ITC = ?

Answer 45

IRC + ITC = 100%

Question 46

What must happen for there to be a reflection when sound strikes a boundary with normal incidence?

Answer 46

Different impedances on either side of the boundary

Question 47

What equation is used to determine the IRC?

Answer 47

IRC% = [Z2-Z1/Z2+Z1]^2 x 100

Question 48

How can we determine what % of intensity is transmitted?

Answer 48

Whatever sound is not reflected must be transmitted | IRC + ITC = 100%

Question 49

Can we predict if and how much reflection or transmission will occur with oblique incidence?

Answer 49

No

Question 50

What two physical principles will apply to reflectin with oblique incidence?

Answer 50

Conservation of energy | Reflection angle=incidence angle

Question 51

What is refraction?

Answer 51

A change in direction of wave propagation when traveling from one medium to another

Question 52

What two conditions must be met for refraction to occur?

Answer 52

If there is oblique incidence | If there are different propagation speeds of two media

Question 53

What is Snell's Law?

Answer 53

sin (transmission)/sin (incident) = speed of medium 2/speed of medium 1

Question 54

What does Snell's Law describe?

Answer 54

The physics of refraction

Question 55

What happens to angle of transmission if Speed 2 = Speed 1?

Answer 55

no refraction | transmission angle = incident angle

Question 56

What happens to angle of transmission if Speed 2 > Speed 1?

Answer 56

transmission angle greater than incident angle

Question 57

What happens to angle of transmission if Speed 2 < Speed 1?

Answer 57

transmission angle less than incident angle