Interaction Of Sound And Media Chapter 6 Flashcards
What is attenuation? Aka?
The decrease in strength (intensity, power, and amplitude) of a sound wave as it travels. (Unrelated to speed). The further sound travels, the more attenuation occurs.
What are the 3 components of attenuation?
- Absorption
- Scattering
- Reflection
What does absorption mean in terms of attenuation?
Primary, sound converted into heat
What units is attenuation measured in? What is true about these values?
Decibels; always negative since attenuation causes intensity to decrease
How does more or less attenuation affect distance and frequency?
Less attenuation = shorter distance, lower frequency, More attenuation = longer distance, higher frequency.
How does more or less attenuation affect distance and frequency?
Less attenuation = shorter distance, lower frequency, More attenuation = longer distance, higher frequency.
What is the difference in attenuation in media like air, lung/bone, water, blood?
Air= much more attenuation than in soft tissue, gel is used to remove air. Lung and bone = more than soft tissue, bone absorbs and reflects, lung scatters. Water = much less than soft tissue. Blood = less than soft tissue. Air»Bone and Lung»_space; soft tissue»_space; water
what is the relationship between frequency, attenuation, and penetration in soft tissue? What does this relationship mean for attenuation, and how it affects this?
Lower frequency results in less attenuation. Thus, we penetrate further with lower frequency sound. Attenuation limits the max imaging depth from which meaningful reflections are obtained.
When does reflection occur?
when propagating sound energy strikes a boundary bw 2 media and some returns to the transducer.
what is specular reflection?
reflections from a smooth reflector (mirror) are specular and return in one direction
when else do specular reflections occur?
when the wavelength is much smaller than the irregularities in the boundary.
when do the strongest reflections occur?
specular reflectors are well seen when sound strikes the boundary at 90 degrees/ perpendicular.
what is diffuse reflection or backscatter? when does this occur? how would you describe the reflected sound?
the reflection of sound generally back towards the transducer but in a number of directions; occurs when a boundary is rough; reflected sound is disorganized and random
when else can diffuse reflection/backscatter occur?
when the boundary has irregularities that are approximately the same size as the sound’s wavelength.
what is scattering? does anything affect this?
the distribution of sound randomly in all directions. Higher frequency sound scatters to a greater extent.
what is Rayleigh scattering? what does this mean?
if a reflector is much smaller than the wavelength of sound, sound is uniformly distributed in all directions. As frequency increases, scattering increases.
what is organized reflection called? disorganized reflection? organized scattering? disorganized scattering?
specular; diffuse/backscatter; Rayleigh; Scatter
what is attenuation coefficient? what is this used for? what units are used here?
the amount of attenuation per cm. Way to report attenuation without dealing with how far sound travels. dB/cm
what is the relationship of attenuation coefficient and frequency? how is this portrayed in soft tissue?
as frequency increases, the attenuation coefficient increases. Higher frequency = more attenuation per cm, shows why higher frequency sound cannot penetrate deep. = lower frequencies used to image deeper in the body.
how does attenuation coeff change as the sound wave travels?
remains the same
in soft tissue, attenuation coeff is:
1/2 of the transducer’s frequency.
0.5 db/cm/MHz
what is impedance? what units is it measured in? typical values?
a number associated with a medium. This is calculated not measured; Rayls (Z); bw 1,250,00 and 1,750,000 rayls (1.25-1.75 Mrayls)
what does reflection of an ultrasound wave depend on?
different acoustic impedances of the media on either side of the boundary.
what is the equation for impedance?
impedance (rayls) = density (kg/m cubed) X Propagation speed (m/s)
what are two examples of oblique incidence?
anything other than 90 degrees; acute or obtuse angles
what is incident intensity? units?
intensity of the sound wave prior to striking a boundary. w/cm squared
what is reflected intensity? units?
intensity that after striking a boundary changes direction and returns back from where it came; w/cm squared
what is transmitted intensity? units?
intensity that after striking a boundary continues on on the same general direction that it was originally traveling; w/cm squared
what is the equation for incident intensity?
incident intensity = reflected intensity + transmitted intensity
what exists at a boundary in terms of intensities?
conservation of energy
what is intensity reflection coefficient (IRC)? units? typical values?
the percentage of the US intensity that bounces back when the sound strikes a boundary. unitless. range from 0-100% or 0-1.0
what is intensity transmission coefficient (ITC)? units? typical values?
the percentage of the incident intensity that after striking a boundary, continues on in the same direction it was originally traveling. unitless. range from 0-100% or 0-1.0
what 2 things are true about the boundary between 2 media in regards to intensity?
- IRC + ITC = 100%
- when reflected and transmitted intensities are added, the result is the incident intensity.
how are intensities and coefficients reported?
W/cm squared; coeff= percentages
in the following boundaries what is the reflection percent? soft tissue-air, soft tissue-bone , soft tissue-soft tissue what does this mean?
soft tissue-air = 99%
soft tissue-bone = 50%
soft tissue-soft tissue= <1% ;there is greatest attenuation at an air-tissue interface and less a bone-tissue interface.
a sound wave w an intensity of 50 W/cm ^2, strikes a boundary and is totally reflected. what is the intensity reflection coefficient?
100%
a sound wave with an intensity of 50 W/cm ^2 strikes a boundary and is totally reflected. what is the reflected intensity?
50W/cm^2
the intensity transmission coeff of sound is 99.9%. what percentage of sound is reflected back to the transducer?
0.1%
when does reflection occur?
occurs only if the two media at the boundary have different acoustic impedances.
what is the equation for intensity reflection coeff? what does this measure?
IRC (%) = [(Z2-Z1)/(Z2+Z1)]^2
;predicts the percentage of sound reflected at a boundary
what is the relationship bw impedance diff and IRC?
with greater impedance diff bw the two media, the IRC increases and the amount of reflection increases
what happens to whatever is not reflected?
it must be transmitted!
how does reflection and transmission occur with oblique incidence?
transmission and reflec may or may not occur with oblique incidence
what 2 things do we know about oblique incidence?
- incident inten= transmitted+reflected
- reflection angle= incident angle (specular reflections arise when the interface is smooth
what is the def of refraction?
refraction is transmission of sound with a bend. It is a change in direction as sound transmits from one medium to another.
what does refraction require?
- oblique incidence
AND - different speeds
when does refraction not occur?
normal incidence or with identical propagation speeds
what is snell’s law? what is the equation?
describes the physics of refraction;
sin trans angle/ sin incident angle = propagation speed 2/ prop speed 1
sooo what is required for reflection w normal incidence? w oblique incidence? what about transmission? refraction?
look for diff impedances; unknown; from reflection info (aka medium impedances); look for oblique incidence and diff speeds
