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)
