Ch 6 Flashcards
Interaction of Sound & Media
Attenuation
- Sound waves weakening as they propagate in a medium
- Decrease in intensity, power, amp
Attenuation Factors
Path Length
Frequency of sound
Attenuation components
Reflection, scattering, absorption
More Attenuation
longer distances, higher frequencies
- bone
Less Attenuation
shorter distances, lower frequencies
- fluids
Rayleigh Scattering
- Redirects sound in equal directions
- Rayleigh scattering ∝ frequency^4
- Omnidirectional
Specular Reflection
Hits smooth boundary, sound reflected in only one direction in organized way
Ex) liver and diaphragm
Diffuse Reflection/ backscatter
Hits irregular surface, sound reflects in several directions
Impedance - define, determined by, equation
- Resistance to sound
- Medium only, calculated not measured
- impedance (rayls) = density (kg/m^3) X prop. speed (m/s)
Intensity Reflection Coefficient (IRC)
% of intensity that bounces back when beam strikes boundary
–very little between 2 soft tissues (less than 1%), greater between tissue and bone
Intensity Transmission Coefficient (ITC)
% of intensity that goes through when beam strikes interface
- -most of sound wave’s intensity is transmitted between two soft tissues (99%+)
- -small % between bone and soft tissue
Refraction
transmission with a bend
Refraction conditions
oblique incidence (not normal) --anything other than 90 degrees
Different propagation speeds of two media (diff. impedances)
Snells law defines this
Distance and Attenuation
Directly related - farther the sound travels the greater the attenuation and the weaker the beam
Frequency and Attenuation
Directly related - attenuation is greater in higher frequency sound than in lower frequency sound
