Interaction of Sound and Media Flashcards
Sound _______ as it propagates through the body
weakens
The weakening of sound as it travels through tissues is called?
attenuation
When sound waves are sent back to the system they are ______.
amplified
What is used to report a change in intensity?
decibels
The log of 1000 is?
3
The log of 100 is?
2
Decibal notation is?
a relative measurement, a comparison, a ratio, logarithmic
Decibels require _____ intensities
2
Positive decibels signify?
an increase or amplification
+3dB means the intensity _______.
doubles
+10 dB means the intensity ________.
increases by ten times
Negative decibels signify?
a decrease in intensity or attenution
-3dB means the intensity _______
reduces to 1/2 its value
-10 dB means the intensity _______
reduces to 1/10 it’s original value
If the original intensity is 2 watts/cm2, and it reduces by 3 dB, what is the final intensity?
1 watt/cm2
If the original intensity is 20 watts/cm2, and it reduces by 10 dB, what is the final intensity?
2 watts/cm2
Attenuation is determined by?
path length (depth) and frequency
Attenuation and path length are _______ related
directly
Attenuation and frequency are _________ related
directly
What three things contribute to attenuation?
reflection, scattering, absorption
Reflection occurs when a boundary is _____
large
What are the two types of reflection?
specular and diffuse
What are the two types of reflection?
specular and diffuse
What is specular reflection? When does it occur?
sound reflects in one direction; boundary is smooth
What is diffuse reflection? When does it occur?
Sound is reflected in more than one direction; boundary is irregular
What is scattering? When does it occur?
random redirection of sound in many directions; boundary is small
Frequency and scattering are _______ related
directly
What is an example of rayleigh scattering?
interaction with red blood cells
What is absorption?
Ultrasonic energy is converted into another energy form, like heat
Absorption is _______ related to frequency
directly
What does the attenuation coefficient represent?
the number of decibels of attenuation that occur when sound travels 1 cm
Unit of the attenuation coefficient?
dB/cm
Unit of attenuation?
dB
Formula for total attenuation
attenuation coefficient x distance
Attenuation coefficient and frequency are _______ related
directly
The attenuation coefficient is _____ the frequency
half
Define half value layer thickness
the distance sound travels to reduce the intensity to 1/2 it’s original value (-3dB)
The HVL depends on?
medium and frequency
Define impedance
the resistance to sound traveling in a medium
What are the units of impedance
rayls (Z)
What is the typical value of the HVL?
0.25-1.0 cm
What can lead to a thin HVL?
high frequency sound or medium with high attenuation rate
What can lead to a thick HVL?
low frequency sound or a media with a low attenuation rate
Define incidence
the angle at which a wave strikes a bondary
What are the three types of incident angles?
acute/oblique, right, obtuse/oblique
Define normal incidence
beam strikes boundary at 90 degrees
Normal incidence is also called?
perpendicular, orthoganal, right angle
Define oblique incidence
Sound beam strikes boundary at angle other than 90 degrees
What is the intensity before a wave strikes a boundary?
incident intensity
What is the intensity of the portion of the wave that bounces back after striking a boundary?
reflected intensity
What is the intensity of the portion of the wave after striking a boundary that continues forward?
Transmitted Intensity
The Incident intensity equals?
Reflected intensity + Transmitted intensity
What is the intensity reflection coefficient (IRC)?
the percentage of intensity that bounces back after striking a boundary
What is the intensity transmission coefficient?
the percentage of intensity that continues forward after striking a boundary
True or False: The IRC is usually larger than the ITC
False; ITC is larger (more transmission than reflection)
IRC + ITC =?
100%
Reflection only occurs when with normal incidence ?
When media have different impedances
True or False: If impedances are identical with normal incidence there is reflection
False; All transmissio
If there are slightly different impedances, there is a _______ amount of reflection
small
If there are largely different impedances, there is a _______ amount of reflection
large
With normal incidence, if two media have identical impedances, all sound is?
Transmitted
With normal incidence, reflection and transmission depends on?
the impedences
True or False: Reflection and transmission cannot be predicted with oblique incidence
True
With oblique incidence, what two principles always apply?
conservation of energy; reflection angle = incident angle
With an oblique incidence, the direction of the reflected echo is?
equal and opposite to the direction of the incidence
What is refraction?
When a beam strikes a boundary at oblique incidence and transmits but changes direction (does not travel straight)
Refraction only happens under what two conditions?
oblique incidence and different propagation speeds of two media
True or False: Refraction can occur with normal incidence
False; only oblique incidence
True or False: Refraction occurs when the speeds of two media are identical
False; different speeds
When the speed of medium 2 is is greater than medium 1, the transmission angle will be ______ than the incident angle
greater
When the speed of medium 2 is less than medium 1, the transmission angle will be______ than the incident angle
less
