Ch. 6 Interaction Of Sound And Media Flashcards
Logarithm
A novel method of rating numbers. Logarithm or log of a number represents the number of 10s that are multiplied to create the original number.
What is the log of 1000, 10000, & 1000000?
For 1000: We multiply 10 by itself 3 times to create the number 1,000, so the log is 3. For 10000: We multiply 10 by itself 4 times to create the number 10,000, so the log is 4. For 1000000: We count the zeros, there are 6 zeros, so the log is 6.
When a wave’s intensity doubles the relative change is ________
Relative change is +3 dB.
Definition of Decibel
A relative measurement, comparison, ratio, and logarithmic.
What do positive decibels report?
Signals are increasing in strength or getting larger.
What do negative decibels report?
Describe signals that are decreasing in strength, or getting smaller.
When a wave’s intensity increases 10-fold the relative change is ________
Relative change is +10 dB.
When a wave’s intensity is reduced to ½ its original value the relative change is ________
-3 dB
When a wave’s intensity reduced to 1/10 its original value the relative change is ________
-10 dB
Definition of attenuation
The decrease in intensity, power, and amplitude as sound travels.
Attenuation is determined by
Path length and frequency of sound.
Attenuation is measured by
In decibels and reported as a relative change, not as an absolute change.
3 Processes that contribute to attenuation
Reflection, scattering, and absorbtion.
Two types of reflection
Specular and diffuse
Rayleigh Scattering
A special form of scattering that occurs when the structures dimensions are much smaller than the beams wavelength. This redirects the sound wave equally in all directions.
Attenuation Coefficient
The number of decibels of attenuation that occurs when sound travels one centimeter.
Attenuation Coefficient in soft tissue
The attenuation coefficient (dB/cm) and frequency (MHz) are directly related. The attenuation coefficient is one=half the frequency.
What mathematical equation is used to solve for total attenuation in soft tissue?
Atten. Coef. (dB/cm) = frequency (MHz)/ 2; Atten. Coef. = 0.5 dB/cm/MHz
Half – Value Layer Thickness defined
The distance sound travels in a tissue that reduces the intensity of sound to one-half its original value.
Half – Value Layer Thickness units
Centimeters, or any other unit of length.
Half – Value Layer synonyms
Penetration depth, depth of penetration, and half-boundary layer.
Half – Value Layer dependent on what?
The medium and frequency of sound.
Impedance defined
The acoustic resistance to sound traveling in a medium. Calculated by multiplying the density of a medium by the speed at which sound travels in the medium.
Reflection & Transmission
Impedance units
Rayls, often represented by the letter z.
Impedance determined by what?
Associated with the medium only. It is calculated, not measured.
Impedance synonyms
Characteristic impedance
A tissue’s impedance is ____.
Calculated, not measured.
Impedance formula
impedance (rayls) = density (kg/m^3) x prop. Speed (m/s)
Incidence defined
A sound pulse strikes many tissue interfaces as it travels in the body. The angle at which the wave strikes the boundary determines the behavior of the pulse.
Acute angle
Less than 90 degrees
Right angle
Exactly 90 degrees
Obtuse angle
Greater than 90 degrees
Normal Incidence defined
Means that the incident sound beam strikes the boundary at exactly 90 degrees.
Synonyms for normal incidence
Perpendicular, orthogonal, right angle, and 90 degrees.
Oblique Incidence defined
Occurs when the incident sound beam strikes the boundary at any angle other than 90 degrees.
Oblique Incidence synonyms
Means not at right angles or non-perpendicular
Incident Intensity defined
Is the sound waves intensity immediately before it strikes a boundary.
Reflected Intensity defined
The intensity of the portion of the incident sound beam that, after striking a boundary, returns from where it came.
Transmitted Intensity defined
The intensity of the portion of the incident beam that, after striking a boundary, continues forward in the same general direction that it was traveling.
All intensities are measured in ____________ .
W/cm^2
Intensity Reflection Coefficient (IRC) defined
Is the percentage of the intensity that bounces back when a sound beam strikes the boundary between the two media.
Intensity Transmission Coefficient (ITC) defined
The percentage of intensity that passes in the forward direction when the beam strikes an interface between two media.
100% = inten. Reflection coef. (IRC) (%) + intensity trans. Coef. (ITC) (%)
