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what does an ellipse measure?
area
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"increase by a factor of" means
multiply
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decrease by a factor of means
divide
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10^-2 = hundreth
centi = c
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10^-3 = thousandth
milli = m
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10^-6 = millionth
micro = μ
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10^-9 = billionth
nano = n
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acoustic propagation properties
the effects of the medium upon the sound wave
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biologic effect
the effects of the sound wave upon the biologic tissue
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sound
- a type of wave that carries enery, not matter from place to place
- must travel through a medium
- is mechanical, longitudinal wave
- travels in a straight line
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compressions
are areas of increased pressure or density
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rarefactions
are areas of decreased pressure or density
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three acoustic variables
- pressure = concentration of force w/i area force / area units= pascal (Pa)
- density = concentration of mass w/i volume units = kg/cm^3
- distance = measure of particle motion units = cm, ft, miles
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transverse wave
particles move in a perpendicular direction (right angles or 90) to the direction of the wave
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sound is longitudinal wave
particles move in the same direction as the wave
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acoustic parameters
- period
- fequency
- amplitude
- power
- intensity
- speed
- wavelength
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period
- the time required to complete a single cycle (the start of a cycle to the start of the next)
- units = any unit of time
- determines by = sound source
- can not be changed by sonographer
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frequency
- number of certain events that occure in a particular time duration
- units = per second, 1/second = Herts (Hz)
- determined = sound source
- can not be changed by sonographer
- frequency affects penetration and axial resolution
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ultrasound
- a wave with a frequency exceeding 20,000Hz (20 kHz)
- frequency is so high that is not audible
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audible sound
heard by man, frequencies btwn 20Hz and 20,000Hz
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infrasound
- sound with frequencies less than 20Hz
- frequency is so low that is not audible
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frequency and period
- are reciprocals
- frequency (Hz) x period (sec) = 1
- period (sec) = 1/ frequency (Hz)
- frequency (Hz) = 1/period (sec)
- when one goes down the other goes up
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amplitude
- the difference btwn the average value and the maximum value of an acoustic variable. the variation of an acoustic variable
- units = same as acoustic variables
- pressure=pascals
- density=grams/cubic cm
- particle motion=cm,inches,units of distance
- may be expressed in decibels (dB)
- determined by sound source (initially)
- can be changed by sonographer
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power
- the rate that work is performed, or the rate of energy transfer
- units = watts
- determined by sound source (initially)
- can be changed by sonographer
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power & amplitude
power is porportional to the wave's amplitude squared
- ex: if amplitude is tripled, the power is increase by a factor of nine:
- 3 x 3 = 9
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intensity
the concentration of energy in a sound beam. the beam's power devided by the beam's cross-sectional area. intensity depends upon bothe the power and the cross-sectional area of the beam
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