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Understanding Ultrasound Physics CH.3

Sidney K. Edelman, THIRD EDITION
STUDY
PLAY
Parameters
DESCRIBE features of a sound wave:(7)
period, frequency, amplitude, power, intensity, wavelength and propagation speed
Parameters
Can be determined by the SOUND SOURCE or the MEDIUM
Sound Source
Is the ULTRASOUND SYSTEM and TRANSDUCER
Some parameters are set by the machine
Some parameters can be adjusted by the sonographer, while others can not.
Medium
The tissue through which the sound is traveling/
Some parameters are determined by the medium
1. Period
The TIME it takes a wave to vibrate a single CYCLE or
The TIME from the START of a cycle to the START of the next cycle.

UNITS:
any unit of TIME, such as seconds, hours, days, ms(milliseconds)
Period
Determined by: SOUND SOURCE
Adjustable: NOT ADJUSTABLE
2. Frequency
The number of CYCLES that occurs in ONE SECOND or
The number of particular EVENTS that occurs in a specific DURATION of time

UNITS: Hertz(Hz)=per second
1 cycle/second=1 hertz
Frequency
Determined by: SOUND SOURCE
Adjustable: NOT ADJUSTABLE

Frequency affects penetration and image quality
The relationship between PERIOD and FREQUENCY
INVERSELY RELATED:
-As frequency INCREASES, period DECREASES
-As frequency DECREASES, period INCREASES
and
RECIPROCAL: Period x Frequency = 1
Frequency Spectrum of Sound
1. INFRASOUND- less than 20 Hz
2. AUDIBLE SOUND - between 20Hz-20,000 Hz (20kHz)
3. ULTRASOUND - greater than 20,000 Hz (20kHz)
Three "Bigness" Parameters
1. Amplitude
2. Power
3. Intensity
Describe the SIZE, MAGNITUDE, or STRENGTH of a sound wave
3. Amplitude
The "bigness" of wave- measured from the middle/undisturbed value to the maximum value in either direction- up (peak) or down (trough)

UNITS: Can have units of any of the acoustic variables (pressure- pascals, density- g/cm3, distance- cm, inches etc and decibels, dB)
Amplitude
Determined by: SOUND SOURCE
Adjustable: YES, a control on the machine allows the sonographer to alter amplitude
Peak-to-Peak Amplitude
The total measurement between the MAXIMUM and MINIMUM values of the wave
It is 2x the value of amplitude
4. Power
RATE of ENERGY TRANSFER or the RATE at which WORK is performed

UNITS: Watts (w)
Power
Determined by: SOUND SOURCE
Adjustable: YES
The relationship between POWER and AMPLITUDE
DIRECTLY RELATED- When power increases, so does the amplitude

Power is proportional to the wave's AMPLITUDE SQUARED
5. Intensity
Concentration of energy in a sound beam. Depends on the POWER and the AREA over which the power is applied

UNITS: Power (w)/ Area (cm2)
Intensity
Determined by: SOUND SOURCE
Adjustable: YES
Amplitude, Power, and Intensity
are DIRECTLY RELATED
When intensity increases, so does amplitude and power etc
Relationship between INTENSITY AND POWER
PROPORTIONAL-
If the wave's power is doubled, the intensity is doubled
Relationship between INTENSITY and AMPLITUDE
Intensity is proportional to the waves AMPLITUDE SQUARED
If a wave's amplitude is doubled, the intensity is increased to 4x its original value
6. Wavelength
The DISTANCE or LENGTH of one complete CYCLE

UNITS: any unit of distance or length (mm, meters, etc.
Wavelength
Determined by: BOTH the SOURCE and MEDIUM
Adjustable: NOT ADJUSTABLE
Relationship between WAVELENGTH and FREQUENCY
INVERSELY RELATED
As frequency increases, wavelength decreases, (as long as the wave remains in one medium)

-the lower the frequency, the longer the wavelength
7. Propagation Speed
DISTANCE that a sound wave travels through a MEDIUM in ONE SECOND

UNITS: m/s meters per second, mm/micro second, or any distance divided by time
Propagation Speed
Determined by: ONLY the MEDIUM
Adjustable: NOT ADJUSTABLE

-Regardless of the frequency, all sound travels at the same speed through any specific medium
Slowest to Fastest propagation
A (air) FAT (fat) WAITER (water) SERVED (soft tissue) BLOODY (blood) MASHED (muscle) BONE (bone)
Sound travels fastest in solids, slower in liquids, and slowest in gases
Speed of Sound in Soft Tissue
1540 m/s or 1.54 mm/micro seconds

speed(m/s) = frequency(Hz) x wavelength (m)
What characteristics of a medium affect the speed of sound in that medium?
1. Stiffness
2. Density
Stiffness
Ability of an object to RESIST COMPRESSION:

a stiff material will retain its shape, whereas a non-stiff material will change its shape
Relationship between STIFFNESS and SPEED
DIRECTLY RELATED:
The stiffer it is, the faster it goes
The opposite of stiff is elastic and compressible
Density
Describes the relative weight of a material.

-The higher the density, the higher the weight
Relationship between DENSITY and SPEED
INVERSELY RELATED:
As materials become more dense (heavier) the speed of sound in the material decreases

When density increases, speed decreases

Materials that are STIFF, but NOT DENSE will have the fastest speed.