Ultrasound Physics Ch 3

parameters

describe features of a sound wave

source of sound

ultrasound system or transducer

period

time it takes a wave to vibrate a single cycle

Units: units of time (microseconds, seconds, hours, days)

period is determined by:

the sound source only

not adjustable by the sonographer

frequency

number of particular events that occur in a specific duration of time or the number of cycles that occurs in one second

Units: Hz

frequency is determined by:

sound source only

infrasound/infrasonic

less than 20 Hz

audible sound

between 20 Hz and 20 kHz

ultrasound/ultrasonic

greater than 20kHz

relationship of period and frequency

inversely related, reciprocals

three parameters that describe the size of a sound wave

amplitude

power

intensity

amplitude

the “bigness” of a wave

amplitude is determined by:

initially the sound source, but decreases as sound propagates through the medium

adjustable by the sonographer

peak-to-peak amplitude

twice the amplitude

power

rate of energy transfer

Units: watts

power is determined by:

initially the sound source, but decreases as sound propagates through the medium

adjustable by the sonographer

relationship of power and amplitude

power is proportional to amplitude²

intensity

concentration of energy in a sound beam

intensity formula

intensity = power/area

Units: watts/cm²

intensity is determined by:

initially by the sound source

adjustable by sonographer

relationship between intensity and power

intensity is proportional to power

relationship between intensity and amplitude

intensity is proportional to amplitude²

wavelength

distance or length of one complete cycle

wavelength is determined by:

sound source and the medium

not adjustable by the sonographer

wavelength and frequency are:

inversely related

wavelength of 1MHz sound in soft tissue

1.54 mm

difference between wavelength and period

wavelength is the distance

period is the time it takes

rule that defines the relationship between wavelength and frequency

wavelength (mm) = 1.54 mm/micro second divided by frequency (MHz)

propagation speed

rate at which a sound wave travels through a medium

Units: m/s, mm/micro second

propagation speed is determined by:

only by the medium

not adjustable by sonographer

speed of sound in soft tissue

1,540 m/s

speed of sound in bone

3,500 m/s

speed of sound in air

330 m/s

two characteristics that affect speed

stiffness

density

stiffness

ability of an object to resist compression

density

relative weight of a material

relationship of speed and stiffness

directly related

relationship of speed and density

inversely related

bulk modulus

same as stiffness

opposite of stiff

elasticity and compressibility

adjustable parameters

amplitude

power

intensity

non-adjustable parameters

period

frequency

wavelength

speed

parameters determined by sound source

period

frequency

amplitude

power

intensity

parameters determined by medium

speed

parameters determined by both

wavelength