Chapter 3: Describing Sound Waves

____ describe features of a sound wave

parameters

the ____ of a sound wave is the ultrasound system and transducer

source

the tissue is also called the ____

medium

the time it takes a wave to vibrate a single cycle, or the time from the start of one cycle to the start of the next cycle

period

period is reported in units of ____

time

typical value of period in ultrasound

0.06 to 0.5 µs

period is determined by ____

sound source only

is period adjustable?

no

the number of particular events that occur in a specific duration of time

frequency

frequency is reported in units of ____

per second, hertz, or Hz

Hertz means ____

per second

typical value of frequency in ultrasound

2 MHz to 15 MHz

frequency is determined by ____

sound source only

is frequency adjustable?

no

sound wave less than __ Hz is below the threshold of human hearing and cannot be heard

20 Hz

sound with a frequency of less than 20 Hz

infrasound

sound with a frequency between 20 Hz and 20 kHz

audible sound

sound with a frequency greater than 20 kHz

ultrasound

period and frequency are ____ related and are also ____

inversely; reciprocals

when two reciprocals parameters are multiplied together, the result is __

1

period x frequency = __

1

three parameters describe the size, magnitude, or strength of a sound wave:

• intensity

• power

• amplitude

____ is the “bigness” of a wave. it is the difference between the maximum value and the average or undisturbed value of an acoustic wave. it is also the difference between the minimum and average value of the acoustic variable

amplitude

____ can have units of any of the acoustic variables

• pressure - pascals

• density - g/cm³

• particle motion - cm, inches - any distance

amplitude

amplitude, in relative terms can also be expressed in ____

decibels (dB)

typical value of amplitude

1 MPa to 3 MPa

amplitude is determined by ____

sound source only

is amplitude adjustable?

yes

the difference between maximum and minimum values of an acoustic variable

peak-to-peak amplitude

the rate of energy transfer or the rate at which work is performed; also describes the “bigness” of a wave

power

power has units of ____

watts

typical value of power

0.004 to 0.090 watts (4 to 90 milliwatts)

power is determined by ____

sound source only

is power adjustable?

yes

when power increases or decreases, so does ____

amplitude

when amplitude increases or decreases, so does ____

power

power is proportional to the waves ____ squared

squared

the concentration of energy in a sound beam

intensity

to calculate intensity, divide the beam’s ____ by the beam’s ____

power; cross-sectional area

units of intensity are ____

watts/cm²

typical value of intensity

0.01 to 300 W/cm²

intensity is determined by ____

sound source only

is intensity adjustable?

yes

the distance or length of one complete cycle

wavelength

units of wavelength

mm, meters, or any other unit of length

typical value of wavelength

0.1 to 0.8 mm

wavelength is determined by ____

sound source and medium

is wavelength adjustable?

no

wavelength and frequency are ____ related

inversely

the lower the frequency, the ____ the wavelength

longer

in soft tissue, sound with a frequency of 1 MHz has a wavelength of ____

1.54 mm

to find the wavelength of a sound wave in soft tissue, divide 1.54 mm by the ____

frequency in MHz

wavelength (mm) = 1.54 mm/µs divided by ____

frequency (MHz)

shorter wavelengths are created with ____ frequency sound which usually produces higher quality images with greater detail

higher

the rate at which a sound wave travels through a medium

propagation speed

speed is measured in units of ____

meters per second, mm/µs, or any distance divided by time

typical value of propagation speed

500 m/s to 4000 m/s depending on the tissue through which it is traveling

propagation speed is determined by ____

medium

is propagation speed adjustable?

no

speed of sound in soft tissue

1,540 m/s or 1.54 mm/µs

speed of sound in lung tissue

500 m/s

speed of sound in fat tissue

1,450 m/s

speed of sound in liver tissue

1,560 m/s

speed of sound in blood

1,560 m/s

speed of sound in muscle tissue

1,600 m/s

speed of sound in tendon tissue

1,700 m/s

speed of sound in bone

3,500 m/s

speed of sound in air

330 m/s

speed of sound in water

1,480 m/s

speed of sound in metals

2,000 to 7,000 m/s

sound travels fastest in ____, slower in ____, and slowest in ____

solids; liquids; gases

speed = ____ x ____

frequency (Hz) x wavelength (m)

two characteristics of a medium that affect the sound of speed

stiffness and density

describes the ability of an object to resist compression

stiffness

describes the relative weight of a material

density

stiffness and speed are ____ related

directly

____ is the same as stiffness

bulk modulus

____ and ____ are the opposite of stiff

elasticity and compressibility

density and speed are ____ related

inversely

as materials become more dense (heavier), the speed of sound in the material increases/decreases

decreases

if two media are equally stiff, which medium will have a lower speed?

the denser medium

in relation to stiffness and density, which factor has a greatest influence on propagation speed?

stiffness