Describing Sound Waves

Chapter 3

What do parameters do?

describe the features of a sound wave

What is the source of a sound wave?

the ultrasound system & transducer

True or False: Transducer values cannot be changed

true

Period:

time it takes a wave to vibrate a single cycle

A period is measured in…

seconds

Are periods adjustable?

no

A period is determined by…

sound source only

Frequency:

number of cycles that occurs in a second

Frequency is reported in…

Hz

The typical values for frequency are…

2-15 MHz

Frequency is determined by…

sound source only

Is frequency adjustable?

no

Frequency affects…

penetration & image quality

If you are scanning superficial…

a higher frequency is used

If you are scanning deep…

a lower frequency is used

Humans cannot hear sound above…

20,000 Hz

What is the relationship between frequency & period?

inversely related

A higher frequency =

a shorter period

A lower frequency =

a longer period

Hertz:

events/cycles per second

100 Hz means…

100 cycles per second

What are the 3 bigness parameters?

amplitude, power, & intensity

What purpose do the 3 bigness parameters serve?

describe size, magnitude, or strength of a sound wave

Amplitude:

the bigness (height) of a wave

Amplitude is measured in…

pascals; pressure

Can amplitude be adjusted?

yes

Amplitude is determined by…

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

What is the difference between amplitude & peak to peak amplitude?

amplitude is measured from the middle value to the maximum value

peak to peak amplitude is twice the value of the amplitude

Power:

the rate of energy transfer at which work is performed

Power is measured in…

watts

Power describes…

the bigness of a wave

Power is determined by…

sound source initially, but decreases as the sound beam travels through the body

Is power adjustable?

yes

How are power & amplitude related?

directly related

When power increases…

amplitude increases

When power decreases…

amplitude decreases

Intensity:

the concentration of the energy in a sound beam; cone-shaped

Intensity is measured in…

watts per square centimeter; (W/cm²); power over area

Intensity is determined by…

sound source initially, but changes as sound propagates through the body

Is intensity adjustable?

yes

How are intensity & power related?

directly proportionate

How are intensity & amplitude related?

directly proportionate

How are intensity & frequency related?

not related

Wavelength:

the length of one complete cycle

Wavelength is measured in…

mm/m

Wavelength is determined by…

both sound source & medium

Is wavelength adjustable…

no

What is the difference between wavelength & period?

wavelength- describes the length of a cycle

period- describes the time it takes to complete a cycle

What is the relationship between wavelength & frequency?

inversely related

When frequency increases…

wavelength decreases

When frequency decreases…

wavelength increases

The lower the frequency…

the longer the wavelength

The wavelength of 1 MHz sound in soft tissue is…

1.54 mm

If the frequency is twice that of another, the wavelength…

will be half as much

Why is wavelength important in diagnostic ultrasound?

shorter wavelengths produce higher quality images with greater detail

Always use the…

highest frequency transducer possible

What is the trade off for using a higher frequency transducer?

less penetration & more absorption of the sound beam

Propagation speed:

the rate at which a sound wave travels through a medium

Propagation speed is measured in…

meters per second (m/s)

Propagation speed is determined by…

only the medium

Is propagation speed adjustable?

no

What characteristics of a medium determine the speed of sound in that medium?

stiffness & density

Stiffness:

the ability of an object to resist compression

Density:

the relative weight of a material

What is the relationship between stiffness & speed?

directly related

The stiffer the material, the speed of sound in the material…

increases

How are density & speed related?

inversely related

As materials become more dense, the speed of sound in the material…

decreases

Sound travels faster in media with…

low density

If two media are equally stiff…

the denser medium will have a lower speed

Between density & stiffness, which factor has the greatest influence on speed?

stiffness

What is the opposite of bulk modulus (stiffness)?

elasticity & compressibility

Materials that are stiff, but NOT ___ will have the fastest speed.

dense

Materials that are not stiff AND very ___ will have the slowest speed.

dense

What five parameters are all INITIALLY determined by the sound source only?

period, frequency, amplitude, power, & intensity

Period & frequency are…

reciprocals

Amplitude, power, & intensity are grouped together because they all describe…

the magnitude of the sound wave

Wavelength is the ONLY parameter determined by…

both the sound source & medium

Speed is the ONLY parameter determined by…

solely the medium through which the sound wave travels