Chapter 3: Describing Sound Waves

What Identifies a sound wave

oscillation of acoustic variables

What are acoustic variables

pressure (Pa), density (kg/cm³), and distance (cm, mm)

What do parameters do

describe features of a sound wave

What are the 7 parameters

period

frequency

amplitude

power

intensity

wavelength

speed

What is period (T)

the time it takes a wave to vibrate one complete cycle, reported in units of time (seconds, hours, etc)

What is period determined by and is it adjustable

the source only

is not adjustable by sonographer

Typical period range for clinical imaging

0.06-0.5 µs

What is frequency (Hertz/Hz)

Number of particular events that occur in a specific duration of time, reported in “units per second”.

In ultrasound, its the number of cycles that occur in 1 second

What is the clinical imaging (ultrasound) range of frequency and what do these values mean

Generally ranges from 2MHz-15/20MHz

Higher frequency creates better quality image with less penetration

Lower frequency has more penetration but lower quality image

(general rule: the more superficial a structure is, the higher frequency you want to use)

What is frequency determined by and is it adjustable

It is determined by source alone

Sonographers cannot adjust frequency, it is only changed by switching which transducer is being used

What is infrasonic/infrasound frequency

frequency of a sound wave <20Hz is below threshold of hearing (inaudible)

What is audible sound frequency

humans can hear sound frequencies between 20Hz-20kHz

What us ultrasonic/ultrasound frequency

frequency so high that humans cannot hear it is higher that 20kHz

Relationship between period and frequency

they are inversely related and reciprocal. when frequency increases, period decreases. when frequency decreases period increases

What are the 3 bigness parameters

amplitude, power, and intensity

What is amplitude

the strength or “bigness” of a wave. (the difference between the max value (peak) and the baseline, or the min value (trough) and baseline). it is reported in units of various acoustic variables (pressure (Pa), density (g/cm³), distance cm))

What is the pressure amplitude range for clinical imaging

1MPa-3MPa

What is amplitude determined by and is it adjustable

It is determined by the sound source but it decreases as it propagates (travels through) the body.

It is adjustable by the sonographer (gain/TGC)

What is peak to peaks amplitude

the difference/measurement between the peak of a wave and the trough. its usually double the value of the amplitude

What is Power

the rate of energy transfer (from transducer to body) or rate at which work is being performed. it is reposted in units of watts (W)

What is the range of power for clinical imaging

typically ranged from 0.004-0.09 watts (or 4-90 milliwatts)

What is power determined by and is it adjustable

It is determined by sound source only and decreases as it propagates through the body. the rate of decrease depends on characteristics of the medium it is traveling through and soundwave.

Sonographers can adjust the initial power of a wave

How are power and amplitude related

they both describe size of magnitude of a wave.

power is proportional to waves amplitude squared (power ∝ amplitude²)

(value of power is the squared value of amplitude)

What is Intensity

It is the concentration of energy in a sound beam. it is reported in units of watts per squared centimeter (W/cm²) where W is from power and cm² is from beam area

How to calculate intensity

Divide beams power by beams area

intensity (W/cm²) = power (W) / area (cm²)

intensity relates to how the power in a wave spreads/is distributed in a space

Clinical imaging intensity range

ranges from 0.01-300 W/cm²

What is intensity determined by and is it adjustable

It is determined by sound source alone and changes as sound propagates through the body. the rate of change depends on characteristics of sound waves and the medium.

initial intensity is adjustable by the sonographer

How is intensity and power related

intensity is proportional to power (intensity ∝ power).

when wave power is doubled, intensity is doubled

How is intensity and amplitude related

Intensity is proportional to waves amplitude squared (value of intensity is the squared amplitude value)

(intensity ∝ amplitude²)

when a wave amplitude is doubled, the intensity is increased by 4 times the original value

What is wavelength (λ)

It is the distance or length of one complete cycle. it is reported in units of distance (mm, meters, etc).

Clinical imaging wavelength range

in soft tissue, it typically ranges from 0.1-0.8mm

What is wavelength determined by and is it adjustable

it is the only parameter determined by both sound source and medium.

it is not adjustable by the sonographer

Difference between wavelength and period

Wavelength refers to length/distance of a cycle while period refers to the time it takes to complete a cycle.

they are differentiated by their units

Relationship between wavelength and frequency

as long as a wave remains in one medium, wavelength and frequency are inversely related. (as frequency increases, wavelength decreases).

the longer the frequency, the longer the wavelength

Determining Wavelength in soft tissue

in soft tissue, sound with a frequency of 1MHz has a wavelength of 1.54mm.

wavelength = 1.54mm/µs / frequency (MHz)

What is propagation speed

the rate at which a sound wave travels through a medium. it is reported by distance divided by time (m/s, mm/µs)

Speed of sound in lung

500m/s

Speed of sound in fat

1450m/s

Speed of sound in soft tissue (average)

1540m/s

Speed of sound in liver

1560m/s

Speed of sound in blood

1560m/s

Speed of sound in muscle

1600m/s

Speed of sound in tendon

1700m/s

Speed of sound in bone

3500m/s

Speed of sound in air

330m/s

Speed of sound in water

1480m/s

Speed of sound in metals

2000-7000m/s

Where does sound travel fastest/slowest

it travels fastest in solids and slowest in gases

What is propagation speed determined by and is it adjustable

is it determined only by the medium that its traveling through (the stiffness and density). all sound regardless of its frequency travels at the same speed through a specific medium (a sound with 3MHz frequency and a sound with 5MHz frequency travel at the same propagation speed in the same medium).

It is not adjustable by the sonographer

Stiffness and density relation with speed

Stiffness and speed are directly related

Density and speed and inversely related

What is stiffness (Bulk Modulus)

the ability of an object to resist compression. a stiff material will retain its shape when pressure is applied

What is the opposite of stiffness

elasticity and compressibility

What is density

the relative weight of a material. as a material becomes more dense, the speed of sound decreases.

Does stiffness or density have a greater influence on speed

stiffness. air has exceedingly low stiffness, therefore sound travels slowly in air.