Ultrasound Physics: Seven Parameters of Sound Waves

These flashcards cover the essentials of describing sound waves in ultrasound physics, including definitions, units, typical values, and mathematical relationships for the seven primary parameters.

How many parameters are required to completely characterize a sound wave?

The values of seven parameters are required.

What are the seven parameters that describe sound waves?

1. period, 2. frequency, 3. amplitude, 4. power, 5. intensity, 6. wavelength, 7. speed.

How is the period of a sound wave defined?

Period is 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.

What are the typical values for period in diagnostic ultrasound?

$0.06$ to $0.5\,\mu s$, which can also be written as $6 \times 10^{-8}$ to $5 \times 10^{-7}\,\text{seconds}$.

Is the period of a wave determined by the sound source or the medium?

Period is determined by the sound source only, not by the medium.

Can the sonographer adjust the period when using a basic ultrasound system?

No, the sonographer cannot change the period while using a basic ultrasound system with a particular transducer.

What is the definition of frequency?

Frequency is the number of particular events that occur in a specific duration of time; in ultrasound, it is the number of cycles that occurs in one second.

What units are used to report frequency?

Frequency is reported in units of per second, $1/\text{second}$, hertz, or $Hz$.

What is the typical range of frequencies used in clinical imaging?

$2\,MHz$ to $15\,MHz$ ($2$ million to $15$ million per second).

How is sound classified as infrasonic or infrasound?

Sound with a frequency less than $20\,Hz$, which is below the threshold of human hearing.

What frequency range is defined as audible sound?

Between $20\,Hz$ and $20,000\,Hz$.

Define ultrasonic or ultrasound waves.

Sound with frequencies higher than $20,000\,Hz$ ($20\,kHz$), which humans cannot hear.

What is the relationship between frequency and period?

They are inversely related and reciprocals. When multiplied together, their result is $1$ ($\text{Period} \times \text{Frequency} = 1$).

What are the three parameters that describe the 'bigness' or magnitude of a sound wave?

1. amplitude, 2. power, 3. intensity.

How is amplitude defined?

Amplitude is the difference between the maximum value and the average (undisturbed) value of an acoustic variable, or the difference between the minimum and average value.

What are the possible units for amplitude?

Amplitude can have units of any acoustic variable: pressure (pascals), density ($g/cm^3$), or particle motion (distance units like $cm$ or inches). It can also be expressed in decibels ($dB$).

What is the difference between amplitude and peak-to-peak amplitude?

Amplitude is measured from the baseline to the maximum or minimum, while peak-to-peak amplitude is the difference between maximum and minimum values, making it twice the value of amplitude.

Define power in the context of sound waves.

Power is the rate of energy transfer or the rate at which work is performed.

What is the mathematical relationship between power and amplitude?

Power is proportional to the wave's amplitude squared ($\text{power} \propto \text{amplitude}^2$).

If the amplitude of a wave is increased by a factor of $3$, how much does the power increase?

The power increases by a factor of $9$ ($3^2 = 9$).

What is the definition of intensity and its formula?

Intensity is the concentration of energy in a sound beam. It is calculated as: $\text{intensity (W/cm}^2\text{)} = \frac{\text{beam's power (W)}}{\text{area (cm}^2\text{)}}$.

How is intensity related to power and amplitude?

Intensity is proportional to power ($\text{intensity} \propto \text{power}$) and proportional to amplitude squared ($\text{intensity} \propto \text{amplitude}^2$).

Define wavelength.

Wavelength is the distance or length of one complete cycle.

What determines the wavelength of a sound wave?

Wavelength is the only parameter determined by both the source and the medium.

How are wavelength and frequency related in a single medium?

They are inversely related. As frequency increases, wavelength decreases.

What is the rule for finding the wavelength of a sound wave in soft tissue?

\text{wavelength (mm)} = \frac{1.54\,\text{mm/\mu s}}{\text{frequency (MHz)}}.

Define propagation speed.

Propagation speed is the rate at which a sound wave travels through a medium.

What is the average speed of sound in soft tissue?

$1,540\,m/s$, $1.54\,mm/\mu s$, or $1.54\,km/s$.

What characteristics of a medium determine the speed of sound?

Stiffness and density.

How does stiffness affect the speed of sound?

Stiffness and speed are directly related; as a material becomes stiffer, the speed of sound in it increases.

How does density affect the speed of sound?

Density and speed are inversely related; as materials become more dense (heavier), the speed of sound decreases.

What are synonyms or related terms for stiffness?

Bulk modulus is the same as stiffness. Elasticity and compressibility are the opposites of stiffness.

In which state of matter does sound travel fastest, and in which does it travel slowest?

Sound travels fastest in solids, slower in liquids, and slowest in gases.

Which of the three 'bigness' parameters can a sonographer adjust?

Amplitude, power, and intensity can all be adjusted by the sonographer.