Wave Parameters

 

What are Acoustic Variables and Wave Parameters?

Acoustic variables are those things which change due to a mechanical interaction of the sound wave with a medium and include pressure, density, temperature, and particle motion.

Wave parameters are the group of characteristics that identify a wave. They are shown in the figure at the below. From these components, one can usually derive other wave properties (i.e. period, power and intensity) based on known equations.

Frequency (f)– the number of cycles per second, measured in Hertz (Hz). Clinical ultrasound uses a frequency of 2-10 Megahertz. The range of human hearing spans 20Hz to 20kiloHz.

Period (P) - The reciprocal of frequency (1/f) which is the time for one cycle. Clinical ultrasound uses periods of 0.1 to 0.5 microseconds. Both frequency and period are determined by the source of ultrasound only.

Wavelength (ƛ)– the length/distance of once cycle. Clinical ultrasound uses 0.15 to 0.8mm wavelengths. Shorter wavelengths produce higher resolution pictures. It is determined by the source of ultrasound (with a given f) and the medium (with a given velocity of propagation)

Amplitude (A)– the height of the wave. Equal to the (maximum-minimum)/2. For sound this is measured in decibels (dB) or pascals (Pa). Normal values are 1-3 megapascals for clinical ultrasound. The term transmit voltage is also sometimes used to describe amplitude and is measured in volts. Amplitde shows up as a change in the brightness on the doppler tracings.

Propagation velocity (C)– how fast the wave is moving in the specific medium. Propagation velocity is dependent on the medium and is related to the stiffness of the medium and something called the bulk modulus (directly related to how stiff the medium is ) by the equation:

C = square root (bulk modulus/density)


In general stiffer material has a faster propagation velocity (because of higher bulk modulus)
Denser materials also have a faster propagation velocity (despite density showing up in the denominator of the equation above). This is because the more dense the material, the greater also is the bulk modulus, and hence propagation velocity, or C is higher. The key velocity to know is that in the medium of the body, ultrasound has a velocity of 1540m/sec.


Air (and other low density substances in the body) has a very low propagation velocity and low bulk modul, making it a poor medium for ultrasound. Other materials are shown below:

 

Medium Propagation Velocity (meters/second)

Air

347

Average Soft Tissue

1540

Lung

500

Blood

1560

Bone

4080

Water

1485

Fat

1440

 

Important relationships to know are the following
C = f*ƛ
The most clinically relevant average propagation velocity in the medium of the body is where C =1540 m/sec and therefore this equation simplifies to
ƛ = 1.54 mm/microsec

Thus given the wavelength of the wave in mm you should be able to calculate the frequency in MHz and vice versa. Also using the equation:
f = 1/P
Given the frequency you should be able to calculate the period.