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. Although sound is a longitudinal wave, for simplicity it is depicted below as a transverse wave. 

Wave Properties.jpg

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

 

Period (P) - The reciprocal of frequency (1/f) which is the time it takes for one wave 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 wave 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. Amplitude 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 and density of the medium. 
In general stiffer material has a faster propagation velocity. Denser materials also have a faster propagation velocity. The key velocity to know is that in the tissues of the body, ultrasound has an average  velocity of 1540m/sec.


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.