Acoustic contrast factor

The acoustic contrast factor is a number used to describe the relationship between the densities and the sound velocities (or, equivalently because of the form of the expression, the densities and compressibilities) of two media. It is most often used in the context of biomedical ultrasonic imaging techniques using acoustic contrast agents and in the field of ultrasonic manipulation of particles much smaller than the wavelength using ultrasonic standing waves. In the latter context, the acoustic contrast factor is the number which, depending on its sign, tells whether a given type of particle in a given medium will be attracted to the pressure nodes or anti-nodes.

Example - particle in a medium

Given the compressibilities $\beta _{m}$ and $\beta _{p}$ and densities $\rho _{m}$ and $\rho _{p}$ of the medium and particle, respectively, the acoustic contrast factor $\phi$ can be expressed as

$\phi ={\frac {5\rho _{p}-2\rho _{m}}{2\rho _{p}+\rho _{m}}}-{\frac {\beta _{p}}{\beta _{m}}}$

For a positive value of $\phi$, the particles will be attracted to the pressure nodes, and vice versa.

See also

• Acoustic impedance
• Acoustic tweezers