Acoustic contrast factor

In acoustics, the acoustic contrast factor is a number that describes the relationship between the densities and the sound velocities of two media, or equivalently (because of the form of the expression), the relationship between 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 (acoustophoresis) 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.