# Acoustic contrast factor

Given the compressibilities ${\displaystyle \beta _{m}}$ and ${\displaystyle \beta _{p}}$ and densities ${\displaystyle \rho _{m}}$ and ${\displaystyle \rho _{p}}$ of the medium and particle, respectively, the acoustic contrast factor ${\displaystyle \phi }$ can be expressed as
${\displaystyle \phi ={\frac {5\rho _{p}-2\rho _{m}}{2\rho _{p}+\rho _{m}}}-{\frac {\beta _{p}}{\beta _{m}}}}$
For a positive value of ${\displaystyle \phi }$, the particles will be attracted to the pressure nodes, and vice versa.