text: The force on a small particle in a sound wave is given by:
  where V is the volume of the particle, and P is the acoustic pressure gradient on the bubble.
  Assuming a sinusoidal standing wave, the time-averaged pressure gradient over a single acoustic cycle is zero, meaning a solid particle (with fixed volume) experiences no net force. However, because a bubble is compressible, the oscillating pressure field also causes its volume to change; for spherical bubbles this can be described by the Rayleigh–Plesset equation. This means the time-averaged product of the bubble volume and the pressure gradient can be non-zero over an acoustic cycle. Unlike acoustic radiation forces on incompressible particles, net forces can be generated in the absence of attenuation or reflection of the sound wave.
  The sign of the force will depend on the relative phase between the pressure field  and the volume  oscillations. According to the theory of forced harmonic oscillator the relative phase will depend on the difference between the bubble resonant frequency and the acoustic driving frequency.