Acoustic lubrication or sonic lubrication occurs when sound (measurable in a vacuum by placing a microphone on one element of the sliding system) permits vibration to introduce separation between the sliding faces. This could happen between two plates or between a series of particles. The frequency of sound required to induce optimal vibration, and thus cause sonic lubrication, varies with the size of the particles (high frequencies will have the desired, or undesired, effect on sand and lower frequencies will have this effect on boulders).