When applied to a sound wave through a medium of a fluid like air, particle velocity would be the physical speed of a parcel of fluid as it moves back and forth in the direction the sound wave is travelling as it passes.
Particle velocity should not be confused with the speed of the wave as it passes through the medium, i.e. in the case of a sound wave, particle velocity is not the same as the speed of sound.
The wave moves relatively fast, while the particles oscillate around their original position with a relatively small particle velocity.
In applications involving sound, the particle velocity is usually measured using a logarithmic decibel scale called particle velocity level.
The particle displacement of a progressive sine wave is given by where It follows that the particle velocity and the sound pressure along the direction of propagation of the sound wave x are given by where Taking the Laplace transforms of
Sound velocity level, denoted Lv and measured in dB, is defined by[1] where The commonly used reference particle velocity in air is[2] The proper notations for sound velocity level using this reference are Lv/(5 × 10−8 m/s) or Lv (re 5 × 10−8 m/s), but the notations dB SVL, dB(SVL), dBSVL, or dBSVL are very common, even though they are not accepted by the SI.