Sound waves are characterized by compression and expansion of the medium as sound energy moves through it. This represents the pressure component of sound. At the same time, there is also back and forth motion of the particles making up the medium (particle motion).

  
     Sound is a longitudinal wave. Animation credit: Dan Russell, Grad. Prog. Acoustics, Penn State, http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html

     Particle motion is described by displacement, velocity, and acceleration.

     Sound intensity depends on both particle velocity ( ) of a sound wave is defined as the amount of energy transmitted through a unit area per unit time in the direction that the sound wave is traveling. It is given by the product of the pressure and velocity of the sound wave:

                     
     The particle velocity and pressure in a sound wave are related. For a plane wave the relationship is: