The two forms of energy in a wave are kinetic and potential. Denoting them at a particular time and position by and , respectively, we can write them in terms of velocity and wave impedance as follows:
More specifically, and may be called the acoustic kinetic energy density and the acoustic potential energy density, respectively.
At each point in a plane wave, we have (pressure equals wave-impedance times velocity), and so
where denotes the acoustic intensity (pressure times velocity) at time and position . Thus, half of the acoustic intensity in a plane wave is kinetic, and the other half is potential:^{B.30}
Note that acoustic intensity has units of energy per unit area per unit time while the acoustic energy density has units of energy per unit volume.