The intensity in dB is
[tex]
I = 20 log_{20} \left[ \frac{P}{P_{0} \right][/tex]
Where [tex]P[/tex] is the pressure of the wave in pascals and [tex]P_{0} = 2x 10^{-5} \frac{N}{m^2}[/tex].
Actually I've got a conflicting source that says "Intensity is a measure of the sound energy that passes through a given area each second."