Sound pressure level and sound intensity level

[hanson]

Hi all.
A very simple question: what is the difference between sound pressure level and sound intensity level?
It seems to me that sound presssure level also measure the "energy" since it is p^2, which shall be related to energy...
If it is so, it seems that sound pressure level shall be the same as sound intensity level, isn't it?
Please help.

 

[FredGarvin]

SPL is the actual pressure fluctuation that is measured. That is the pressure fluctuation due to the traveling wave. Sound intensity is a is the rate of flow of energy flow through a unit area as defined by

$$\overrightarrow{I}=\frac{1}{T}\int_{0}^{T} p \overrightarrow{u} dt = \frac{1}{2}Re\left[p \overrightarrow{u}\right]$$