Sound intensity is defined as the sound power per unit area. Although the units for sound intensity are technically watts per meter squared, it is much more common for it to be referred to as decibels, dB.
It is one tenth of a bel on the logarithmic intensity scale. A decibel is a ratio of the observed amplitude, or intensity level to a reference, which is 0 dB.
Hence, the S.I. unit of sound intensity is decibels.