From WebRef.org

Ten times the logarithm of the ratio of the space and time average of squared sound pressure to the squared reference sound pressure, the space average being taken over the total volume of the room, except for the regions of the room where the direct field of the source and the near field of the boundaries are of significance. Unit, decibel (dB).

Sponsor: From A to Z we've got your perscription at 1-800-Get-Lens.