A logarithmic unit of sound intensity as computed from the sound pressure level. The reference level is a pressure of 20 micropascals. If sound waves exert a pressure of P pascals, the sound intensity is 100 + 20·log10(P/2) dB spl
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}