This article is from the Acoustics FAQ, by Andrew Silverman with numerous contributions by others.
A sound level meter is the principal instrument for general noise measurement. The indication on a sound level meter (aside from weighting considerations) indicates the sound pressure, p, as a level referenced to 0.00002 Pa.
Sound Pressure Level = 20 x lg (p/0.00002) dB
Peak levels are occasionally quoted. During any given time interval peak levels will be numerically greater, and often much greater than the (rms) sound pressure level.
 
Continue to: