How is sound measured ?

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.