Acoustics expert witness Angelo J. Campanella, P.E., Ph.D. FASAA describes how sound is measured:
A sound level meter (SLM) is the principal instrument for general noise measurement. The indication on a SLM (aside from weighting considerations) indicates the sound pressure, p, as a level referenced to 0.00002 Pa, calibrated on a decibel scale.
Sound Pressure Level = 20 x lg (p/0.00002) dB
Often, the “maximum” level and sometimes the “peak” level of the sound being measured is quoted. During any given time interval the peak level will be numerically greater than the maximum level and the maximum level will be numerically greater than the (rms) sound pressure level;
Mr. Campanella is principal of Campanella Associates, campenellaacoustics.com.