Sound pressure level is a measure of the RMS average pressure deviation from still air over some period,
integrated by an exponential “time-weighting” function. SPL is stated in decibels, referenced to 20 micro
Pascals (μPa) – the approximate threshold of audibility for humans – such that 94 dB SPL ≈ 1 Pascal.
SPL Time Weighting
SPL is exponentially time-averaged using Fast or Slow “time weighting.” Fast and Slow time weighting
are implemented as first order low pass filters with time constants of 1/8 second (125ms) and one full second (1000ms) respectively. Older sound level meters may also include an “Impulsive” time-weighting option with a
very short time constant. More recent sound level meter (SLM) standards replace this option with “Peak
C,” i.e., C-weighted sound level without explicit time integration.
SPL metrics in Smaart labeled "Peak" are instantaneous, and SPL Measurements in Smaart that have no indication of weighting (C, A, C-A, Octave) are unweighted - also referred to as "Z" weighted.
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