Fast Time Weighting (Clause 5.6)

Fast weighting applies an exponential time constant of 125 ms to the squared sound pressure signal. This corresponds to the detector reaching within 1 dB of a steady-state signal in approximately 200 ms. Fast weighting is the default choice for most environmental and occupational noise measurements, providing a good balance between response speed and display stability.

Time-weighted levels using Fast are denoted with subscript "F" — for example, LAF (A-weighted, Fast) or LCF (C-weighted, Fast).

Slow Time Weighting (Clause 5.6)

Slow weighting uses a 1000 ms time constant, making the detector response eight times slower than Fast. It produces a more stable reading for relatively steady noise sources. Slow weighting is useful when the Fast reading fluctuates too rapidly to read, though modern digital instruments typically compute both simultaneously.

Slow-weighted levels use subscript "S" — LAS, LCS, LZS.

Impulse Time Weighting (Clause 5.6)

Impulse weighting is asymmetric: the attack time constant is 35 ms (detector reaches within 1 dB of a continuous tone in approximately 50 ms), while the decay time constant is 1500 ms (level drops at approximately 2.9 dB per 1500 ms, or about 4.3 dB/s). This asymmetry captures the peak of impulsive events while allowing the display to settle slowly enough to be read.

Note that Impulse weighting is distinct from peak measurement. Peak captures the true instantaneous peak of the waveform; Impulse weighting is a slower exponential detector with fast attack.

Practical Selection Guide

Fast — default for occupational noise per OSHA 1910.95, environmental surveys, and general-purpose monitoring
Slow — useful for steady industrial sources, HVAC system measurements, and when display stability is needed
Impulse — required by some European regulations for impulsive noise assessment, construction sites, and shooting ranges

Tolerance Requirements

Class 1 instruments must reproduce the time-weighted level of a 4 kHz tone burst within specified limits: for Fast weighting, the displayed level of a 200 ms burst must be within -1.1 to +0.4 dB of the steady-state level. Class 2 tolerances are wider.

SonaVyx Implementation

The SonaVyx SPL meter implements all three time weightings as single-pole IIR filters applied to the squared, frequency-weighted signal. You can switch between F, S, and I in real time. The noise dose calculator uses Slow weighting for TWA computation per OSHA convention. For continuous monitoring over longer periods, see the environmental noise monitor which logs Leq values with configurable intervals.