Sound level meters

Sound level meters are electronic instruments for measuring sound level expressed in decibels (dB), used for environmental acoustics, occupational hygiene, environmental quality, statutory inspections and scientific research. They pick up sound waves with a high-grade prepolarized microphone, amplify the signal, apply frequency weighting filters (A, C, Z), integrate the signal over time to calculate equivalent levels, and show the results on a highly legible digital display. The class of the meter (1 = laboratory, 2 = field) is defined by IEC 61672 and determines the instrument’s accuracy.

Accuracy classes

IEC 61672 defines two classes: class 1 (precision, for laboratory measurements and for inspections with legal standing – stricter tolerances on accuracy, linearity and frequency response); class 2 (general purpose, for field measurements and standard applications – wider tolerances but still suitable for most industrial and occupational applications). Class 1 meters generally cost more (3-5 times a class 2 unit) but are the reference for legal applications, expert reports and disputes. Class verification is carried out with standardized laboratory tests.

Frequency weightings

Sound level meters apply frequency weighting filters to reflect human perception of sound: A weighting (dB(A)) – the most widely used filter, approximating the response of the human ear at low to medium intensities (for environmental and occupational acoustics); C weighting (dB(C)) – flatter at low frequency, used for peak measurements and impulsive sounds; Z weighting (dB(Z)) – linear, unweighted, used for spectral analysis and vibration measurements. The time integration characteristics are: Fast (125 ms, for statutory environmental measurements); Slow (1000 ms, for occupational exposure measurements); Impulse (35 ms, for transient sounds).

Acoustic parameters

The parameters typically calculated by sound level meters are: SPL (Sound Pressure Level, the instantaneous level, dB); Leq (equivalent level, the average energy equivalent over the time interval, fundamental for cumulative exposure); Lmin, Lmax and Lpk (minimum, maximum, peak); the percentile statistics L10, L50, L90 and L95 (for statistical analysis of environmental noise); spectral analysis in octave or third-octave bands (to identify the tonal components of noise); and automatic calculation of Daily Noise Exposure (LEX,8h) for eight-hour occupational exposure.

Environmental acoustics

For environmental acoustics (in Italy, Law 447 of 26/10/1995 and subsequent decrees) daytime (06:00-22:00) and night-time (22:00-06:00) Leq measurements apply, with limits that depend on the acoustic zoning of the area. UNI 11143 governs architectural acoustics measurements. Characterizing noise sources (industrial plant, traffic, commercial activities) requires extended measurements (24 hours, 7 days) with datalogging meters and advanced statistical analysis. In Italy, the Ministerial Decree of 16/03/1998 prescribes the official measurement techniques.

Occupational acoustics

For occupational noise exposure, Italian Legislative Decree 81/2008 sets limit values: a daily exposure level LEX,8h ≥ 80 dB(A) → obligations to inform and train; LEX,8h ≥ 85 dB(A) → mandatory hearing protection and audiometric monitoring; LEX,8h ≥ 87 dB(A) → the absolute exposure limit. Measurements are taken with integrating meters worn by the worker (personal dosimeters) or placed at representative workstations, over complete work shifts. ISO 9612 governs the measurement procedure.

Reference standards

The main standards are: IEC 61672 (technical specifications for sound level meters); IEC 61252 (personal dosimeters); ISO 9612 (occupational noise exposure); ISO 1996 (acoustics – environmental noise measurement); UNI 11143 (architectural acoustics); in Italy, the Ministerial Decree of 16/03/1998 (official measurement procedures) and Legislative Decree 81/2008 (workplace safety). Sound level meters are calibrated annually with ISO 17025 certified acoustic calibrators at a fixed frequency (typically 94 dB @ 1 kHz).