ISO 1996

ISO 1996 is the international standard for describing, measuring and assessing environmental noise: the noise from roads, railways, aircraft, industry and construction as heard at dwellings and other sensitive places. It is not an instrument standard, but it determines which instrument is needed and how it must be used, and several analysers implement its procedures directly. It is in two parts, and it is the reference behind most national environmental noise procedures.

The parts

• ISO 1996-1:2016, Basic quantities and assessment procedures, defines the descriptors (L_Aeq, L_den, L_dn, L_AFmax, sound exposure level), the concept of a rating level with adjustments for tonal, impulsive and low-frequency character and for time of day, the classification of impulsive sources, and the principles of assessing long-term annoyance.
• ISO 1996-2:2017, Determination of sound pressure levels, sets out how to measure or calculate the descriptors: instrumentation, measurement positions, meteorological conditions, measurement duration, extrapolation to long-term values, residual-sound correction and the estimation of measurement uncertainty. Its annexes contain the one-third-octave and narrow-band methods for identifying tonal sound.
• ISO/PAS 1996-3:2022 provides an objective method for assessing the prominence of impulsive sound using the sound level time history, filling a gap that Part 1 left to judgement.

Descriptors

The central quantity is the A-weighted equivalent continuous level over a stated reference time interval, with L_den and L_dn as 24-hour composites that penalise evening and night periods, and L_AFmax and sound exposure level for single events. The rating level L_r is the measured level plus adjustments, and the standard describes adjustments for tonality, impulsiveness and, in some national practice, low-frequency content, without fixing their size; that is left to national regulation, where 3 to 6 dB is typical. Definitions of each term are in the glossary.

Instrumentation requirements

ISO 1996-2 requires a sound level meter meeting IEC 61672-1 class 1 with a calibrator to IEC 60942 class 1, checked before and after each measurement series; a drift between the two checks of more than 0.5 dB invalidates the measurements. Where spectra are needed for tonality or for calculations, filters to IEC 61260-1 class 1 are expected. Microphones are to be fitted with windscreens, and for long-term unattended monitoring the complete outdoor system is to be verified, which is where the remote check methods discussed in specifying a monitoring station come from.

Positions, conditions and duration

Tonality and impulsiveness in instruments

Two of the standard’s procedures appear as instrument functions. The simplified one-third-octave method of ISO 1996-2 compares the level in a band with the average of its two neighbours and declares a prominent tone when the difference exceeds a frequency-dependent threshold; it needs stored one-third-octave spectra for the period of interest. The engineering method uses narrow-band (FFT) analysis and critical-band masking to compute the tonal audibility, and an instrument offering it must store FFT spectra at a stated resolution. The objective impulsiveness method of ISO/PAS 1996-3 works on the time history of A-weighted levels at short intervals. Instruments that cite ISO 1996 in their standards lists usually mean one of these functions; the table below quotes the statement as the manufacturer makes it.

Relationship to national procedures

Most countries publish their own environmental noise measurement procedures, which adopt the descriptors of ISO 1996-1 and vary the details of ISO 1996-2: measurement heights, reference periods, adjustment values and the handling of weather. The EU Environmental Noise Directive uses L_den and L_night as defined in ISO 1996-1 for strategic mapping. When a measurement will be assessed under a national procedure, that procedure governs; ISO 1996 is the reference it leans on and the fallback where it is silent. The guide How to choose a sound level meter lists the parameters environmental work requires.

Instruments citing ISO 1996

The table lists every instrument in the database whose manufacturer-published standards list mentions ISO 1996, with the function as the manufacturer describes it. An instrument that does not appear may still be entirely suitable for ISO 1996 measurements; the citation indicates that a specific procedure from the standard is implemented in firmware or software.