Uncertainties in Environmental Noise Assessments - ISO 1996 - Effects of Instrument Class and Residual Sound
ISO/DIS 1996-2.2 (2005) “Acoustics — Description, assessment and measurement of environmental noise — Determination of environmental noise levels” contains guidelines on assessing the uncertainties of the determined sound pressure levels. This depends on the sound source, measurement time interval, weather conditions, distance from the source, measurement method and instrumentation. Guidelines on estimating the measurement uncertainty are given. Four main sources of uncertainty (reproducibility, operating conditions, weather and ground conditions, and residual sound) are combined to determine the overall uncertainty. Reproducibility represents the influence of the operator, equipment at the same place under constant conditions. A value for IEC 61672-1 “Electroacoustics – Sound level meters – Part 1: Specifications” class 1 instrumentation is given. Operating conditions are determined from minimum 3, preferably 5, measurements under repeatable conditions and at a position where variations in meteorological conditions have little influence on results. Uncertainty due to weather and ground conditions depends upon the measurement distance and the prevailing meteorology. A method using a simplified meteo window is provided. Uncertainty due to residual sound varies depending on the difference between measured total values and the residual sound but no more specific guidelines on determining the uncertainty due to residual sound have been developed. This paper proposes a method compliant with ISO 1996 and shows initial results of investigations to determine the effects of instrumentation class. It will be shown that the choice of instrumentation greatly affects the uncertainty due to residual sound as this approaches the specific sound, and thus is an important influence on overall uncertainty.