Study of the uncertainty of façade sound insulation measurements: Analysis of the ISO 12999-1 uncertainty proposal

Abstract

The result of a measurement is an approximation of the true value of the measurand and is complete when it is accompanied by its uncertainty. Building acoustics requires an assessment of the uncertainties that is understandable and corresponds closely to reality. The new standard ISO 12999-1 gives general uncertainty values for typical sound insulation measurements. The objective of this work is to determine the most accurate method for evaluating the uncertainty of in situ façade sound insulation measurements for both the Dls,2m,nT values in one-third octave bands and the global magnitude Dls,2m,nT,w. We establish a comparative analysis between the uncertainty values proposed by the standard ISO 12999-1, the uncertainty values deduced from interlaboratory activities and the average uncertainties deduced from individual calculations for each in situ measurement. The comparison is based on uncertainty calculations for around 1000 in situ façade sound insulation measurements made by our laboratory. The uncertainties given by the standard ISO 12999-1 for Dls,2m,nT in one-third octave bands are, at low and high frequencies, higher than the values deduced from individual calculations and smaller than the calculated values at mid frequencies. We believe that for Dls,2m,nT, it is advisable wherever possible to make individual uncertainty calculations for each in situ façade measurement. Nevertheless, the comparison with ISO 12999-1 points to the suitability of including some uncertainty components in the calculation, such as the related to the measurement procedure or source directivity, which are not normally considered in the uncertainty evaluation. Although we have clearly endorsed the use of individual calculations to determine the uncertainty of the sound insulation magnitude in one-third octave bands, we believe the uncertainty proposed by standard ISO 12999-1 could be a more realistic and reasonable approximation for the global magnitude Dls,2m,nT,w. Therefore if an individual calculation of the uncertainty of the global magnitude is required the Monte Carlo simulation has been shown to be a good method.