On the uncertainty of sound reduction index measurements from inter-laboratory tests

Test reports of building acoustics measurements cannot be considered complete without declaring the associated uncertainty. Two dominant methods exist for calculating the uncertainty, i.e. the empirical approach, standardized in ISO 5725, and the modelling approach, standardized in ISO/IEC Guide 98-3. The state of the art is so that this uncertainty must be assessed using the standard deviation of reproducibility obtained from multiple Inter-Laboratory Tests (ILTs), whose average values are standardized in ISO 12999-1. However, here three standard deviations are given: the usual repeatability and reproducibility standard deviations and an additional new standard deviation ?R95, neither defined nor used before. In the paper, it is shown that ?R95 causes an over-coverage in the frequentist sense, while the natural choice is the reproducibility standard deviation, both for laboratory and field measurements. Moreover, it is remarked that for products for which a specific inter-laboratory test (test code) already exists, a smaller uncertainty can be used, and this may produce an unbalanced situation on the market.