28.07.2010
A compilation of many round robin data has revealed that the correlation between one-third octave bands is of major importance for the uncertainty of the A-weighted sound power levels.
The A-weighted sound power level is the most important quantity to describe the sound emission of technical sound sources. A number of standardized measurement procedures exist for its determination (ISO 3741 – 3747, ISO 9614-1,-2,-3). In the past, uncertainties were assigned to the individual procedures which were mainly based on experience. Some of these values have now been confirmed by a compilation of many round robin data. It also became clear that for most of the measurement procedures, no round robin data are currently available. Thus, the known associated uncertainties can be interpreted as estimates only.
It was furthermore investigated in what way the uncertainty of the A-weighted sound power level is made up of the uncertainties of the one-third octave bands. Thereby, strong correlations of non-predictable magnitude have emerged. The empirical correlation coefficient, which is a measure for the impact of the correlation between the frequency bands on the A-weighted sound power level, reaches values between -0.3 and 1.0. Since, in practical cases, no knowledge of the correlation is available, a full positive correlation should be assumed. The real uncertainties will then always be smaller than this estimate.
Figure 1: Empirical correlation coefficient for the calculation of the uncertainty of A-weighted sound power levels from frequency bands obtained from round robin data.
Contact person:
Volker Wittstock, Dept. 1.7, WG 1.72, E-mail: volker.wittstock@ptb.de