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Extending the frequency range does not increase the uncertainty of single number values in building acoustics

05.10.2011

An evaluation of many interlaboratory tests yielded the result that there was no noticeable change in the uncertainty of single number descriptors in building acoustics through the inclusion of the frequency bands from 50 to 80 Hz and 4 and 5 kHz, even though these bands are considered to have the largest uncertainties.

The insulation of airborne sound depends strongly on frequency. To communicate and declare product properties and to predict sound insulations, quantities are needed which describe the airborne sound insulation as one number. The necessary rating of sound insulation spectra is described in ISO 717 which is now being revised under the auspices of PTB. Originally, only frequencies between 100 Hz and 3.15 kHz were considered. Some current investigations show that an increase in the frequency range increases the correlation between the single number descriptor and the subjective response significantly. Before the frequency range could be extended it had to be clarified how the uncertainties of the single number values will be affected.

A comprehensive data base with interlaboratory test results is available at PTB which could be used to investigate the uncertainty of single number values. It turns out that the uncertainties remain largely unchanged by the extension of the frequency range (Figure 1). There are numerous examples showing a decrease in uncertainty. This can be explained by the fact that the single number value is a weighted mean value from all frequency bands. If this mean value is now calculated from 21 instead of 16 band values, the uncertainty of the mean value decreases. In some cases, the uncertainty increases. These are building elements which have insulation deficiencies at single frequencies in the added frequency range. The single number value is then dominated by very few band insulations with large uncertainties. The information for the user in these foreseeable cases is that the safety margin must be large enough. Uncertainties are thus not in opposition to the extension of the frequency range [1].

Figure 1: Uncertainty of the single number value Rliving determined by interlaboratory tests for frequency ranges 100 Hz - 3.15 kHz and 50 Hz - 5 kHz.

References:

[1] W. Scholl, J. Lang, V. Wittstock: Rating of Sound insulation at Present and in Future. The Revision of ISO 717. Acta Acustica united with Acustica. Vol. 97 (2011) 686 - 698

Contact person:

Volker Wittstock, Dept. 1.7, WG 1.72, E-Mail: volker.wittstock@ptb.de