Field measurements of airborne sound insulation; Differences in results for methods prescribed in current standards at different degrees of furnishing and a possible new approach

Abstract

There is a widespread belief among acousticians that the airborne sound insulation measured between empty rooms systematically results in lower values than when furnished. No studies investigating the phenomenon were found and since inspection measurements are mostly performed in empty rooms, the difference, if it exists, would need to be quantified and addressed. Since regulations in Sweden, in many cases, require measurements down to lower frequencies than in other countries, and the theoretical framework on which existing methods are based is generally less valid at lower frequencies, there is reason to believe that the problem could be more common in Sweden. In addition, a variety of measurement methods are described in current standards, of which only a few are regularly used. No studies have been found where the methods are systematically tested and compared. Methods based on measurements and further processing of impulse responses, using deterministic signals, are very useful in certain situations but are, to the author’s knowledge, almost never used. This is probably mainly because methods based on manual scanning paths with a hand-held analyzer are more time-efficient in field measurements. The main objective of this study is to identify whether furnishing leads to systematic deviations in measured sound insulation. This is achieved by testing and comparing several of the methods described in the current standards. A possible explanation for the error could be that the sound pressure levels are measured with the loudspeaker positioned in the source room, while the reverberation times are measured with the loudspeaker in the receiving room. There is reason to believe that the differences in the sound fields for these two cases reduce the compatibility between these measurements, especially at low frequencies. Since methods based on impulse responses measured with swept-sine signals provide superior signal-to-noise ratios that far exceed the other methods, the reverberation time can possibly be measured without moving the loudspeaker from the source to the receiving room. If that would be possible, both the sound level differences between the rooms and the reverberation time, i.e. all measurements required to evaluate the sound insulation, could be measured from a single sine sweep. If a foldable measurement stand was developed, which quickly could spread out a number of microphone positions in a space, field measurement times could be drastically reduced. However, the method where the reverberation time is measured with the loudspeaker in the source room was shown to perform significantly worse than default methods. Nevertheless, the method can potentially be used for simpler survey measurements, without further development. Systematic differences, between furnished unfurnished rooms, were demonstrated for the weighted standardized level differences. However, contrary to what was expected, the differences do not arise as a result of deviations for the lower 1/3-octave bands. Rather, the differences arise as a result of deviations for the 1/3-octave bands 200 Hz, 250 Hz and for some higher frequency bands. Hence, the results do not support the suspicion that measurements according to the Swedish regulations, which includes frequency bands below 100 Hz, result in systematic deviations greater than what would be the case in other countries.