Measurement of element normalized level difference of small building elements with intensity technique

A laboratory method to evaluate the sound insulation of small building elements was developed. In the method, sound intensity measurements are applied and the results are expressed in terms of the element normalized level difference. There are special requirements concerning the installation and operation of small building elements in measuring their sound insulation. The consequences of the requirements to the measurement method are taken into account. Also the special demands caused by the small size of the object are taken into account. A supplement, considering the general usefulness of the element normalized level difference (or unit sound insulation) in evaluating the sound insulation of partitions, is included in the method. The supplement can be applied also to ISO 140-10 and NT ACOU 037. The classical form of the Waterhouse correction, the purpose of which is to take account of the higher energy density near room boundaries, has been developed to an improved formula, which is a function of room modal density. The Waterhouse correction can be determined for each room by measuring or calculating its modal density. The improved form of the Waterhouse correction normally differs from the traditional one at third octave bands with centre frequencies less than 100 Hz. There is a tendency for some measurement methods in building acoustics to be used in an extended frequency range down to a third octave band with a centre frequency of 50 Hz. With that kind of extended frequency range, the refinement of the Waterhouse correction has an obvious effect. The Waterhouse correction of the receiving room should be subtracted from the result of traditional measurements of the sound reduction index. This is especially important if the results are compared with those of intensity measurements. No Waterhouse correction is needed for the source room. The measurement of sound insulation by the intensity technique needs no Waterhouse corrections. Experimental measurements were carried out according to this method and according to ISO 140-10. Both of the methods give quite similar results with an accuracy of 1 dB or better at a frequency range where the flanking transmission is not very important and where both of the methods give valid results. The effects of flanking transmission on the sound insulation measurement results can be diminished by using intensity technique. Very remarkable flanking transmission may, however, make the results of the intensity technique invalid.