1.1 General This part of ISO15186 specifies a sound intensity method to determine the sound reduction index and the element-normalized level difference of building elements at low frequencies. This method has significantly better reproducibility in a typical test facility than those of ISO140-3, ISO140-10 and ISO15186-1. The results are more independent of the room dimensions of the laboratory and closer to values that would be measured between rooms of volume greater than 300m3. This part of ISO15186 is applicable in the frequency range 50Hz to 160Hz but is mainly intended for the frequency range 50Hz to 80Hz. NOTE For elements faced with thick, porous absorbers, the recommended frequency range is 50Hz to 80Hz. The main differences between the methods of ISO15186-1 and ISO15186-3 are that in ISO15186-3 the sound pressure level of the source room is measured close to the surface of the test specimen, and the surface opposite the test specimen in the receiving room is highly absorbing and converts the room acoustically into a duct with several propagating cross-modes above the lowest cut-on frequency. The results found by the method of ISO15186-3 can be combined with those of ISO140-3 and ISO15186-1 to produce data in the frequency range 50Hz to 5000Hz.1.2 Precision The reproducibility of this intensity method is, for all frequencies, estimated to be equal to or better than that found with the method of ISO140-3 at 100Hz. Some comparisons of data obtained with the methods of this part of ISO15186 and ISO140-3 are given in annex B.