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This part of ISO 15186 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 ISO 140-3, ISO 140-10 and ISO 15186-1. 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 300 m3. This part of ISO 15186 is applicable in the frequency range 50 Hz to 160 Hz but is mainly intended for the frequency range 50 Hz to 80 Hz. NOTE For elements faced with thick, porous absorbers, the recommended frequency range is 50 Hz to 80 Hz. The main differences between the methods of ISO 15186-1 and ISO 15186-3 are that in ISO 15186-3 a) the sound pressure level of the source room is measured close to the surface of the test specimen, and b) 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 ISO 15186-3 can be combined with those of ISO 140-3 and ISO 15186-1 to produce data in the frequency range 50 Hz 5 000 Hz.