Method of Inverse-Square Law Qualification of Hemi-Anechoic Chambers for Parallelepiped Measurement Surfaces in ISO 3744

When determining sound power levels according to ISO 3744 (grade 2, engineering method), the environmental correction K2 is applied. One available measurement surface defined by this standard is a parallelepiped surface of which size and K2 depend on the size of noise source. Although this parallelepiped surface is essential in the ITT equipment industry, the methods for determining K2 are limited and difficult to apply to it. Alternatively, the standard also allows "K2 = 0 dB" if the measurement surface is included within the testing volume qualified per ISO 3745 (grade 1, precision method) by evaluating differences from the inverse-square law. Once a chamber is qualified by this method, "K2 = 0 dB" is allowed to adopt at all times regardless of the sizes of noise sources. Consequently, testing laboratories qualify their chambers per ISO 3745 by necessity. However, in ISO 3745:2012 FDAM 1 under discussion, a hemisphere shape is only considered for the measurement surface; meaning its qualification could not necessarily qualify parallelepiped surfaces adequately. To address this situation and intended for future ISO 3744 amendments, this paper considers an inverse-square law qualification method which references ISO 26101 and features another set of traverses dedicated to parallelepiped surfaces.