Method of inverse-square law qualification of hemi-anechoic chambers for parallelepiped measurement surfaces in ISO 3744

摘要

When determining sound power levels according to ISO 3744 (grade 2, engineering method), the environmental correction K_2 is applied. One available measurement surface defined by this standard is a parallelepiped surface of which size and K_2 depend on the size of noise source. Although this parallelepiped surface is essential in the ITT equipment industry, the methods for determining K_2 are limited and difficult to apply to it. Alternatively, the standard also allows "K_2 = 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, "K_2 = 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.