Environmental-correction factors for typical industrial workrooms

ISO Standard 3746 presents methods by which the sound-power level of a noise source can be determined from measurements of sound-pressure level made over a survey surface (and vice versa) in arbitrary acoustical environments. Measured levels, which are higher than the required free-field levels because of reflections from the room surfaces, are corrected using an environmental-correction factor. The extent to which measured levels are higher than free field is also affected by source directivity. Here, the ‘‘effective’’ environmental-correction factor, including source directivity effects, is considered. According to the standard, the environmental-correction factor is calculated using the Sabine theory, which assumes a diffuse sound field and low surface absorption. However, the sound field in typical industrial workrooms may not be diffuse, for reasons related to the room shape and the distribution of surface absorption; thus the calculated correction factor may be inaccurate. This was demonstrated, in the case of vertically directional sources, by a series of sound-power- and pressure-level measurements made on a calibrated sound-power source and on two packing machines in workrooms of different dimensions, heights, and ceiling absorptions. The effective correction factor was found to be incorrect by as much as 4 dB. In order to investigate the accuracy of the correction method and, if necessary, obtain more accurate correction factors for typical workroom configurations, ray-tracing predictions, and 1:8-scale-model measurements of the effective environmental-correction factor were made for the cases of omnidirectional and directional sources. Correction factors calculated according to the standard were found to be accurate within 1 or 2 dB in the case of omnidirectional sources, but to be significantly underestimated in the case of vertically directional sources. Tables giving the average effective environmental-correction factors for machines (survey surfaces) of different sizes, in 5- to 10-m-high rooms with omnidirectional or vertically directional sources and without or with ceiling absorption, are presented.