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 le vel made over a survey surface (and vice versa) in arbitrary acoustica l environments. Measured levels, which are higher than the required fr ee-field levels because of reflections from the room surfaces,- are co rrected using an environmental-correction factor. The extent to which measured levels are higher than free held is also affected by source d irectivity. Here, the ''effective'' environmental-correction factor, i ncluding source directivity effects, is considered. According to the s tandard, the environmental-correction factor is calculated using the S abine theory, which assumes a diffuse sound field and low surface abso rption. However, the sound field in typical industrial workrooms may n ot be diffuse, for reasons related to the room shape and the distribut ion of surface absorption; thus the calculated correction factor may b e inaccurate. This was demonstrated, in the case of vertically directi onal sources, by a series of sound-power- and pressure-level measureme nts made on a calibrated sound-power source and on two packing machine s in workrooms of different dimensions, heights, and ceiling absorptio ns. The effective correction factor was found to be incorrect by as mu ch as 4 dB. In order to investigate the accuracy of the correction met hod and, if necessary, obtain more accurate correction factors for typ ical workroom configurations, ray-tracing predictions, and 1:8-scale-m odel measurements of the effective environmental-correction factor wer e made for the cases of omnidirectional and directional sources. Corre ction factors calculated according to the standard were found to be ac curate 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 1 0-m-high rooms with omnidirectional or vertically directional sources and without or with ceiling absorption, are presented. (C) 1995 Acoust ical Society of America.