THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

9.20 Sound Power Measurement in a Diffuse Field 205Table 9.2. Microphones Array Positions a in a Free Field According toISO 3745.No. x/r y/r z/r1 −0.99 0 0.152 0.50 −0.86 0.153 0.50 0.86 0.154 −0.45 0.77 0.455 −0.45 −0.77 0.456 0.89 0 0.457 0.33 0.57 0.758 −0.66 0 0.759 0.33 −0.57 0.7510 0 0 1.011 0.99 0 −0.1512 −0.50 0.86 −0.1513 −0.50 −0.86 −0.1514 0.45 −0.77 −0.4515 0.45 0.77 −0.4516 −0.89 0 −0.4517 −0.33 −0.57 −0.7518 0.66 0 −0.7519 −0.33 0.57 −0.7520 0 0 −1.0a This table lists the Cartesian coordinates (x, y, z) with the origin located at the centerof the source. The vertical axis is perpendicular to the horizontal plane z = 0.result of sound waves reflecting back and forth between room surfaces until theydie out. A steady noise source generates sound and builds up the sound pressurelevel from ambient level in the room, until an equilibrium sound pressure levelis reached. This occurs when the new acoustic energy emanating from the sourceoffsets the dissipation of reflected sound energy in the slightly elastic reflectingsurfaces (however hard they may be) and in the slightly viscous air of the chamberand through energy leakage from the room. The applicable international standardsfor measuring the sound power levels in reverberation chambers are ISO 3741and ISO 3742, the former giving details for broadband noise and the latter fordiscrete-frequency and narrow-band sources. The relationship between the soundpower of a source and the average reverberant sound level can be determined andallows calculation of the sound power.The procedure for measuring the sound power level in an echoic chamber maybe summarized as follows:1. The reverberation time T 60 is measured by using a standard technique prescribedby ISO 354. 33 Reverberation time T 60 for an enclosure is the time interval required for a sound pressure level todrop 60 dB after the sound source has been stopped. More detail treatment of T 60 is rendered inChapter 11.