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

202 9. Sound-Measuring InstrumentationFigure 9.21. Locations of the microphones on the surface of an imaginary hemispheresurrounding a source whose sound power level is to be measured according to ISO 3744.The directivity index DI of the source may be computed from measurements ina semianechoic chamber from the following equation:DI = L pi − ¯L p + 3 dB (9.22)where L pi is the sound pressure level measured at point i, located on the measuringsurface and defining the direction along which the DI is desired at a distance fromthe source.9.19 Measurement of Sound Power Level in a Free Field(Full Anechoic Chambers)The procedure for measuring sound power in a free field is basically the sameas that for a free field with a reflecting surface (semianechoic condition), withsome modifications. In this case, the source is centered in a hypothetical sphereof radius rand surface area S = 4πr 2 . The sound pressure levels are measured atspecific points on the spherical surfaces; these points are stipulated by ISO 3745and shown in Figure 9.23, and defined in terms of Cartesian coordinates in Table9.2. Equations (9.20) and (9.21) also apply to yield the average sound pressurelevel ¯L p and the sound powerL w . The surface area ratio S/S 0 in Equation (9.21)now refers to two spheres, the hypothetical one (of radius r) used for placementof measuring sensors and the other a reference sphere with an area of unity, with