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

9.18 Measurement of Sound in a Free Field over a Reflecting Plane 201chamber, the directivity is readily apparent, owing to the absence of reflections.But in a highly reflective environment, such as that of an echo chamber, the multiplereflections that occur render the directivity less important and the sound fieldbecomes more uniform.9.18 Measurement of Sound in a Free Field over aReflecting Plane (Semianechoic Chamber)We can summarize the measurement procedure in a free field over a reflectingplane as follows:1. The source is surrounded with an imaginary surface of area S, either a hemisphereof radius r or a rectangular parallelepiped.2. The area of the hypothetical surface is calculated. S = 2πr 2 in the case ofa hemispherical surface, and S = ab + 2(ac + bc) in the case of the parallelepipedhaving length a, width b, and height c.3. The sound pressure is measured at designated points of the imaginary surface.4. The average sound pressure level ¯L p is computed from the measured results ofthe previous step. This is found from[¯L p = 10 log1NN∑10 L i /10i=1](9.20)where N is the total number of measurements and L i denotes the measuredvalue of the SPL at the designated point i.5. The sound power level is then calculated from the following:L w = ¯L p + 10 log(S/S 0 ) (9.21)where S 0 is the reference area of 1 m 2 .The above procedure applies only if the source is not too large, i.e., the radiusr of the hypothetical hemisphere should be at least 1 m and at least twice thelargest dimension of the source (or the perpendicular distance between the sourceinside the imaginary parallelepiped and a measurement surface is 1 m), and thebackground noise level is more than 6 dB below that of the source. The rectangularparallelepiped setup is preferred for large rectangular sources.Figure 9.21 shows the designated points on the hemispherical surface wherethe microphones are located. The corresponding points for the rectangular parallelepipedare given in Figure 9.22. These designated points are associated withequal areas on the surface of the hemisphere or the rectangular parallelepiped. TheSPL is usually measured at the designated points with A-weighting or in octaveand partial octave bands, with the meter set in the slow-response mode.The applicable international standards for acceptable sound power measurementtechniques under semianechoic conditions are given in ISO 3744 and ISO 3745.Adjustments in the values of the measured SPL should be made for the presenceof background noise. Equations (9.20) and (9.21) are used to convert the measurementsinto the desired values of averaged SPL, ¯L p , and sound power level L w .