Effect of directivity on the sound field produced by a source above a reflecting plane

In describing equipment sound emissions, it is desired to measure its noise radiation spectra with the equipment located in an anechoic or reflection-free environment. In practice, however, there are a number of situations where sound measurements are made with the equipment in hemi-anechoic space, in which case the equipment is located on a reflecting plane, but in an otherwise reflection-free environment. One problem associated with measurements made in hemi-anechoic environments is that the sound spectra measured under these conditions are not necessarily the same as the spectra that would be produced by the source in the absence of the reflecting plane. Investigators have sought to reconstruct the spectra that would be measured in anechoic space from measurements of spectra made with the source in a hemi-anechoic environment by assuming the equipment to be a point source located above a reflecting plane. In this paper, we examine the effect of directivity on measurements made in a hemi-anechoic environment by computing and measuring the sound field produced by a collinear array of five point sources. Our results show that source directivity has a considerable effect on the radiated sound pressure levels. Finally, the maximum sound pressure levels occur when the major lobe in the source directivity is directed toward the reflecting plane.