Active control of extended acoustic sources in a half-space

Many noise sources, such as an electronic transformer, are located on or near an acoustically rigid planar surface, such as a concrete floor. Thus far, however, most of the active noise control research has been based on the free-field assumption. The work presented here investigates the effect the presence of a rigid plane has on the active noise control of an acoustic source with characteristic dimensions comparable to the acoustic wavelength at the frequency of interest. It is shown that when the source is located a distance of one wavelength or more from the plane, the presence of the plane may be neglected. However, the minimum radiated power computed using a free space active analysis can be significantly greater than that predicted using a half space active analysis for distances less than one wavelength. As a result, using the simpler free space analysis techniques leads to less effective control for distances less than one wavelength.