Abstract From:
W. Steve Shepard Jr. and Kenneth A. Cunefare, "Active control of extended acoustic sources in a half-space," J. Acoust. Soc. Am.,
96(4), pp. 2262-2271 (1994).
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 affect 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.
Shown below is a discretized box model with 100 elements and 286 nodes. The entire upper surface is vibrating as the noise source. All other sides of the box are rigid except the six active source elements designated S1 to S6.
An active noise control analysis was performed to minimize the total acoustic power with the box in proximity to an acoustically rigid plane. The orientation of the plane was changed to determine when the presence of the plane caused the solution to significantly vary from the free-field solution.