Effects of reflective ground on the actively created quiet zones

Previous studies on the local active control of noise in free space show that the size of the quiet zone and the total sound power output of the control system are mainly determined by the spatial arrangement of primary sources, secondary sources, and error sensors. When a nearby reflective surface is introduced, the size of the quiet zone and the total power output of the system will be affected. The research presented in this paper examines the influence of the presence of a nearby reflective plane on the control system with different alignments to the reflective surface. It is found that the reflective surface significantly affects the size of the quiet zone and the sound power output of the system, as compared with the same control system in free space. The effect is related to two important parameters of the control system: the heights of the sources and the error sensor above the surface and the distances between the primary and secondary sources and between the secondary source and the error sensor. While the quiet zones created by most arrangements of the control system in half-space are smaller than those for free space, the quiet zones created by the arrangements of the control system perpendicular to the reflective surface may be larger than those for free space. (C) 1998 Acoustical Society of America. [S0001-4966(98)05001-2].