Actively created quiet zones by multiple control sources in free space

Local attenuation of sound pressure, which creates a zone of quiet, seems to be the only option if primary and secondary sources are well separated. Two major indexes of local control, the size of the quiet zone and the total sound power output, are mainly determined by the arrangement of primary sources, secondary sources, and error sensors. It has been found that the size of the quiet zone created by the control system with multiple secondary sources and error sensors is not simply the summation of quiet zones created by each individual control source. The interactions of the control sources, which are determined by the configuration of the system, play a very important role in the system performance. In this paper, a typical control arrangement with multiple secondary sources and multiple error sensors equally spaced in two parallel lines is examined. Results from computer simulation and experiments in an anechoic chamber show that there exists a range of optimal spacing for the secondary sources and error sensors. The control system will create the largest quiet zone and suffer the least increase of total sound power output in the optimal configuration.