This invention relates to a system for masking conversation in an open plan office. A conventional generator of electrical random noise currents feeds its output through adjustable electric filter means to speaker clusters in a plenum above the office space. Each cluster has two speakers in a trigonal prism-shaped cabinet, symmetrically disposed about an axis of symmetry, oriented to be vertical. The speaker arrangement is such that, throughout the open plan office area, including below speaker clusters, the background sound energy level horizontally in the open plan office space has a generally constant value except for a modification which provides for a quiet region below a cluster. By proper spacing of clusters horizontally along the plenum region, transmission of background masking sound through an acoustic ceiling throughout an open plan office space, conversation in one region of the office cannot be distinguished at a distance of the order of about 15 feet or more.