This chapter discusses the control of sound propagation in free fields. It considers two types of primary sound fields, one is that from plane waves, and the other is the sound field generated by point monopole sources. The chapter also discusses the control of sound propagation through a finite size aperture where there is open space on both sides of the aperture. Planar virtual sound barriers are used to control sound propagation from one side of a plane. The virtual sound barriers are usually far away from the primary sound sources, so the secondary sources can only interact with the primary sound field and have little effect on the primary sources. The primary sound field can be plane waves, or waves generated by point monopole sources, or a combination of the two. The boundary control system can achieve more consistent sound reduction performance than the surface control system over a wider frequency band.