Previous work has demonstrated that sound radiation through a cavity opening can be reduced with secondary sources at the edge of the opening, but the error microphones are implemented over the entire opening, which might affect the natural ventilation, lighting, and especially the access through the opening in some applications. A boundary error sensing arrangement is proposed and investigated in this paper. It is found that a double-layer error microphone arrangement achieves better performance than a single-layer one. Although its performance is not as good as the arrangement with error microphones distributed over the entire opening, it is preferable in some applications because it does not block the opening. It is also found that there exists an upper-limit frequency for the systems with error microphones installed at the edge, which is related to the size of the opening and can be increased by adding more layers of error microphones at the edge. This work demonstrates the possibility of developing an almost invisible virtual sound barrier system that can block sound transmission through an opening without affecting its functionalities.