Apertures, like vents of air-conditioning systems and slits of partially opened windows or doors are very common in the buildings, but the transmission loss performances are strongly influenced by the presence of these apertures. This paper proposes an analytical method to calculate the sound radiation from inside an enclosure through an aperture of finite thickness to the free field based on the modal superposition method and Kirchhoff-Helmholtz equation. The active noise control technique is then applied on the aperture to reduce noise transmitted through it to the outside. Different configurations of secondary sources and error microphones are investigated. It is found that sound radiation can be effectively reduced with an active noise system at the edge of the aperture, and the upper limit frequencies of effective control are discussed.