The power transformer noise is dominated by the fundamental frequency and its harmonics, and most of the noise components to be controlled are in the low-frequency range. Sound barriers are built around outdoor transformers to block the transformer noise. There has been a lot of research for developing a special window that has both good noise reduction performance and sufficient ventilation for providing fresh air. A mixed-error approach has been proposed to reduce the computational complexity of multichannel active noise control systems for mitigating noise passing through open windows. Research on the active control of sound via open windows can be divided into two categories. The first one is the application of the virtual sound barriers, which control sound transmission through window openings directly by installing secondary sources on the openings to reduce sound propagation through the openings. The second one is the Active noise control (ANC) plenum windows, which apply ANC on the plenum structures transformed from opening windows.