Hybrid virtual sound barrier systems use different acoustic elements at the quiet zone boundary, where sound field control is obtained by integrating functions of passive elements with different impedance and adaptive active elements. Virtual sound barrier systems are completed active control systems, which adopt loudspeaker and microphone arrays and use signal processing techniques to control the sound field. In some virtual sound barrier systems, the transfer functions among the actuators and error sensors have similar characteristics, so a simplified plant estimation technique might be applied by exploiting the spatial-temporal structure of the multiple secondary propagation paths. The loudspeakers and microphones are on the surface of the hemisphere, and the space between the loudspeakers can be empty to let air and light pass through. The chapter also presents some closing thoughts on the key concepts discussed in the preceding chapters of this book.