Frequency Selective Surface

Radio signals cannot travel thought solid metal walls. If one would like to completely block all radio signals from entering and leaving a room, one need to build all six walls with metal. This is obviously not an ideal way of construction.

Is it possible to build a room that only blocks one type of radio signals but allow other types to travel through? Sure, a convenient way would be to attach Frequency Selective Surfaces to your existing walls. 

Frequency Selective Surfaces are basically "wallpapers" consist of metallic loop patterns. This size, shapes, spacing between these patterns are calculated to filter out radio signals of a certain frequency range. For example, radio signals transmitting at frequency A can travel through these wallpapers but signals at frequency B cannot. 

Suppose a cinema is installed with frequency selective surfaces that are designed to block cellphone signals. Once the doors are closed, all cellphones within that cinema would have no reception. The ushers who carries walkie-talkies can still communicate with the outside world. 

Suppose a house owner would like to protect his home wifi network from his neighbors. With frequency selective surfaces installed, signals from his wifi router cannot leave his house. He will still be able to receive phone calls on his cellphone.

Cool idea, isn't it? Search online and learn more.