An enclosure designed to attenuate electromagnetic fields either inside or outside the area. There are two basic shielded enclosure types: (1) modular/bolted demountable; and (2) welded metal enclosure. The latter is built in place and is either free standing or becomes a part of the building structure. Different types of shielding materials can be used for both shielded enclosure types but must be properly specified so that the constructed shield performs its intended purpose. The finished shielded enclosure must be properly tested (Testing Standards: IEEE 644, IEEE 299, Mil Std 285, NSA 65-6) to ensure compliance with the desired level of shielding performance. Penetrations in shielded enclosures, such as doors, air vents, and electrical feedthroughs, are specially designed to maintain this attenuation.