A British firm has developed a one millimeter thick wall covering that can stop specific radio frequencies, while letting others through. Costing about $75 per square foot, the initial offering of FSS (frequency selective surface) material is tuned to keep Wi-Fi (wireless network) signals from getting out, but not others (like cell phones). This prevents unauthorized users from getting into your wireless network. FSS requires a small amount of electrical power to work, thus the power can be turned off if you want to let the blocked signals out. Naturally, FSS can be manufactured to keep out other types of signals. This could include those that escape from a PC keyboard when it is operating. This is useful if the PC is being used on classified information.