EM shielding (electromagnetic shielding) is the practice of surrounding electronics and cables with conductive or magnetic materials to guard against incoming or outgoing emissions of electromagnetic frequencies (EMF).
EM shielding is conducted for several reasons. The most common purpose is to prevent electromagnetic interference (EMI) from affecting sensitive electronics. Metallic mesh shields are often used to protect one component from affecting another inside a particular device. In a smartphone, for example, a metallic shield protects electronics from its cellular transmitter/receiver. Radiation shields in mobile phones also decrease the amount of radio frequency (RF) energy that might be absorbed by the user.
To increase the security of air gapped systems, EM shielding is advised. Conventionally, physical isolation and a lack of external connectivity have been considered adequate to ensure their security. However, proof-of-concept attacks have demonstrated that acoustical infection can be enabled by exploiting the electromagnetic emanations of the system’s sound card.
Air-gapping is used in the military, government and financial systems like stock exchanges. The measures are also used by reporters, activists and human rights organizations working with sensitive information.
A number of different materials and techniques are used for EM shielding. Wires may be surrounded by a metallic foil or braid shield to block errant EMI from the cased wires. Audio speakers often have inner metallic casing to block EMI produced by the drivers so they don’t affect TVs and other electronics. Complete continuous enclosure is not necessary so long as any openings are smaller than the electromagnetic waves that need to be blocked.
Special conductive paints can be used to prevent the EMF from networks escaping the originating business to prevent eavesdropping or wireless attacks. These techniques are like a miniature Faraday cage, which can prevent signal corruption that would cause electronics to perform unexpectedly.
Electronics may also have connections filtered for EMI by use of electronic components like capacitors, ferrules and grounded wires to minimize the effects of EMI noise -- even twisting wires together with grounds can reduce lower interference.
Magnetic materials must be used for EM shielding in environments where the magnetic fields are slowly varied below the 100Khz range as a Faraday cage-type solution is ineffective in that situation. With magnetic materials, the EMI is drawn into the magnetic field of the shielding.