shielding

To keep the rain off, you stand under an umbrella; to keep stray electric fields off a delicate signal, you wrap it in a grounded sheet of metal. Shielding is exactly that: a conductive barrier, a foil wrap, a braided cable screen, or a metal box, placed between a circuit and the interference around it and connected to ground, so that stray fields end on the shield instead of on your signal. It is the metal coat that keeps a sensitive wire's signal in and the world's electrical noise out.

For electric fields the mechanism is simple: a grounded conductor intercepts the stray capacitive coupling and drains it to ground, so almost no field reaches the wire inside, which is the Faraday-cage effect. That is why a microphone cable has a braided screen around its inner conductor, grounded at one end. For magnetic fields a thin shield does much less at low frequency; only at higher frequencies do eddy currents induced in the shield create an opposing field that cancels the intruder. So a shield that is excellent against radio and against capacitive hum may do almost nothing against a 50/60 Hz magnetic field.

Shielding has practical rules that beginners get wrong. A shield only works if it is tied to the right reference, usually grounded at ONE end for a low-frequency cable (grounding both ends can itself create a ground loop), but often at both ends for high-frequency work. Any gap, seam, or slot in a shielded enclosure leaks fields, and a slot behaves like a slot antenna at the frequency whose wavelength matches it. Honest note: shielding treats the symptom, and reducing the loop area and impedance that pick up the noise in the first place is usually cheaper and more reliable than wrapping everything in metal.