 The effectiveness of mu-metal shielding decreases with the alloy's permeability, which drops off at both low field strengths and, due to saturation, at high field strengths.Thus, mu-metal shields are often made of several enclosures one inside the other, each of which successively reduces the field inside it.Because mu-metal saturates at such low fields, sometimes the outer layer in such multilayer shields is made of ordinary steel.