Mass loaded vinyl, or MLV, is a heavy, limp vinyl sheeting material that usually has been impregnated with metal particles to increase its mass. Mass loaded vinyl is used for adding mass to walls and ceilings as part of a soundproofing or sound control scheme. Often mass loaded vinyl is sandwiched between sheets of other material, such as between two layers of drywall, to create wall and ceiling covering. In some cases, mass loaded vinyl is denser (heavier per square foot) than sheet lead. A great example of mass loaded vinyl is Auralex SheetBlok.