Other than reflection and absorption, a mechanism of shield¬ing is multiple reflections, which refer to the reflections at vari¬ous surfaces or interfaces in the shield. This mechanism requires the presence of a large surface area or interface area in the shield. An example of a shield with a large surface area is a porous or foam material. An example of a shield with a large interface area is a composite material containing a filler, which has a large sur¬face area. The loss due to multiple reflections can be neglected