A cloaking device is one where the purpose of the transformation is to hide something, so that a defined region of space is invisibly isolated from passing electromagnetic fields (see Metamaterial cloaking) or sound waves. Objects in the defined location are still present, but incident waves are guided around them without being affected by the object itself. Along with this basic " cloaking device ", other related concepts have been proposed in peer reviewed, scientific articles, and are discussed here. Naturally, some of the theories discussed here also employ metamaterials, either electromagnetic or acoustic, although often in a different manner than the original demonstration and its successor, the broad-band cloak.