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In this paper, we study approximate cloaking of active devices for theHelmholtz equation in the whole space of dimension 2 or 3 using the schemeintroduced by Kohn, Shen, Vogelius, and Weinstein in\cite{KohnShenVogeliusWeinstein}. More precisely, we assess the degree ofinvisibility, determine the limit of the field inside the cloaked and cloakingregions, and show that the scheme is unstable with respect to the materialparameters. As a consequence, we obtain some feasible properties of "perfect"cloaking.

Approximate cloaking for the Helmholtz equation via transformation optics and consequences for perfect cloaking