Controlling and limiting the vibration of thin plates has a multitude of applications in structural acoustics. It is a subject
that has received great attention in the literature where a variety of techniques have been proposed to reduce the level of
structural vibration and noise. See for example[1], the text by Fuller et al. [2]and for an early example see e.g. Olson[3].A
closely related topic that has emerged over the last decade is that of cloaking, the effect of causing a region to be unseen by
incoming waves in the sense that the scattering is zero in all directions, see[4]for a review. Two principal techniques have
been proposed: passive and active. Passive cloaking[5–7]requires complex metamaterials in order to guide waves around
some volume of space, or around a region in a plate. Passive cloaking methods have been successfully developed and
demonstrated for flexural waves in thin plates. Thus, Stenger et al.[8]adapted the design proposed in[9]to make a freespace flexural wave cloak in a thin PVC plate. The flexural wave cloak was demonstrated at acoustic frequencies, and
exhibited the largest measured relative bandwidth (more than one octave) of reported free-space acoustic cloaks.
In contrast to passive methods,active cloakinguses sources of sound to achieve wave cancellation. It does not involve or
call for unusual materials or structural modifications. Active cloaking does however require knowledge of the incident field
in order to activate wave sources that nullify the total field in a given region. Importantly the sources must be non-radiating.
Miller[10] first proposed the notion of actively cloaking a region by measuring particle motion near the surface of the
cloaked zone while simultaneously exciting surface sources where each source amplitude depends on the measurements at all
sensing points. Complete suppression of sound in a finite volume in an unbounded domain can be achieved using a continuous
distribution of monopoles and dipoles with source amplitudes given by the Kirchhoff–Helmholtz integral formula[11].The
main disadvantage of solutions based on the Kirchhoff–Helmholtz integral is the difficulty of realizing in practice acoustically
transparent sensor and actuator surfaces. It would be better to replace the surfaces by finite sets of discrete sensors and