  In elasticity, the design of a cloaking for an inclusion or a void to leave a
vibrational field unperturbed by its presence, so to achieve its invisibility,
is a thoroughly analyzed, but still unchallenged, mechanical problem. The
'cloaking transformation' concept, originally developed in electromagnetism and
optics, is not directly applicable to elastic waves, displaying a complex
vectorial nature. Consequently, all examples of elastic cloaking presented so
far involve complex design and thick coating skins. These cloakings often work
only for problems of unidirectional propagation, within narrow ranges of
frequency, and considering only one colaked object. Here, a new method based on
the concept of reinforcement, achieved via elastic stiffening and mass
redistribution, is introduced to cloak multiple voids in an elastic plate. This
simple technique produces invisibility of the voids to flexural waves within an
extremely broad range of frequencies and thus surpassing in many aspects all
existing cloaking techniques. The proposed design principle is applicable in
mechanical problems ranging from the micro-scale to the scale of civil
engineering. For instance, our results show how to design a perforated
load-bearing building wall, vibrating during an earthquake exactly as the same
wall, but unperforated, a new finding for seismic protection.
