Cloaking obstacles using synchronization

Obstacles in physical space produce a characteristic signature in the scattered field when exposed to incident waves, enabling their active or passive localization from field measurements. In waveguides, obstacles can be inhomogeneities such as cross-section variations or side branches. Such geometric perturbations cause reflections and absorption of traveling waves in the channel, decreasing their signal strength in the propagation direction and scattering waves back towards the sender. To mask these effects, we present a nondispersive method which can completely compensate losses incurred by absorption while simultaneously canceling reflections. It is based on the synchronization of outgoing waves from self-oscillations in the obstacle with the incident waves, and illustrated with an acoustic experiment. The latter is based on the use of a police whistle attached to the obstacle in a one-dimensional waveguide, causing a pure whistling tone whose energy can be harnessed via synchronization with a sufficiently strong and in-tune incident wave. An analytical model for the proposed nonlinear mechanism is derived and validated by experiments. The proposed quasipassive, all-acoustic concept has direct applications in acoustic metamaterials for cloaking and may be translated to electromagnetic and photonic applications with saturable gain elements.