Acoustic cloaking via a hybrid passive–active technique

Both passive and active methods have been employed extensively in recent times to create the effect of acoustic cloaking, with obstacles being made near-invisible over specific frequency ranges. Passive methods typically rely on metamaterials with complex microstructures, whereas active methods require an external power supply. Furthermore, a large number of active sources and sensors are typically required for active control. In this work, a hybrid method that combines passive and active approaches is developed. The passive acoustic cloak is designed theoretically via layered shells with alternating, functionally graded properties. The layer properties are informed by coupling transformation acoustics with homogenization theory and require unphysical values on the inner cloak wall. As such, some scattering occurs from the obstacle to be cloaked as well as due to the discontinuity in material properties across the shell boundaries. Using distributed monopole control sources, active control is then employed to nullify the non-zero scattered field outside the passive cloak domain. Effective cloaking is achieved using the hybrid passive–active control system.