Acoustic cloak designed by topology optimization for acoustic–elastic coupled systems

By including acoustic-elastic interactions in a topology optimization based on the covariance matrix adaptation evolution strategy, we developed acoustic cloaks of optimal design that render an object unobservable through airborne and water-borne sounds. This strategy helps in exploring optimal topologies that minimize the scattering of airborne and water-borne sounds around acoustic cloaks made from acrylonitrile butadiene styrene copolymers frequently used as ink in 3D printers. By applying level set methods, our designed cloaks are expressed as iso-surfaces representing the sharp structural boundaries between the acoustic and elastic media. On these boundaries, conditions that couple the elastic and acoustic aspects of the scattering are imposed. Furthermore, our cloak designs ensure that scatterers are unobservable to sound waves of multiple frequencies incident from various angles by minimizing the fitness incorporating multiple objective functions under several structural symmetries.