  Conventional sound shielding structures typically prevent fluid transport
between the exterior and interior. A design of a two-dimensional acoustic
metacage with subwavelength thickness which can shield acoustic waves from all
directions while allowing steady fluid flow is presented in this paper. The
structure is designed based on acoustic gradient-index metasurfaces composed of
open channels and shunted Helmholtz resonators. The strong parallel momentum on
the metacage surface rejects in-plane sound at an arbitrary angle of incidence
which leads to low sound transmission through the metacage. The performance of
the proposed metacage is verified by numerical simulations and measurements on
a three-dimensional printed prototype. The acoustic metacage has potential
applications in sound insulation where steady fluid flow is necessary or
advantageous.
