Experimental determination of pressure-dependent stiffness of a nonlinear acoustic metamaterial

The field of acoustic metamaterials (AMM) involves the design of subwavelength structures to create material properties exceeding those of traditional composite materials. One such structure developed recently, known as a “Buckliball,” displays strong pressure-dependent stiffness [Proc. Natl. Acad. Sci. USA 109, 5978 (2012)]. The unique response of these structures results from element geometry: spherical elastomeric shells with thin, patterned circular membranes whose skeleton buckles when subjected to a differential pressure. We present measurements of the properties of a nonlinear effective medium consisting of Buckliballs immersed in water. The experimental apparatus consists of four elements: (1) a pressure vessel, which contains (2) a resonance tube filled with Buckliballs suspended in water, (3) an air-coupled ultrasonic system to monitor water height in the resonance tube, and (4) acoustic excitation and measurement instrumentation to obtain the frequency dependent acoustic response of the Bucklib...