Fig. 1. Structure and theoretical model of composite acoustic metamaterial: (a) The top micro-perforated panel (thickness , diameter , perforation rate ); (b) multiple coiled FP channels; (c) schematic of the hybrid metamaterial absorber composed of a microperforated panel (MPP) as a top face sheet and coiled-up Fabry–Perot (FP) channels with folding number n; (d) an approximate analytical two dimensional (2 D) model of one unit cell of a space-coiled metamaterial. All the widths of the channels in the YZ plane are . The height of the channel along the Z axis is .

Fig. 2. Sound velocity and sound pressure in a sound absorber when a 230 Hz sound wave is introduced: (a) Sound velocity distribution (m/s); (b) sound pressure distribution (Pa). The absorbers are constructed using the coiled-up channel with geometric parameters: , and and .

Fig. 3. Sound absorption characteristics of composite sound absorption structure: (a) Zero and pole distributions of reflection coefficients on complex frequency plane; (b) sound absorption performance curve. The dotted line is the theoretical solution and the solid line is the numerical solution, the black dotted line is the theoretical solution of each unit (Gradually increase the equivalent sound absorption length from left to right); (c) real and imaginary parts of relative impedance.

Fig. 4. Quasi-perfect sound absorption characteristics of composite sound absorption structure: (a) Zero and pole distributions of reflection coefficients on complex frequency plane; (b) sound absorption performance curve, The dotted line is the theoretical solution and the solid line is the numerical solution; (c) real and imaginary parts of relative impedance.