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(Color online) (a) Scheme of the AOA. The region is the shell with radially distributed bulk modulus. The central cavity is usually filled with dissipative medium. (b) Three-dimensional scheme of the structure. (c) Scheme of the acoustic ray trajectory in the structure. The incident acoustic beam (the red line) is attracted to the center spirally.
(Color online) (a)–(c) Intensity fields and (d)–(f) pressure fields when a Gaussian beam is incident on different regions of the structure: (a), (d) beam is incident at a general position; (b), (e) beam is tangential with the boundary; (c), (f) beam points to the center. The horizontal and vertical axes denotes the distance in centimeter.
(Color online) A plane wave with wavelength is incident on (a) the AOA and (b) the bare absorptive core. The horizontal and vertical axes denotes the distance in centimeter.
(Color online) (a) Pressure distribution when a low-frequency plane wave is incident on the AOA. The wavelength is set to 24.7 cm. (b) Frequency dependence of the absorption capacity. The blue and red lines denote the absorbed acoustic energy of the AOA (Fig. 3(a)) and the bare core (Fig. 3(b)), respectively. The interval of any two adjacent frequency points along each line is 1/3 of an octave. The horizontal and vertical axes denotes the distance in centimeter.
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