Fig. 1. Boundary conditions for the electromagnetic and acoustic waves on a thin plate.(a) Electric and magnetic fields matching. (b) Pressure and velocity matching.

Fig. 2. Normalized impedance versus the hole diameter d at a frequency of 10 kHz.b=t=100 μm.

Fig. 3. Normalized impedance of micro-perforated plate.d=38.5 μm, b=t =100 μm.

Fig. 4. Acoustic Salisbury and Jaumann absorbers.(a) Schematic of the configuration of the multilayered acoustic absorbers. (b) Absorption spectra of the acoustic Salisbury and Jaumann absorbers.

Fig. 5. Angular dependence of the Jaumann absorber.As the incidence angle is increased from the 0° to 80°, the absorption valley position at about 14 kHz shifts to higher frequencies, accompanying with a small decrease of the absorption efficiency.

Fig. 6. Acoustic coherent perfect absorber.(a) Absorption spectra of the coherent perfect absorber. (b) Normalized impedance at different frequencies.

Fig. 7. Variation in the absorption under different coherent conditions.(a) Absorption versus frequency f with path difference of l=0.2 m. (b) Absorption versus l at f = 10 kHz.

Fig. 8. Schematic of the acoustic multilayer.In each layer, there are two counter-propagating waves with amplitudes of A and B. These amplitudes are connected with matrix shown in the text.