Causal nature of the acoustic response, for any materials or structures, dictates an inequality that relates the absorption spectrum of the sample to its thickness. We present a general recipe for constructing sound-absorbing structures that can attain near-equality for the causal relation with very high absorption performance; such structures are denoted "optimal." Our strategy involves using carefully designed acoustic metamaterials as backing to a thin layer of conventional sound absorbing material, e.g., acoustic sponge. By using this design approach, we have realized a 12 cm-thick structure that exhibits broadband, near-perfect flat absorption spectrum starting at around 400 Hz. From the causal relation, the calculated minimum sample thickness is 11.5 cm for the observed absorption spectrum. We present the theory that underlies such absorption performance, involving the evanescent waves and their interaction with a dissipative medium, and show the excellent agreement with the experiment.

We report the experimental realization of perfect sound absorption by sub-wavelength monopole and dipole resonators that exhibit degenerate resonant frequencies. This is achieved through the destructive interference of two resonators' transmission responses, while the matching of their averaged impedances to that of air implies no backscattering, thereby leading to total absorption. Two examples, both using decorated membrane resonators (DMRs) as the basic units, are presented. The first is a flat panel comprising a DMR and a pair of coupled DMRs, while the second one is a ventilated short tube containing a DMR in conjunction with a sidewall DMR backed by a cavity. In both examples, near perfect absorption, up to 99.7%, has been observed with the airborne wavelength up to 1.2 m, which is at least an order of magnitude larger than the composite absorber. Excellent agreement between theory and experiment is obtained.Total absorption of sound using subwavelength structures or materials has always been a challenge, since the linear dynamics of dissipative systems dictates the fractional power to be linearly proportional to the elastic deformation energy [1], which is negligible in the sub-wavelength scale. To enhance the dissipation, it is usually necessary to increase the energy density, for example, through resonances. However, in an open system, radiation coupling to resonances is an alternative that can be effective in reducing dissipation. In previous studies, by utilizing localized subwavelength resonances, membrane-type metamaterial [2][3][4][5][6][7], containing decorated membrane resonator (DMR) with tunable weights, has shown efficient and flexible capability in low frequency sound absorption [8]. A balance between dissipation and scattering at resonance has been found for optimum absorption [9]. More recently, a perfect absorber has been realized by hybridizing DMR's two resonances through coupling via a thin gas layer. Through interference, waves reflected from such DMR have been shown to completely cancel that from a reflective wall placed a short distance (about 1/133 of airborne wavelength) behind the DMR [9,10]. Meanwhile, the coherent perfect absorber (CPA) in optics shows that the scattering waves at resonance can be cancelled when another counterpropagating coherent light wave, with specific phase and intensity, interferes with the incident beam, thereby leading to total absorption [11][12][13][14][15][16]. Recent efforts have also been made for its analogy in acoustics [17][18][19][20]. However, except for some theoretical attempts in acoustic [21] and numerical studies in optics [22], up to now no perfect absorber has been experimentally realized that intrinsically eliminates all the scattered waves, thereby realizing total absorption regardless of the incident direction, and with no need for a control wave.In this article, we advance the idea of creating a total acoustic absorption unit comprising a monopole (symmetric under mirror reflection) and a dipole (anti-symmetric) resonator that are re...