## 1 Introduction  
Acoustic absorption is a major topic in acoustics, mainly because acoustic absorbers are broadly used in industry (e.g., in building and civil engineering applications). In these situations, a typical configuration is to place the materials at the walls of a room, the non-reflecting acoustic material being rigidly backed. The classical approach to designing such materials is to produce a porous frame whose scale of porous cavities is on the order of the thermal and viscous boundary layers of the filling host fluid (typically air) [1]. When a porous foam layer is rigidly backed, absorption might be large for frequencies only above the so-called quarter wavelength resonance of the backed layer. Thus, the main limitation of porous material-based absorbers yields in the sound speed inside the absorbers, which is on the same order as the one in the air medium: to efficiently absorb low-frequency sound, the thickness of the layer must be large, because at these frequencies the wavelength in air is on the order of several meters. Peculiar absorption properties are also encountered in composite structures like double porosity materials [2], dead-end porosity materials [3; 4], or chirped layered structures [5]. However, it is of special interest to design materials and structures in the form of thin panels (i.e., much smaller than the characteristic wavelength). In recent years, metamaterials have been used to design sub-wavelength absorbers. These specific materials are artificial structures composed of an arrangement of resonant unit cells--smaller than the characteristic wavelength--that present effective properties not observed in the materials that compose the structure. Examples of such absorbers are metaporous materials [6; 7; 8; 9], metamaterials composed of membrane-type resonators [10; 11; 12; 13], Helmholtz resonators (HRs) [14; 15; 16; 13], and quarter-wavelength resonators (QWRs) [4; 17; 18; 19]. These last types of metamaterials [4; 15; 16; 17; 18] make use of strong dispersion, giving rise to slow-sound propagation inside the material. Using slow sound results in a decrease of the cavity resonance frequency, and hence the structure thickness can be reduced to the deep-subwavelength regime [18]. Materials that present perfect absorption are of special interest. This occurs when the leakage of the structure is exactly compensated by the intrinsic losses of the system (i.e., when the material is critically coupled with the exterior media) [13]. For a rigidly backed material, this reduces to the well-known impedance matching conditions. However, the critical coupling conditions can only be fulfilled at a single particular incidence angle for a particular frequency in locally reacting materials, because the effective acoustic impedance does not depend on the direction of propagation. This can reduce the efficiency of the material when used in situations where the incoming acoustic waves impinge the structure in a broad range of angles [1; 20]. In this work, we present an ultra-thin acoustic metamaterial presenting perfect absorption at selected angles and frequencies. Thus, it produces an acoustic iridescent effect due to its selective and perfect absorption at particular angles and frequencies. Light iridescence is commonly observed in nature in structured natural phononic materials as occurs in beetles and butterfly wings, fish scales, or bird feathers [21; 22], producing strong reflections at selected angles. It has also been proposed to generate acoustic iridescent materials using perforated plates [23]. The structures proposed in this work produce iridescence through perfect absorption at selected angles and frequencies. The specific features of the angle-dependent absorption are illustrated by using the complex frequency plane representation of the reflection coefficient and by showing the trajectories of the different zeros of the reflection coefficient in the complex frequency plane.