Ultra-thin low frequency perfect sound absorber with high ratio of active area

Abstract : A concept of ultra-thin low frequency perfect sound absorber is proposed and demonstrated experimentally. To minimize non-linear effects, an high ratio of active area to total area is used to avoid large localized amplitudes. The absorber consists of three elements: a mass supported by a very flexible membrane, a cavity and a resistive layer. The resonance frequency of the sound absorber can be easily adjusted just by changing the mass or thickness of the cavity. A very large ratio between wavelength and material thickness is measured for a manufactured perfect absorber (ratio = 201). It is shown that this high sub-wavelength ratio is associated with narrowband effects and that the increase in the sub-wavelength ratio is limited by the damping in the system. Sound absorption is of great interest to engineers who want to reduce sound reflection in room acoustics or reduce sound emissions. There is a need for innovative acoustic absorbent materials, effective in low frequencies while being able to deal with spatial constraints present in real applications. Innovative ultra-thin materials are also useful tools for the scientific community to manipulate sound waves and obtain negative refraction [1, 2], sub-wavelength imaging [3, 4], cloaking [5], etc. Traditional sound absorption structures use perforated and micro-perforated panels covering air or porous materials. [6, 7]. These materials have a low reflection of normal incident waves at frequencies such that the wavelength (λ = c 0 /f where c 0 and f are the sound velocity in air and the frequency) is about four times the thickness of the material H leading to a sub-wavelength ratio r H = λ/H 4. There has been a very significant reduction in the thickness of the absorbent materials [8] by using space-coiling structures [9-12] of by using slow-sound inside the material [13-15]. Nevertheless, all these structures have a front plate with small holes leading to a very low open area ratio (σ = area of the orifices on the total area). In this case, since the velocity in the orifices is equal to the acoustic velocity in the incident wave divided by σ, some non-linear effects can easily occur in the orifices when the amplitude of the incident wave becomes large enough [16]. Moreover , in many engineering applications, a grazing flow is present and its effect on the impedance of the material is inversely proportional to σ [17]. For instance, it was experimentally shown [18] that the efficiency of a thin slow-sound metamaterial with σ = 0.023 was divided by 100 in the presence of a grazing flow with a Mach number of 0.2. Therefore, in the case of high sound levels or in the presence of flow, the additional constraint of having a high open area ratio must be added in the design of structures that are thin and absorbent at low frequency. From this perspective, the use of elastic membranes and decorated elastic membranes as sound absorbers is * yves.auregan@univ-lemans.fr an interesting option. [19-21]. In most of the studied structures, the membrane represents a large part of the active surface and the added mass (platelets) is of smaller size. The maximum absorption appears for an hybrid resonance [20] due to the interaction between two modes of coupled system consisting of the membrane, the platelet and the air cavity. By a proper arrangement of the various parameters very impressive results can be obtained both in reflection [20] and in transmission [22]. At the resonance frequency, the three main parameters of membrane absorbers (equivalent mass, stiffness and damping) are very sensitive to the characteristics of the membrane. By changing one of the properties of the membrane (its tension, its mass density,...) or the size of the added mass, the three equivalent parameters are simultaneously changed. Cavity Membrane Mass Resistance FIG. 1. Sketch of an ultra-thin low frequency (UTLF) res-onator. This letter presents an ultra-thin low frequency (UTLF) resonator for which the three parameters (mass,