This article presents a thin-film acoustic metamaterial that can absorb low-frequency airborne sound at selective resonance frequencies ranging from 100–1,000 Hz. The material consists of an elastic membrane decorated with asymmetric rigid platelets, which induce flapping motion that leads to large elastic curvature energy density at their perimeter regions. This results in strong absorption, with samples achieving nearly unity absorption at frequencies where the relevant sound wavelength in air is three orders of magnitude larger than the membrane thickness. The system behaves like an open cavity, with the overall energy density in the membrane being two-to-three orders of magnitude larger than the incident wave energy density at low frequencies. The research demonstrates that by using thin elastic membranes decorated with designed patterns of rigid platelets, the resulting acoustic metamaterials can absorb 86% of the acoustic waves at ~170 Hz, with two layers absorbing 99% at the lowest frequency resonant mode as well as at higher frequency modes. The sample is thus acoustically 'dark', at those frequencies. Finite element simulations of the resonant mode patterns and frequencies are in excellent agreement with the experiments. The study shows that the absorption is due to the high elastic curvature energy density within the perimeter regions of the platelets, which is minimally coupled to the radiation modes. This leads to strong absorption similar to a cavity system, even though the system is geometrically open. The absorption frequency tunability is shown to be dependent on the mass of the platelets and the separation between them. The energy density enhancement factor (EDEF) is defined as the ratio of the average energy inside the sample to the energy density of the incident wave, and is shown to be high at resonances. The research also compares the performance of the dark acoustic metamaterials with reflective metamaterials, showing that the dark metamaterials achieve much higher absorption. The study highlights the potential applications of the dark acoustic metamaterials in noise reduction, acoustic quality tuning, and environmental noise abatement. The results demonstrate the effectiveness of the proposed acoustic metamaterials in achieving near-unity absorption of low-frequency sound.This article presents a thin-film acoustic metamaterial that can absorb low-frequency airborne sound at selective resonance frequencies ranging from 100–1,000 Hz. The material consists of an elastic membrane decorated with asymmetric rigid platelets, which induce flapping motion that leads to large elastic curvature energy density at their perimeter regions. This results in strong absorption, with samples achieving nearly unity absorption at frequencies where the relevant sound wavelength in air is three orders of magnitude larger than the membrane thickness. The system behaves like an open cavity, with the overall energy density in the membrane being two-to-three orders of magnitude larger than the incident wave energy density at low frequencies. The research demonstrates that by using thin elastic membranes decorated with designed patterns of rigid platelets, the resulting acoustic metamaterials can absorb 86% of the acoustic waves at ~170 Hz, with two layers absorbing 99% at the lowest frequency resonant mode as well as at higher frequency modes. The sample is thus acoustically 'dark', at those frequencies. Finite element simulations of the resonant mode patterns and frequencies are in excellent agreement with the experiments. The study shows that the absorption is due to the high elastic curvature energy density within the perimeter regions of the platelets, which is minimally coupled to the radiation modes. This leads to strong absorption similar to a cavity system, even though the system is geometrically open. The absorption frequency tunability is shown to be dependent on the mass of the platelets and the separation between them. The energy density enhancement factor (EDEF) is defined as the ratio of the average energy inside the sample to the energy density of the incident wave, and is shown to be high at resonances. The research also compares the performance of the dark acoustic metamaterials with reflective metamaterials, showing that the dark metamaterials achieve much higher absorption. The study highlights the potential applications of the dark acoustic metamaterials in noise reduction, acoustic quality tuning, and environmental noise abatement. The results demonstrate the effectiveness of the proposed acoustic metamaterials in achieving near-unity absorption of low-frequency sound.