We investigate a meta-absorber composed of impedance-matched rods for the broadband acoustic absorption. However, with a slow longitudinal wave speed, the absorber’s density is high to keep the characteristic impedance matching with that of water. To obtain a lightweight high efficiency absorber, we analyze the influence of characteristic impedance and loss factor on absorption behaviors and find that the characteristic impedance matching is not necessary and the low characteristic impedance structure can also attain the efficient absorption performance, and the high dissipation can reduce the density required for an efficient absorption. Based on this, we propose a meta-absorber composed of low characteristic impedance rubber rods in parallel coated by a rubber cover layer and backed with a stepped resin base. By the combination of the transfer matrix method and transmission line impedance transfer equation, the acoustic performance of meta-absorber is analyzed. Compared with the characteristic impedance matching rods, the low characteristic impedance rods can release restrictions on design to attain better acoustic behaviors with a lower density. Further, the integration of rods with designed lengths can motivate coherently coupled weak resonances to obtain a broadband absorption. Finally, an optimized meta-absorber is tested in a water-filled impedance tube and the experimental results agree well with the theoretical results which can achieve a broadband low frequency quasi-perfect absorption ($\alpha >0.9$) in a deep subwavelength region ($\lambda$/158 at 950 Hz) with the density close to that of water. Our work finds a new mechanism that low characteristic impedance absorbing materials can achieve the high efficiency absorption with a slow longitudinal wave speed and low density and promise broad applications in the sound stealth of marine vehicles.

我们研究了一种由阻抗匹配棒组成的超吸收器，用于宽带吸声。然而，纵波速度较慢，吸收体的密度较高，以保持特性阻抗与水的特性阻抗匹配。为了获得轻量化的高效吸收器，我们分析了特性阻抗和损耗因子对吸收行为的影响，发现特性阻抗匹配不是必需的，低特性阻抗结构也可以获得高效的吸收性能，高耗散可以降低有效吸收所需的密度。基于此，我们提出了一种由低特性阻抗橡胶棒组成的元吸收器，该橡胶棒由橡胶覆盖层平行包裹并背衬阶梯状树脂基底。结合传递矩阵法和传输线阻抗传递方程，分析了元吸波体的声学性能。与特性阻抗匹配棒相比，低特性阻抗匹配棒可以放开设计限制，以更低的密度获得更好的声学性能。此外，设计长度的棒的集成可以激发相干耦合的弱共振以获得宽带吸收。最后，优化后的元吸波器在充水阻抗管中进行了测试，实验结果与理论结果吻合良好，可以实现宽带低频准完美吸收（与特性阻抗匹配棒相比，低特性阻抗匹配棒可以放开设计限制，以更低的密度获得更好的声学性能。此外，设计长度的棒的集成可以激发相干耦合的弱共振以获得宽带吸收。最后，优化后的元吸波器在充水阻抗管中进行了测试，实验结果与理论结果吻合良好，可以实现宽带低频准完美吸收（与特性阻抗匹配棒相比，低特性阻抗匹配棒可以放开设计限制，以更低的密度获得更好的声学性能。此外，设计长度的棒的集成可以激发相干耦合的弱共振以获得宽带吸收。最后，优化后的元吸波器在充水阻抗管中进行了测试，实验结果与理论结果吻合良好，可以实现宽带低频准完美吸收（$\alpha >0.9$）在深亚波长区域（$\lambda$/158 (950 Hz)，密度接近水。我们的工作发现了一种新的机制，低特性阻抗吸波材料可以实现低纵波速度和低密度的高效吸收，并有望在海洋车辆的声音隐身中得到广泛应用。