A lightweight waterborne acoustic meta-absorber with low characteristic impedance rods

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.