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Schematic of synthesis route and optical image of LRPW. (a) Synthesis scheme. We produced three kinds of woodpile structure layers utilizing steel rods with three different diameters (steel square mesh sizes: , , and ). First, we coated a thin layer of soft PU on three kinds of woodpile structure based on the volume ratio of soft PU and steel rod. Steel woodpile layers coated with soft PU were stacked in sequence under uncured soft PU condition. After total drying of this soft PU layer, the uncured hard PU was filled into the remaining space. (b) Three kinds of steel rod coated with soft PU has been defined as , , , , , and according to the volume ratio of soft PU and steel rods. (c) The optical photo shows that sample is homogeneous solid material.
Comparison of underwater sound absorption coefficients for LRPW and other materials that were fabricated with the same dimensions in 5–30 kHz. Other materials included component materials of LRPW (soft PU and hard PU), aluminum foam 1 (the average pore size was 4 mm) and aluminum foam 2 (the average pore size was 0.9 mm). LRPW owns high average sound absorbing coefficient in 5–30 kHz, which could not be achieved by other materials.
The model simplification process of LRPW. (a) Vertical view of 2 cm thick sample. Three sizes of steel woodpile layers coated with soft PU were stacked in sequence from perspective images. (b) In this picture, the space perspective effect shows that two-dimensional unit similar to that of LRPC exists in LRPW. According to the mass-spring model, two-dimensional unit of LRPC can be simplified to the double-oscillator model. (c) The right red solid square indicates a unit cell of LRPW. In this picture, the space perspective effect indicates a unit cell of 2 cm thick sample in the vertical direction. According to the mass-spring model, this unit cell can be simplified as ten vertical units connected by a fictitious spring.
Schematic of different locally resonant units and spectral characteristics. (a) Two-dimensional LRPC units only have relevant one band gap under full elastic scattering condition. (b) The LRPW unit of 2 cm thickness corresponds to nine band gaps of 6.3–54 kHz. (c) The measurement acoustic absorbing spectrum is mostly covered by band gaps spectrum of LRPW unit.
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