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Acoustic metamaterials made of foam metal.

Although foam metal has good mid-frequency sound absorption performance as a porous material, it still has limitations due to its low absorption coefficient in low-frequency sound absorption and narrow mid-to-high frequency sound absorption band. At the same time, the existing foam metal sound absorption structure has not been able to achieve the goal of high sound absorption performance with low thickness. To address these issues, foam aluminum is used as the base material, and by combining the characteristics of porous materials and resonant structure sound absorption, the high permeability of the porous material with narrow slits or micropores will cause mesoscopic pressure changes within the porous material. This results in macroscopic flow being dominated only by the flow within the pores, while the microscale region does not participate in the macroscopic flow but compresses some of the equivalent media which generates a new dissipative effect related to the mesoscopic structure and the properties of the micropores of the material. As a result, acoustic materials with this structure may have better sound absorption effects and can establish two types of acoustic structures suitable for wideband sound absorption and low-frequency sound absorption. Furthermore, the acoustic structure materials have been developed to achieve a wideband sound absorption structure with smaller thickness and an acoustic structure that is more suitable for low-frequency sound absorption, and by doing so these materials provide a more accurate and suitable solution for low-frequency and wideband sound absorption.

Silver Foam (2)

The research on folded narrow seam materials mentioned above demonstrates that this structure can improve the sound absorption characteristics of porous materials and achieve low-frequency noise reduction. In terms of the structure and parameters of the material, the loosely porous structure of foam aluminum has a lower flow resistance, allowing low-frequency sound waves to enter the sound-absorbing framework of the porous material. The sound absorption mechanism mainly depends on the properties of the porous material itself, which is consistent with the high permeability situation in the dual-porosity theory. Additionally, the phenomenon of the sound absorption coefficient significantly increasing with frequency indicates that foam aluminum, rather than the folded narrow seam structure, plays a major role in sound absorption. On the other hand, for foam aluminum with smaller pore size and denser pore distribution, it has a higher flow resistance. Therefore, this low permeability situation makes it difficult for low-frequency sound waves to enter the framework of the porous material. Instead, they enter through the narrow seams and accumulate sound energy within the folded structure. When the sound energy reaches a certain level, resonance occurs and this frequency range is also where the best sound absorption effect is observed. As the frequency increases, high-frequency sound waves with shorter wavelengths can enter the matrix of the porous material, where they are dissipated by thermal friction and viscosity. However, when the wavelength is small enough, the sound energy within this frequency range is difficult to accumulate energy within the folded narrow seam, resulting in the absence of resonance, which leads to a decrease in the absorption coefficient.

Copper Foam
Copper Foam
  1. The double-layer rear air layer foam metal sound-absorbing structure with excellent sound absorption performance for high-frequency noise was designed by combining foam metal porous materials with excellent sound absorption performance and the resonance effect of the Helmholtz resonator cavity. After structural optimization design, the average sound absorption coefficient in the frequency range of 500Hz-6300Hz can reach 0.82, achieving wideband sound absorption.By combining the sound absorption characteristics of foam aluminum and the characteristics of narrow slit cavity resonance, an acoustic metamaterial structure with low-frequency sub-wavelength sound absorption was obtained. It is a folded narrow slit dual porosity structure based on foam aluminum as the matrix. This structure can achieve perfect sound absorption below 500Hz and sub-wavelength absorption of sound waves. The optimized design of the structure can improve the low-frequency sound absorption coefficient by 13.4% and 8.9% compared to the unoptimized structure.
  2. The factors that influence the sound absorption performance of wideband sound absorption structures with a rear cavity of double-layer foam metal panels include the type of foam metal, the thickness of the metal layer, and the thickness of the air layer. Materials with larger porosity and lower flow resistance are suitable for placement in the first layer, while materials with higher flow resistance are suitable for placement in the second layer. As for the thickness of the metal layer, it is advantageous to have a thicker first layer and a thinner second layer to improve the overall wideband sound absorption. The cavity depth should be kept shallow in the first layer and deeper in the second layer for better wideband sound absorption.
  3. The sound absorption performance of the constructed folded narrow slit foam aluminum sound absorption structure is mainly influenced by the type of substrate material, the number of unit cells, and several important geometric dimensions.
  4. The folded narrow slit structure constructed using porous materials with higher flow resistance and denser porosity has a significantly improved low-frequency sound absorption performance. On the other hand, foam aluminum with a looser pore structure and lower flow resistance has a very weak effect on improving low-frequency sound absorption. The number of unit cells mainly controls the peak value and peak width of the low-frequency sound absorption peak by controlling the effective length of the slit, and this control is almost unaffected by the substrate material. More unit cells will result in lower frequency sound absorption peaks and narrower peak widths. The geometric dimensions that have a significant impact on low-frequency sound absorption are the folding height (t) and the slit width (d). Increasing the folding height will narrow the sound absorption peak width and shift it towards lower frequencies, while increasing the slit width will not affect the corresponding frequency of the sound absorption peak, but the peak value will increase slightly.

Copper Foam 1 (6)

Picture of Lu

Lu

Our materials research team from Tsinghua University postdoctoral researcher lin and Harbin Institute of Technology researcher Mu, Nanjing University of Technology researcher Wei, they share their expertise in foam metal materials article.

About HGP

WE were established in 2003, located in the Gaoxin Zone of Guangdong-Guangxi Cooperation Special Experimental Zone, covering an area of 70 mu, with a plant of about 30,000 square meters, with more than 170 employees, is an advanced new material technology enterprise integrating research and development, production and sales.

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