“…Based on the measured average fibre diameter and gap, five JCA parameters of the melamine foam are obtained by the bottom-up method (Perrot et al, 2008a, 2008b). The bottom-up method has also been used by other researchers to extract the five parameters of porous materials successfully (Ji et al, 2019a; Ji et al, 2019b; Sun et al, 2022a; Yuan et al, 2022) and these five parameters are listed in Table 1.Figure 6.The photograph and microstructure of the melamine foam.…”
Acoustic liners take an essential role in the noise attenuation in ducts. In this work, the multimodal method based on the finite-difference method is extended to predict the acoustic field in a rectangular duct lined with a finite porous material and validated by the experiment. A modified immersed interface method is developed for the air–porous interface problem in the extended multimodal method without flow. When the flow exists, the treatments of the air–porous interface problem and the continuity of pressure between the wall and liner are also given. Three ways of describing the porous liner using the surface impedance at normal incidence named normal impedance, the surface impedance at grazing incidence named shear impedance and a cavity filled with the equivalent fluid of porous material are introduced. The comparison among the three ways reveals that the extended method of treating liners using a cavity filled with the equivalent fluid is more accurate for the acoustic evaluation of porous liners. The analysis finds that the shear impedance can reasonably present the influence of porous material during the comparisons of transmission loss curves of the liners with different lengths and depths at different Mach numbers. In most cases, the prediction by the shear impedance is closer to that simulated by the cavity filled with porous material than the normal impedance. Normal impedance is hardly utilized to describe the porous material and is only reliable when the cavity is very short and deep.
“…Based on the measured average fibre diameter and gap, five JCA parameters of the melamine foam are obtained by the bottom-up method (Perrot et al, 2008a, 2008b). The bottom-up method has also been used by other researchers to extract the five parameters of porous materials successfully (Ji et al, 2019a; Ji et al, 2019b; Sun et al, 2022a; Yuan et al, 2022) and these five parameters are listed in Table 1.Figure 6.The photograph and microstructure of the melamine foam.…”
Acoustic liners take an essential role in the noise attenuation in ducts. In this work, the multimodal method based on the finite-difference method is extended to predict the acoustic field in a rectangular duct lined with a finite porous material and validated by the experiment. A modified immersed interface method is developed for the air–porous interface problem in the extended multimodal method without flow. When the flow exists, the treatments of the air–porous interface problem and the continuity of pressure between the wall and liner are also given. Three ways of describing the porous liner using the surface impedance at normal incidence named normal impedance, the surface impedance at grazing incidence named shear impedance and a cavity filled with the equivalent fluid of porous material are introduced. The comparison among the three ways reveals that the extended method of treating liners using a cavity filled with the equivalent fluid is more accurate for the acoustic evaluation of porous liners. The analysis finds that the shear impedance can reasonably present the influence of porous material during the comparisons of transmission loss curves of the liners with different lengths and depths at different Mach numbers. In most cases, the prediction by the shear impedance is closer to that simulated by the cavity filled with porous material than the normal impedance. Normal impedance is hardly utilized to describe the porous material and is only reliable when the cavity is very short and deep.
“…Owing to the existence of pores, porous metallic materials give them the dual attributes of being structural and functional [1,2], which has attracted much attention in recent years. With many extraordinary characteristics, such as low relative density, large specific surface area, lightweight, sound insulation, and energy absorption, porous metals are widely applied in lightweight [3,4], medical implants [5][6][7], filtration and separation [8,9], sound absorption and noise reduction [10][11][12], heat exchange [13][14][15] and battery catalysis [16,17]. Therefore, a porous structure with channeled pores and pore sizes less than 100 µm plays a particularly important role in physical and chemical performance owing to the excellent connectivity and high specific surface area of the micropores.…”
Laser powder bed fusion can fabricate porous structures through lattices, but the preparation of micropores (<50 μm) with a specific pore distribution remains a challenge. Microporous 316L was fabricated by controlling the melting and solidification behavior of the particles using laser energy. The laser energy density was not a determining factor for the porosity and micropore formation, except for the single-factor condition. The high-speed scanning mode required a higher laser power to disorder the pore distribution, whereas low-speed scanning with a low laser impact on the stacking particles formed organized pores. The hatch distance significantly affected the pore distribution and pore size. The pore distribution in the XY plane was organized and homogenous, with channeled pores mainly interconnected along the laser scanning tracks, whereas in the Z direction, it showed a relatively disordered distribution, mainly linked along the layered direction. The microporous 316L displayed a mean pore size and median pore size of 10–50 μm with a high-percentage size distribution in 1–10 μm, a controllable porosity of 17.06%–45.33% and a good yield strength of 79.44–318.42 MPa, superior to the lattice porous 316L with 250.00 MPa at similar porosity.
“…Traditionally, effective sound-absorbing materials are mainly porous − or fibrous, , but their thickness is often matched with the operating wavelength, which results in a large volume under medium–low frequency and is not conducive to practical applications. Theoretical simulation further indicates the limitations of porous materials .…”
Noise is a threat to human life quality and an invisible killer that causes many chronic diseases. The vast majority of porous sound absorbers are single-phased, which limits their sound absorption potential. Herein, a graphenedecorated porous system (GDPS) prepared using the immersion-hydrothermalfreezing self-assembly method is reported as an efficient sound absorber based on its unique consecutive double three-dimensional (dual-3D) structure. Due to the increased tortuosity and other gain effects, the novel structure can achieve the perfect broadband absorption at a wide bandwidth in which the sound absorption coefficient exceeds 0.9 easily. Within the effective thicknesses, the widest perfect absorption bandwidth of 814−6400 Hz is realized. Moreover, a higher graphene concentration and the addition of a polymer are found to be able to decrease the absorption frequency to the lowest values of 1979 and 1544 Hz, respectively. The design of a unique dual-3D structure opens up new strategies and applications to use graphene aerogels in noise and vibration applications.
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