Acoustic properties of porous microlattices from effective medium to scattering dominated regimes

Microstructure-based modeling to characterize low pore density open-cell foams and its experimental validation

Microlattices with large pore sizes are involved in many multifunctional applications, so it is essential to understand their acoustic properties. However, for these low pore density microlattice foams, the classical homogenization or "equivalent fluid" methods fail abruptly. This paper proposes and discusses a microstructure-based direct fluid model (DFM) that would help to predict the acoustic performance of low pore density periodic open-cell foams with spherical pores. The DFM is simulated directly, including the microscale geometric features inherent in the unit cell. A comparative study is performed for designed three-dimensional (3D) body-centered-cubic (BCC) porous foams having pores per inch (PPI) ranging from 1 to 12 over the frequency range of 500-4100 Hz with equivalent fluid models and experiments. The study shows the extent of deviation in homogenization-based methods from the experiment for PPI < 5. On the other hand, the acoustic performance parameters predicted with the DFM agree well with experiments on 3D-printed samples fabricated by additive manufacturing of varying PPI starting from 1. This study shows that the DFM is a valid method to predict the acoustics of low PPI microlattices. Furthermore, the gradual transition from dissipative to the reactive regime with a decrease in PPI is also brought out.

Effect of the Pore Shape and Size of 3D-Printed Open-Porous ABS Materials on Sound Absorption Performance

Noise has a negative impact on our environment and human health. For this reason, it is necessary to eliminate excessive noise levels. This paper is focused on the study of the sound absorption properties of materials with open-porous structures, which were made of acrylonitrile butadiene styrene (ABS) material using additive technology. Four types of structures (Cartesian, Octagonal, Rhomboid, and Starlit) were evaluated in this work, and every structure was prepared in three different volume ratios of the porosity and three different thicknesses. The sound absorption properties of the investigated ABS specimens were examined utilizing the normal incidence sound absorption and noise reduction coefficients, which were experimentally determined by the transfer function method using a two-microphone acoustic impedance tube. This work deals with various factors that influence the sound absorption performance of four different types of investigated ABS material’s structures. It was found, in this study, that the sound absorption performance of the investigated ABS specimens is strongly affected by different factors, specifically by the structure geometry, material volume ratio, excitation frequency of an acoustic wave, material’s thickness, and air space size behind the tested sound-absorbing materials.

Locally Resonant Phononic Crystals at Low frequencies Based on Porous SiC Multilayer

In this work, a one-dimensional porous silicon carbide phononic crystal (1D-PSiC PnC) sandwiched between two rubber layers is introduced to obtain low frequency band gaps for the audible frequencies. The novelty of the proposed multilayer 1D-PnCs arises from the coupling between the soft rubber, unique mechanical properties of porous SiC materials and the local resonance phenomenon. The proposed structure could be considered as a 1D acoustic Metamaterial with a size smaller than the relevant 1D-PnC structures for the same frequencies. To the best of our knowledge, it is the first time to use PSiC materials in a 1D PnC structure for the problem of low frequency phononic band gaps. Also, the porosities and thicknesses of the PSiC layers were chosen to obtain the fundamental band gaps within the bandwidth of the acoustic transducers and sound suppression devices. The transmission spectrum of acoustic waves is calculated by using the transfer matrix method (TMM). The results revealed that surprising low band gaps appeared in the transmission spectra of the 1D-PSiC PnC at the audible range, which are lower than the expected ones by Bragg's scattering theory. The frequency at the center of the first band gap was at the value 7957 Hz, which is 118 times smaller than the relevant frequency of other 1D structures with the same thickness. A comparison between the phononic band gaps of binary and ternary 1D-PSiC PnC structures sandwiched between two rubber layers at the micro-scale was performed and discussed. Also, the band gap frequency is controlled by varying the layers porosity, number and the thickness of each layer. The simulated results are promising in many applications such as low frequency band gaps, sound suppression devices, switches and filters.