Advancements in 3D print technology now allow the printing of structured acoustic absorbent materials at the appropriate microscopic scale and sample sizes. The repeatability of the fundamental cell unit of these metamaterials provides a pathway for the development of viable macro models to simulate built-up structures based on detailed models of the individual cell units; however, verification of such models on actual manufactured structures presents a challenge. In this paper, a design concept for an acoustic benchmark metamaterial consisting of an interlinked network of resonant chambers is considered. The form chosen is periodic with cubes incorporating spherical internal cavities connected through cylindrical openings on each face of the cube. This design is amenable to both numerical modelling and manufacture through additive techniques whilst yielding interesting acoustic behaviour. The paper reports on the design, manufacture, modelling, and experimental validation of these benchmark structures. The behaviour of the acoustic metamaterial manufactured through three different polymer-based printing technologies is investigated with reference to the numerical models and a metal powder-based print technology. At the scale of this microstructure, it can be seen that deviations in surface roughness and dimensional fidelity have a comparable impact on the experimentally measured values of the absorption coefficient.
Significant potential for acoustic metamaterials to provide a breakthrough in sound attenuation has been unlocked in recent times due to advancements in additive manufacturing techniques. These materials allow the targeting of specific frequencies for sound attenuation. To date, acoustic metamaterials have not been demonstrated in a commercial automotive silencer for performance enhancement. A significant obstacle to the practical use of acoustic metamaterials is the need for low cost and efficient modelling strategies in the design phase. This study investigates the effect of acoustic metamaterials within a representative automotive silencer. The acoustic metamaterial design is achieved using a combination of analytical and finite element models, validated by experiment. The acoustic metamaterial is then compared with commonly used techniques in the silencer industry to gauge the effectiveness of the acoustic metamaterials. COMSOL simulations were used to validate the developed test rig and were compared to experimental results which were obtained using the two-load transmission loss test method. Through this testing method, the implementation of a labyrinthine metamaterial cylinder proved to be a significant improvement in transmission loss within the silencer, with an increase in transmission loss of 40 dB at 1500 Hz. The research has successfully shown that acoustic metamaterials can be used in practical settings, such as an automotive silencer, to improve the overall sound attenuating performance. The described analytical model demonstrates the potential for industrially relevant low cost design tools.
The sound absorptive performance of a proposed “meta-liner” are investigated in this paper. The structure is composed of closely placed plates connected by openings at alternating locations in a stacked format. This system presents multiple band gaps with high absorption and sub-wavelength behaviour (sample thickness equals 0.04 λ), achieved through tortuosity within the design. The acoustic response of the single layer is obtained numerically and with experimental verification under normal incidence. The repeating cellular design allows efficiencies in the viscothermal numerical analysis and using a transfer matrix approach, it is demonstrated that the response of the overall system may be efficiently predicted from a detailed model of a unit cell. Both the transfer matrix method and a full viscothermal model are validated against experimental data as a function of system depth. The analysis gives very satisfactory results which could form the basis for future designs.
Commonly used conventional acoustic porous materials suffer from a limited operational frequency range and great dependency of the sample thickness on their performance. For this reason, acoustic metamaterials have caught the eye of the scientific community with their ability to manipulate and absorb soundwaves despite their subwavelength dimensions. These novel acoustic materials are especially of interest for aerospace and automotive industries. Industrially relevant design tools are required to unlock the potential of these materials and the development of these tools requires benchmark problems. This work analyzes how the additive manufacturing process influences the acoustic performance of a periodic porous acoustic material. To answer this question, samples of a benchmark material were fabricated using selective laser melting with a 0.03 mm layer height. An optical microscope, a confocal microscope and computerized tomography scanner were used to examine manufactured specimens and provide insight into their surface topology. Numerical models of the structure were created. The computational results were compared with the experimental values. A combination of modelling strategies are investigated to incorporate features of the additive manufacture in the prediction of the acoustic behaviour. The observed mismatch between them can only partially be explained by the presence of the surface roughness. This study emphasizes the need to take into account more aspects of the additive manufacturing process when designing acoustic materials.
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