Diffusive surfaces can be optimally designed for both acoustic and aesthetic purposes. Adapting to the parametric demands of interface design, fractals are widely applied as a fusion of mathematical calculation and artistic design. The Sierpinski triangle is a self-similar structure with a more impressive appearance than conventional acoustic diffusers. However, the acoustic performance of Sierpinski fractal patterns has not been considered. This paper proposes a design of an acoustic diffuser based on the construction rules of the Sierpinski triangle to broaden the effective frequency range. The diffuser is made of triangular blocks of different sizes attached to a plane surface. A series of case studies are examined through numerical simulations based on the boundary element method (BEM) to investigate the effects of the number of iterations, the randomness of block arrangements, and the inclination of block tops. The diffusion performance of a conventional quadratic residue diffuser (QRD) is compared to confirm the advantage of the designed diffuser for broadening the effective frequency range. Furthermore, a workflow of the design and evaluation processes is presented to fabricate samples that could be used to tune the design parameters according to their in-field application demands.@en

This paper proposes a new design of an acoustic diffuser based on the construction rules of the Sierpinski triangle in order to broaden the effective diffusion frequency range. The diffuser is made of triangular blocks of different sizes attached to a plane surface. The effects of the number of fractal iterations, the height of triangular blocks, and arrangements of the blocks on the normal-incidence diffusion coefficients in the near field are examined through numerical simulations based on the boundary element method (BEM) in the frequency range of 100 Hz – 5 kHz. Furthermore, measurement results will be presented to validate the diffusion performance presented by the numerical simulations. The diffusion performance of a conventional quadratic residue diffuser (QRD) is compared to confirm the advantage of the designed diffuser for broadening the effective frequency range. It shows that the fractal patterns with various sizes of blocks improve diffusion performance compared to the conventional QRD of the same size, especially in the mid-low frequency range below 1 kHz.@en

This paper proposes a new design of an acoustic diffuser based on the construction rules of the Sierpinski triangle in order to broaden the effective diffusion frequency range. The diffuser is made of triangular blocks of different sizes attached to a plane surface. The effects of the number of fractal iterations, the height of triangular blocks, and arrangements of the blocks on the normal-incidence diffusion coefficients in the near field are examined through numerical simulations based on the boundary element method (BEM) in the frequency range of 100 Hz – 5 kHz. Furthermore, measurement results will be presented to validate the diffusion performance presented by the numerical simulations. The diffusion performance of a conventional quadratic residue diffuser (QRD) is compared to confirm the advantage of the designed diffuser for broadening the effective frequency range. It shows that the fractal patterns with various sizes of blocks improve diffusion performance compared to the conventional QRD of the same size, especially in the mid-low frequency range below 1 kHz.@en

Reliable data on acoustical properties of materials are crucial for the design of a desired acoustic environment as well as to obtain accurate results from acoustic simulations. Although the acoustical properties of materials can be obtained via laboratory measurements, situations where in situ measurements are needed are often encountered. However, in situ measurement methods presented so far are limited by their poor portability or inaccuracies in the low-frequency range. In this work, we propose a characterization method that combines an in situ pressure-velocity (PU) measurement with a model fitting procedure using the Delany-Bazley-Miki impedance model for porous materials. The method uses an optimization routine to find the best match of measured and modelled reflection coefficient values within a given frequency range for the optimization parameters: flow resistivity, panel thickness, and probe-sample distance. The optimal parameter values allow, in turn, calculating the porous panel’s reflection coefficients for a broad frequency range including frequencies below the lower bounds of the optimization frequency range. The sensitivity of the method to panel width, lower bound of fitting frequency range, and to excluding parasitic reflections by time windowing is studied. The study shows that the proposed method provides characterization results in good agreement with reference data for panels of dimensions larger than 1800 mm and that the method is robust for reduction of one dimension of the panel down to 300 mm. It also shows that the model fitting accuracy is best when the frequency range of analysis is restricted to 1000–5000 Hz.@en

Triply periodic minimal surfaces (TPMS), a class of periodic implicit surface with zero mean curvature, are emerging as an excellent solution to create graded porous structures. The grading of a porous layer can enhance and broaden the acoustic absorption in a target frequency range. The porosity gradient within the TPMS structure can be precisely controlled by a mathematical function, straightforwardly allowing for optimization towards the desired absorption. As the first step of the optimization procedure, we present a computational approach to determine acoustic absorption properties of functionally graded TPMS porous structures: the transport parameters of a rigid-frame homogeneous TPMS porous absorber are found by the finite element simulations of three static problems, then the absorption coefficient of the TPMS porous structure with graded porosity along the thickness is calculated with the transfer matrix method. The presented approach is validated by direct numerical simula- tions of the same porous structure.@en

In this work, the vibrations of a structure excited by an impact source are modelled using the time domain nodal discontinuous Galerkin (DG) method, which solves linear elasticity equations. A scaled lightweight wooden floor (LWF) structure which consists of components that differ in their mechanical properties is taken as a case study. The Rankine-Hugoniot jump conditions for piece-wise constant material properties are used to obtain accurate numerical fluxes in the DG method, and their detailed derivation is the main contribution of this work. Free boundary conditions are applied on the surface of the structures, and constant viscous damping force is added to the model to have vibrational energy losses. To validate the numerical results, the mobility of the structure is calculated and compared with experimental data. The agreement is good regarding the natural frequencies, with a maximum absolute difference of less than 11 Hz in the frequency range below 200 Hz. The adopted damping approach is shown to be insufficient to represent a broad frequency range.@en

Accurate modeling of boundary conditions is of critical importance for acoustic simulations. Recently, the time-domain nodal discontinuous Galerkin (TD-DG) method has emerged as a potential wave-based method for acoustic modeling. Although the acoustic reflection behavior of various time-domain impedance boundaries has been studied extensively, the modeling of the sound transmission across a locally-reacting layer of impedance discontinuity is far less developed. This paper presents a formulation of broadband time-domain transmission boundary conditions for locally-reacting surfaces in the framework of the TD-DG method. The formulation simulates the acoustic wave behavior at each of the boundary nodes using the plane-wave theory. Through the multi-pole model representation of the transmission coefficient, various types of transmission layers can be simulated. One-dimensional numerical examples demonstrate the capability of the proposed formulation to accurately simulate the reflection and transmission characteristics of the limp wall and the porous layer, where quantitative error behavior against analytical results is presented. Furthermore, to demonstrate the applicability, two scenarios of two-dimensional acoustic environment are considered. One is the sound transmission between two rooms partitioned by a limp panel and the other is the sound propagation through a transmissive noise barrier. Comparison of the predicted results from the proposed method against the results from the frequency-domain finite element simulations further verifies the formulation.@en

The purpose of this work is to check if additive manufacturing technologies are suitable for reproducing porous samples designed for sound absorption. The work is an inter-laboratory test, in which the production of samples and their acoustic measurements are carried out independently by different laboratories, sharing only the same geometry codes describing agreed periodic cellular designs. Different additive manufacturing technologies and equipment are used to make samples. Although most of the results obtained from measurements performed on samples with the same cellular design are very close, it is shown that some discrepancies are due to shape and surface imperfections, or microporosity, induced by the manufacturing process. The proposed periodic cellular designs can be easily reproduced and are suitable for further benchmarking of additive manufacturing techniques for rapid prototyping of acoustic materials and metamaterials.@en

The purpose of this work is to check if additive manufacturing technologies are suitable for reproducing porous samples designed for sound absorption. The work is an inter-laboratory test, in which the production of samples and their acoustic measurements are carried out independently by different laboratories, sharing only the same geometry codes describing agreed periodic cellular designs. Different additive manufacturing technologies and equipment are used to make samples. Although most of the results obtained from measurements performed on samples with the same cellular design are very close, it is shown that some discrepancies are due to shape and surface imperfections, or microporosity, induced by the manufacturing process. The proposed periodic cellular designs can be easily reproduced and are suitable for further benchmarking of additive manufacturing techniques for rapid prototyping of acoustic materials and metamaterials.@en

Sound transmission reduction is typically governed by the mass law, requiring thicker panels to handle lower frequencies. When open holes must be inserted in panels for heat transfer, ventilation, or other purposes, the efficient reduction of sound transmission through holey panels becomes difficult, especially in the low-frequency ranges. Here, we propose slow-wave metamaterial open panels that can dramatically lower the working frequencies of sound transmission loss. Global resonances originating from slow waves realized by multiply inserted, elaborately designed subwavelength rigid partitions between two thin holey plates contribute to sound transmission reductions at lower frequencies. Owing to the dispersive characteristics of the present metamaterial panels, local resonances that trap sound in the partitions also occur at higher frequencies, exhibiting negative effective bulk moduli and zero effective velocities. As a result, low-frequency broadened sound transmission reduction is realized efficiently in the present metamaterial panels. The theoretical model of the proposed metamaterial open panels is derived using an effective medium approach and verified by numerical and experimental investigations.@en

Sound absorption for a broad frequency range requires sound dissipation. The mechanics of acoustic metamaterials for non-dissipative applications has been extensively studied, but sound absorption using dissipative porous metamaterials has been less explored because of the complexity resulting from the coupling of its dissipative mechanism and metamaterial behavior. We investigated broadband sound absorption by engineering dissipative metaporous layers, which absorb sound by the mechanism of multiple slow waves, and combined local and global resonance phenomena. A set of rigid partitions of varying lengths was elaborately inserted in a hard-backed porous layer of a finite thickness. An effective medium theory was used to explain the physics involved; high performance at a low-frequency range was found to be mainly due to the formation of global resonances caused by multiple slow waves over the thickness of the metaporous layer, while enhancement at a high-frequency range was attributed to the combined effects of the global resonances and the local resonances directly related to the sizes of the inserted partitions.@en

The sound absorption of a porous layer is affected by its thickness, especially in a low-frequency range. If a hard-backed porous layer contains periodical arrangements of rigid partitions that are coordinated parallel and perpendicular to the direction of incoming sound waves, the lower bound of the effective sound absorption can be lowered much more and the overall absorption performance enhanced. The consequence of rigid partitioning in a porous layer is to make the first thickness resonance mode in the layer appear at much lower frequencies compared to that in the original homogeneous porous layer with the same thickness. Moreover, appropriate partitioning yields multiple thickness resonances with higher absorption peaks through impedance matching. The physics of the partitioned porous layer, or the metaporous layer, is theoretically investigated in this study.@en