Hybrid noise control in a duct using a light micro-perforated plate

Inverse design of acoustic metamaterials based on machine learning using a Gauss–Bayesian model

Acoustic metamaterials (AMs) have attracted a substantial amount of attention in recent decades where the parameter design plays an important role. However, conventional design methods generally rely on analytical physical models and require a very large number of evaluations of acoustic performance. Here, we propose and experimentally demonstrate an inverse-design method for AMs based on machine learning using a Gauss–Bayesian model. As a result of the cycle of training and prediction and the use of adaptive acquisition functions, this method allows the parameters of AMs to be efficiently designed for specific functionalities without the need for physical models. Considering the significance of low-frequency ventilated sound absorption, we present a design for a typical acoustic metamaterial absorber with multiple structural parameters that facilitate high sound absorption at low frequencies. In the design process, the parameters were adaptively adjusted to improve the sound absorption performance at low frequencies using only 37 evaluations, and this high absorption performance was verified by the agreement of numerical and experimental results. Because of its low cost, high flexibility, and independence from physical models, this method paves the way for tremendous opportunities in the design of various AMs for particular desired functionalities.

Acoustic behaviors of the microperforated panel absorber array in nonlinear regime under moderate acoustic pressure excitation

The acoustic performance of a microperforated panel (MPP) absorber array in the nonlinear regime is investigated both numerically and experimentally. The MPP absorber array is constructed by three parallel-arranged MPP absorbers with different cavity depths. A finite element model is used to simulate the acoustic response of the MPP absorber array by adopting the nonlinear impedance model. The results show that the absorption of the MPP absorber array is affected by the incident sound pressure when it is beyond around 100 dB. With appropriate structural and perforation property of MPP, the MPP absorber array in non-linear regime outperforms that in linear regime due to the improvement of equivalent acoustic impedance matching with ambient air over wide frequency range. However, when the sound pressure excitation is too high, the local resonance effect of the resonating component MPP absorber is diminished and the sound absorption is decreased. With the carefully chosen properties of MPP, the performance degradation induced by panel vibration can be avoided. An optimal set of MPP properties to avoid the performance degradation induced by panel vibration is determined. The measured normal absorption coefficients of a prototype MPP absorber array compare well with the numerical prediction in both linear and nonlinear regimes.

Sound Insulation in a Hollow Pipe with Subwavelength Thickness

Suppression of the transmission of undesired sound in ducts is a fundamental issue with wide applications in a great variety of scenarios. Yet the conventional ways of duct noise control have to rely on mismatched impedance or viscous dissipation, leading the ducts to have ventilation capability weakened by inserted absorbers or a thick shell to accommodate bulky resonators. Here we present a mechanism for insulating sound transmission in a hollow pipe with subwavelength thickness, by directly reversing its propagating direction via anomalous reflection at the flat inner boundary with well-designed phase profile. A metamaterial-based implementation is demonstrated both in simulation and in experiment, verifying the theoretical prediction on high-efficient sound insulation at the desired frequencies by the resulting device, which has a shell as thin as 1/8 wavelength and an entirely open passage that maintains the continuity of the background medium. We have also investigated the potential of our scheme to work in broadband by simply cascading different metamaterial unit cells. Without the defects of blocked path and bulky size of existing sound insulators, we envision our design will open new route to sound insulation in ducts and have deep implication in practical applications such as designs of ventilation fans and vehicle silencers.

Numerical analyses of the sound absorption of cylindrical microperforated panel space absorbers with cores

Microperforated panels (MPPs) are next-generation absorption materials because they can provide wideband sound absorption without fibrous materials and can be composed of diverse materials to meet global environmental demands. The fundamental absorbing mechanism is Helmholtz-resonance absorption due to perforations and an air cavity. MPPs are typically backed by rigid flat walls, but to reduce the restrictions on the MPP absorber properties, one of the authors has proposed MPP space sound absorbers without backing structures, including three-dimensional cylindrical microperforated panel space absorbers (CMSAs). Advantages of MPPs without backing structures are design flexibility and ease of use. Besides, the absorption characteristics of a CMSA with a core, which has a rigid cylindrical core inside the CMSA, have been experimentally tested, but a method to predict the absorption characteristics is necessary to design CMSAs with cores. Herein the two-dimensional combined Helmholtz integral formulation method is employed, and its prediction accuracy is evaluated by comparing the measured and predicted absorption characteristics of a CMSA with a core. Furthermore, a parametric study with regard to the core size is carried out to investigate the transition of the absorbing mechanism.

Hybrid silencers with micro-perforated panels and internal partitions

A sub-structuring approach, along with a unit cell treatment, is proposed to model expansion chamber silencers with internal partitions and micro-perforated panels (MPPs) in the absence of internal flow. The side-branch of the silencer is treated as a combination of unit cells connected in series. It is shown that, by connecting multiple unit cells with varying parameters, the noise attenuation bandwidth can be enlarged. With MPPs, the hybrid noise attenuation mechanism of the silencer is revealed. Depending on the size of the perforation hole, noise attenuation can be dominated by dissipative, reactive, or combined effects together. For a broadband sound absorption, the hole size, together with the perforation ratio and other parameters, can be optimized to strike a balance between the dissipative and reactive effect, for ultimately achieving the desired noise attenuation performance within a prescribed frequency region. The modular nature of the proposed formulation allows doing this in a flexible, accurate, and cost effective manner. The accuracy of the proposed approach is validated through comparisons with finite element method and experiments.

Modeling vibroacoustic systems involving cascade open cavities and micro-perforated panels

While the structural-acoustic coupling between flexible structures and closed acoustic cavities has been extensively studied in the literature, the modeling of structures coupled through open cavities, especially connected in cascade, is still a challenging task for most of the existing methods. The possible presence of micro-perforated panels (MPPs) in such systems adds additional difficulties in terms of both modeling and physical understanding. In this study, a sub-structuring methodology based on the Patch Transfer Function (PTF) approach with a Compound Interface treatment technique, referred to as CI-PTF method, is proposed, for dealing with complex systems involving cascade open/closed acoustic cavities and MPPs. The co-existence of apertures and solid/flexible/micro-perforated panels over a mixed separation interface is characterized using a compound panel subsystem, which enhances the systematic coupling feature of the PTF framework. Using several typical configurations, the versatility and efficiency of the proposed method is illustrated. Numerical studies highlight the physical understanding on the behavior of MPP inside a complex vibroacoustic environment, thus providing guidance for the practical design of such systems.

On the modeling of sound transmission through a mixed separation of flexible structure with an aperture

Modeling sound transmission among acoustic media through mixed separations, consisting of both rigid/flexible structures with apertures, is a challenging task. The coexistence of both structural and acoustic transmission paths through the same coupling surface adds system complexities, hampering the use of existing sub-structuring modeling techniques when the system configuration becomes complex. In the present work, a virtual panel treatment is proposed to model thin apertures involved in such complex vibroacoustic systems. The proposed virtual panel considers an aperture as an equivalent structural component, which can be integrated with the solid/flexible structure to form a unified compound interface. This allows handling the entire compound interface as a pure structural element, thus providing an efficient and versatile tool to tackle system complexities when using sub-structuring techniques. The accuracy and convergence of the method are investigated and validated, and the effective thickness range allowing for the virtual panel treatments is determined. The capability and the flexibility of the proposed formulation are demonstrated through several numerical examples, with underlying physics being explored.