Porous materials such as glass wool are widely used in aircraft fuselage insulation systems for their sound absorption performance. In operational aeronautical environments, their acoustic performance may differ from that measured under nominal laboratory conditions due to several factors such as protective coverings, installation procedures and moisture variations occurring during flight operations and throughout the aircraft service life. Despite their practical relevance, the effects of these non-nominal conditions on sound absorption variability remain insufficiently characterized. This study investigates the frequency-dependent sound absorption coefficient of aeronautical glass wools under controlled non-nominal conditions by combining impedance tube measurements, machine-learning techniques and Shapley additive explanations (SHAP). The investigated factors include material type, protective layers, relative humidity, humidity cycling and both controlled and operator-dependent mounting configurations. Results indicate that material, layer, relative humidity and humidity cycling significantly influence the acoustic response, with moisture-related effects exhibiting a strongly frequency-dependent behavior concentrated within three distinct frequency bands. Installation-related effects are also found to produce identifiable spectral variations, with operator-dependent mounting conditions generally associated with higher absorption levels and shifts of the peak response toward lower frequencies. The results further indicate that more controlled and uniformly distributed contact conditions improve measurement repeatability and reduce installation-induced variability.

The objective of similitude theory is to establish scaling conditions and laws by which the response of a given system can be scaled to infer that of another, distinct system. Exact similitude laws for the structural acoustic response of thin cylindrical shells exist only in the trivial case where length, radius and thickness are scaled equally, because of the coupling between in-plane and transverse deformations. However, under the bending approximation, which applies when bending wavelengths are one order of magnitude smaller than the length and radius of the shell, thickness can be scaled differently to length and radius. This work focuses on similitude laws and conditions for the structural acoustic response of fluid-loaded cylindrical shells, under the bending approximation. The scaling parameters include the shell dimensions, material parameters and the properties of the acoustic domain. A criterion for validity of the bending approximation based on ring frequencies of the reference and scaled in vacuo shells is proposed. In the case of fluid-loaded cylindrical shells, new similitude conditions and laws are derived for exact scaling of the modal radiation impedance. When these conditions are satisfied, satisfactory re-scaling of both the spatially-averaged vibration response and the radiated sound power is obtained.

Characterising the acoustic properties of porous materials is essential for predicting the effectiveness of acoustic treatments in rooms, vehicles and aircraft interiors. Impedance tube measurements are commonly used to determine acoustic properties such as the sound absorption coefficient (SAC); however, their reproducibility is often affected by non-standardised specimen preparation procedures. In particular, deviations from the nominal tube diameter and other geometric inconsistencies may alter the measured acoustic response. This study investigates the influence of specimen thickness, diameter, rotational angle and material on the SAC curves. A data-driven classification framework is employed to assess whether these factors leave distinguishable signatures in the SAC, using both univariate and multivariate formulations. Model interpretability tools are used to identify the frequency ranges most sensitive to variations of each factor, enabling a physically interpretation of the results. The analysis demonstrates that variations in diameter, material and thickness produce distinct and measurable modifications of the SAC, whereas the rotational angle has a negligible influence. While the effects of thickness and material are well established in the literature, diameter deviations of 2% are shown to significantly alter the acoustic response, highlighting the role of edge-related phenomena. The higher multivariate performance further suggests the presence of interaction among factors that are not captured by single-factor analyses. The SHAP-based analysis, identifies the low-frequency band ( < 600 Hz) as the most sensitive to variation in SAC for the factors under investigation.

This study presents a comprehensive experimental and numerical investigation of 3D-printed, locally reacting acoustic liners that embed coiled quarter-wavelength resonators behind a multi-perforated plate. The liners are tested under grazing incidence in an impedance tube.Numerical benchmarking is first carried out to identify a modelling strategy that achieves an optimal trade-off between physical fidelity and computational cost. The validated strategy is then used to compare three liner designs against impedance tube measurements under grazing incidence. A reference configuration is subsequently analysed in detail to investigate the physical mechanisms governing the multiresonant response, and to investigate the manufacturing effects by comparing different 3D-printing technologies, materials, and wall thicknesses. The results indicate that resonance frequencies are primarily controlled by the geometry of the unit cell, according to theory, whereas transmission loss (TL) amplitude and bandwidth may be influenced by specimen dependent non-ideal effects associated with manufacturing and experimental realization.Finally, water related effects are assessed experimentally through two controlled scenarios: water intrusion into the labyrinth and a surrogate partial occlusion of the perforations aimed at reproducing a distributed reduction of effective open area. Both perturbations substantially alter the acoustic behaviour, leading to resonance shifts and performance degradation.

Space exploration has long captured the human imagination, yet humans have spent only a few cumulative days beyond Earth’s surface during the Apollo program. As today’s missions aim to stay longer on the Moon and Mars, the need for reliable, long-lasting habitation structures has become a key design challenge. Extraterrestrial habitats are engineered systems designed to support human life beyond Earth. This paper provides a multidisciplinary and design-oriented analysis of pressurized inflatable habitats for surface applications on the Moon and Mars, with particular focus on the structural and habitability requirements imposed by partial gravity. The article begins with an analysis of lunar and Martian environments and their impact on habitat design and local resource utilization. It then traces the historical development of inflatable habitats from 1952 to the present, shaped by technological and geopolitical factors. A comparative study of materials, morphology, and deployment strategies follows, while current numerical and experimental validation methods are reviewed to provide a framework for future habitat development.

Impedance tube measurements are a widespread method for determining the sound absorption coefficient (SAC) of porous materials for normal sound incidence. The measurement method is standardised in ISO 10534-2. However, the standards offer limited guidance on sample preparation and mounting. Many impedance tubes have a circular cross-section and can be mounted at various angles. Therefore, it is crucial to investigate the influence of the mounting angle on the SAC and its interaction with diameter imperfections and different thicknesses using different materials. Following the dataset FOAM 01 this work documents the creation of a new dataset (called FOAM 02) containing SAC measurements from the ISO 10534-2 two-microphone method. The following factors are considered: two different materials, four thicknesses, three diameters, and four rotation angles. For each combination of material, thickness, and diameter, three specimens are produced, resulting in 72 specimens. Each specimen is measured three times at each rotation angle, yielding 864 SAC measurements. The dataset contains the SAC measurements and one-hot encoded label vectors, and is publicly available. A cutting device is proposed to saw cylindrical specimens accurately on a band saw with variable thickness and diameter. The workshop drawings of the cutting device are available as supplementary material.

Any transportation vehicle is inevitably subjected to turbulent air-flows, resulting in different challenges, including structural vibrations, noise emissions, structural fatigue, and passenger discomfort. These complex, multidisciplinary issues necessitate the development of efficient solutions for predicting and managing the effects of stochastic loads, such as Turbulent Boundary Layer (TBL) excitation, on diverse engineering structures. In literature, several alternative methodologies have been introduced, all aimed at providing an “off-line” experimental approach to predict vibrational responses without the need for wind tunnel testing. This work investigates into another innovative method known as the Pseudo-Equivalent Deterministic Excitation method (PEDE _M ). It analyses the evolution of PEDE _M over the past decade since its inception at the inaugural FLINOVIA conference in 2013. The study details a methodology for the experimental application of PEDE _M , named X-PEDE _M , along with its inherent properties and advantages. By subjecting this methodology to both experimental and numerical testing, it aims to validate its efficacy. X-PEDE _M offers the potential to predict the structural responses to turbulent air-flows and to manage the experimental set-ups needed for a final testing of any designed structure.

The acoustic modeling of materials is of paramount importance for the accurate prediction of untested configurations and the development of optimal manufacturing strategies. Nevertheless, this task presents significant challenges due to the complexity of the parameters governing materials, such as porous media, each of which demands distinct experimental setups and procedures, often difficult to implement. Inverse methods for parameter estimation rely on physical approximations and require experimental protocols that are highly sensitive to boundary conditions, rendering the process both expensive and time-consuming. This study investigates the efficacy of machine learning techniques, particularly artificial neural networks, in determining the parameters of the Johnson-Champoux-Allard (JCA) model for porous samples, with a specific emphasis on the mutual influence of input features on network performance. Building upon these insights, a hierarchical artificial neural network-based procedure is proposed and validated using experimental data to predict the JCA parameters with minimal error.

Ship machinery generates significant noise levels, mainly including energetic and low-frequency tonal components, posing two issues. The first is linked to potential health and safety problems related to onboard noise, mainly for the crew working in the engine room. The second concern is that the underwater noise generated by machinery can harm marine life. Conventional sound-absorbing materials are hardly efficient in mitigating low-frequency tonal components. This study introduces multistructured acoustic resonators for machinery noise attenuation. These resonators are based on either Helmholtz resonators, labyrinthine quarter wavelength tubes, or spiral quarter wavelength tubes embedded into a broadband soundproofing material. Design elements are provided for each resonator type, and their effectiveness in reducing machinery noise is evaluated using numerical simulations and tests primarily conducted using a low-frequency impedance tube and a reverberant room. The subsequent validation steps and perspectives are finally summarized.

The aim of this work is to provide in-depth examination of metal-composite hybrid gears that successfully merge significant weight reduction with improved or maintained Noise, Vibration, and Harshness (NVH) properties. A nonlinear finite element method was employed for designing hybrid gears, optimizing mass reduction and Static Transmission Error (STE) as main source of excitation. Experimental methods, including static and dynamic transmission errors, run-ups, and transmitted vibrations were analyzed to compare hybrid gear pairs with their steel counterpart. Results demonstrated significant discrepancies in vibration characteristics between standard and hybrid gear pair configurations. Notably, the hybrid configurations, particularly the innovative solution incorporating a viscoelastic layer, showed superior performance in vibration dampening despite being 50% lighter than its steel counterpart. The research underscores the inherent differences between hybrid and standard gears, relating the observed behaviors to static and dynamic excitations and their impact on the energy distribution between meshing harmonics. Findings suggest that hybrid gear pairs offer a potential pathway for improving noise and vibration comfort in mechanical systems without compromising on weight reduction.

The study focuses on the design of an acoustic liner to tackle noise from modern turbofan engines, particularly addressing broadband components. Introducing innovative locally-reacting liner designs based on coiled quarter wavelength tubes, the research compares three proposed models (B, C, D) with a conventional liner (A) available in literature. Sound absorption results in impedance tube reveal that model B, featuring a six-branch labyrinth resonator, outperforms the reference model in sound absorption, notably at lower frequencies, but with an excessive reduction of the perforation ratio. Model C helps reaching the best compromise since it has almost the same perforation ratio with respect to the reference and high sound absorption at lower frequencies with respect to model A. Significantly, model D achieves superior performance with only one-third the thickness of the traditional liner. Grazing impedance tube simulations compare the sound tranmission loss of the proposed models, showing that model C can reach comparable performance but at lower frequencies respect to model A, and that model D can replace model A but with 1/3 of the thickness. The study emphasizes the potential of coiled quarter wavelength tubes in mitigating tonal noise, offering a transformative solution for quieter and more sustainable aviation. Future steps involve sample 3D printing with experimental validation in impedance tube and under grazing conditions, with and without flow, to estimate insertion loss.

Subcritical damage in sandwich panels refers to damage that occurs in the skins or core of the panel due to loading below the ultimate strength of the materials, and it can reduce the strength and durability of sandwich panels, leading to premature and unpredicted failure. In this work, a computational tool was developed for designing sandwich structures and optimizing their subcritical impact response. The model was validated experimentally on sandwich panels comprising an aluminium honeycomb core with carbon fiber reinforced epoxy skins. The study investigates subcritical damage in aluminium honeycomb and CFRP sandwich panels through mechanical experiments and finite element simulations. A novel computational tool was developed to optimize the impact response of these structures. Experimental validation was performed using standardized tests, and numerical simulations were conducted using Abaqus. Key findings indicate that the developed tool accurately predicts damage and stiffness reduction, offering significant potential for optimizing sandwich panel designs.

This study proposes a novel semi-active vibration control technique for mechanical transmission systems. The backbone of current research technology is the adaptive stiffness and damping properties of metal rubber (MR) which plays the role of tuning to the vibrational system eigenmodes. The adaptive properties of MR material were proved by a previous study, but the present research investigates the effect of this feature on the structural response and designs an MR-based semi-active vibration controller for industrial application. For this purpose, the semi-active control strategy is created based on an optimal tuning using experimental harmonic test results. The control strategy is applied in practice using a linear actuator controlled by a double pole double throw (DPDT) relay, and National Instruments hardware and software for reading data and writing the control task. The isolator attachment position is defined through the analysis of eigenmode shapes that are predetermined by numerical simulation. A maximum isolation rate of 55.17% is measured near the vibration isolator position. It is observed that the isolation position has a prominent effect on structure vibration behavior where the elastic wave has been reduced in amplitude by passing the isolator position. The effectiveness of the MR-based semi-active control approach is eminent and its application for vibration isolation of commercial mechanical transmission systems could be studied in the future.

The concept of similitude is now widely applied in the engineering vibroacoustics applications since it is particularly useful when applied to the testing of models with increasing complexity.The similitude schemes allow predicting the dynamic response of an original system by using the information obtained from a similar one (defined as avatar or replica according to the partial or complete degree of similitude).In addition to geometric similarity, the focus on damping similitude has increased in recent years: this because the damping represents a highly relevant property during the design phase of any structure.Every material has an intrinsic damping: for example, generally, the damping in metal structures, as aluminum alloys, is lower and this results in high resonances of vibrations.Moreover, the damping evaluation for composite structures is even more complicated because it depends on concomitant factors: lay-up sequence, the type of fibres and resin and the manufacturing process.In this view, it could be helpful to develop a criterion of design evaluating the damping variation from a structure to another and verifying if this relative damping variation can validate with experimental tests.

Reducing underwater noise pollution from ship machinery is a significant challenge. Ship machinery usually operates at fixed speeds and emits tonal noise with large amplitudes at low frequencies. Conventional soundproofing materials are inadequate for absorbing tonal noise and require large thicknesses at low frequencies. Quarter-wavelength resonators effectively absorb sound at their fundamental frequency and odd harmonics. Still, the applicability of this solution is nevertheless limited by its length requirement, which becomes cumbersome at low frequencies and, thus, large wavelengths. This study explores different structured metamaterial designs based on labyrinth, coiled quarter-wavelength resonators, and hybrid configurations combining glass wool and coiled resonators. Analytical, numerical calculations, and experimental tests are carried out under normal plane wave incidence (using an impedance tube) and a diffuse acoustic field (in a small reverberant cabin). In particular, a numerical optimization based on a periodic unit cell model is used to optimize the hybrid configuration and analyze its behavior under variable plane wave incidence angles. Preliminary tests conducted in a water basin using a small, straightforward aluminum box equipped with some proposed designs indicate reductions in underwater noise levels. The proposed solutions offer limited-cost and compact solutions for mitigating machinery noise and, potentially, the preservation of marine ecosystems.

This work is concerned about employing a semi-active metal rubber (MR)-based isolator to suppress noise and vibration for vehicle transmission systems. The adaptive stiffness and damping properties of MR material play a key role in the semi-active control effect. In this regard, a particular automotive transmission component—precisely, a plate dedicated to accommodating bearings—has been selected as the focus of research to control its noise and vibration. A microcomputer-driven control system is applied, featuring characteristics such as high performance, compact size, lightweight design, and energy efficiency, which make it ideal for testing in the automotive sector. The efficacy of the devised control scheme is assessed through the deployment of accelerometers and sound intensity probes to quantify the vibrations and the radiated acoustic power, respectively. Interesting noise and vibration reduction rates are achieved using both open-loop and closed-loop strategies. This research opens the horizons to employ MR material in a semi-active configuration for industrial requirements surpassing its classical passive implementation, notably in terms of broadband attenuation performance.

Noise pollution is one of the most widespread problems in modern society and constitutes real danger to the human health. In particular, the reduction of interior noise levels in the transportation industries remains a big problem to cope with. Commercial sound absorber materials that have high values of the sound absorption coefficient are usually porous, they exhibit good sound absorption properties in the high frequency range but little in the low and medium frequency range (250-1600 Hz). Same condition but in different range of frequency applies to the other two types of sound-absorbing systems such as resonant cavities and membrane systems. In nature, there is no material that can cover a wide range of frequencies with a good absorption value, which is why metamaterials were born. Acoustic metamaterial are artificial structures, with unconventional effective properties. In these work an attempt was made to join the trends of typical absorption coefficients of the mentioned structures into a single 3D printing sample, characterized by light and equally strong.

This work is centered on the design and development of a novel honeycomb structure configuration that is industrially feasible, aiming to create increase the acoustic performances keeping the mechanical properties. In particular, the goal of this study is to improve the sound absorption coefficient of this structure at lower frequencies. The reference case for this study is the sandwich panels utilized in the construction of aero-engine liners, consisting of a rigid plate at the bottom, a honeycomb core, and a micro-perforated panel as the upper skin to facilitate interaction with flow. In order to ensure the comparability of the proposed configuration with commercially available counterparts, the external macro geometry of the panel remains consistent. Notable variations, as illustrated in the presented models, are confined to the internal structure of the honeycomb. To validate the numerical results obtained in Comsol Multyphisics thus far and to explore the implications arising from the reduction in the number of inlet holes characteristic of this new structure, experimental tests are planned. These tests will serve to corroborate the numerical findings and contribute to a comprehensive understanding of the performance of this innovative honeycomb configuration.