Sound Transmission Loss Research Articles

Computation of the sound transmission loss of heterogeneous periodic structure using the wave finite element-based methodology

In the present study, a Wave Finite Element-based method is introduced to estimate the Sound Transmission Loss (STL) of periodic structures with contrasting properties. This method addresses the precision and computational speed limitations of existing methodologies. It calculates the wave STL by applying a plane wave excitation from the ambient medium to a Unit Cell (UC), which is representative of the periodic structure. The STL under Diffuse Acoustic Field (DAF) is then computed by integrating the wave STL over all possible incidence angle ranges. The advantages of this methodology are demonstrated through various configurations, ranging from simple plates to complex laminated structures. Additionally, the study investigates the Mass Law of STL.

Investigating the acoustic performance of sustainable composites utilizing recycled polypropylene and denim shoddy: fabrication, characterization, and theoretical Modelling

Addressing the urgent need for sustainable materials, this study investigates the development of eco-friendly composites using Recycled Polypropylene (RPP) and Denim Shoddy (DS) for sound insulation. Two configurations were fabricated through compression molding: one with only RPP and the other incorporating a DS interlayer, repurposing waste materials such as post-consumer cosmetic containers and denim textile waste. The investigation evaluated the composites' density and sound transmission loss (STL). Theoretical calculations of STL in diffuse fields across various angles assessed the composites' real-world applicability. Incorporating a DS interlayer significantly improved sound insulation properties, with the composite exhibiting a higher density of 916 kg/m³ compared to 891 kg/m³ for Neat RPP. STL measurements showed that the composite achieved approximately average STL of 39.0 dB under normal incidence and 30.8 dB in the diffuse field, compared to 37.7 dB and 30.6 dB for Neat RPP, indicating a positive correlation between increased density and enhanced sound insulation. These findings highlight the potential of using recycled and waste materials for eco-friendly, sound-insulating material. The developed materials show promising applications in highway noise barriers, generator insulation, and office cabin partitions. This research contributes to sustainable materials science by demonstrating innovative sound-insulating composites' environmental and practical benefits.

Comparison of sound insulation performance between closed-cell foam and automotive carpet

In developing sound insulation materials for automobiles, multiple sound insulation and sound absorption materials are laminated to achieve high performance. Generally, steel sheets, needle felts, and carpets are used to create sound insulation properties. In a luxury car, multiple layers with sound absorption and sound insulation properties are placed on the carpet to add higher sound insulation properties, but this increases the mass of the carpet. To solve this problem, we are investigating the the potential to realize both lightweight and high sound insulation properties using closed-cell form. In this study, we experimentally evaluated the sound transmission loss of closed-cell foam by comparing it with those of carpets with the same configuration used in standard and luxury cars.Furthermore, a numerical study is conducted. As a result, the closed-cell foam obtained higher sound transmission loss than the automobile specification carpet in a wide frequency range from 400 Hz to 5000 Hz. On the other hand, the sound transmission loss decreased by the same degree for both the automotive carpets and the closed-cell foam at 250 Hz.

Investigating the acoustic performance of metamaterials with internal simple expansion chamber structure

The tunable acoustic properties are possessed by acoustic metamaterials (AMMs) with internal configurable structures. Nine samples(B1-B9) were designed in SOLIDWORKS and then fabricated using 3D-printed photopolymer resin featuring an internal Simple Expansion Chamber (SEC) structure. SEC length varied from 2 to 18 mm, maintaining a constant area expansion ratio (m) of 16. Sound Absorption Coefficient (SAC) and Sound Transmission Loss (STL) were measured through the Impedance Tube Apparatus (ITA). The sample with a 4 mm SEC length exhibited a maximum SAC of 0.99 at 4.8 kHz. As SEC length increased peak SAC and peak absorption frequency linearly decreased. Across all samples, STL ranged from 1- 41 dB (0.5-2.4 kHz) and 20-36 dB (2.4-6 kHz) revealing a periodic pattern in the tested frequency range. With SEC increasing from 2 to 18 mm Peak STL decreased from 41.2 to 38.8 dB. This research contributes valuable insights into the design of acoustic materials for effective noise attenuation showcasing the potential for achieving at least a 20 dB reduction in the 2.4-6 kHz range.

Acoustic performances of earthen walls, review of modeling approaches and tools

Earth construction benefits of a renewed interest particularly for environmental reasons. However, due to its mainly artisanal construction process, acoustic characterization data of earthen walls are quite rare which can limit their use for projects involving acoustic requirements. Moreover, the great diversity in materials as well as in related construction techniques could result in a large spread in terms of acoustical performances. In this context, acoustic characterizations and modelings are necessary to provide a better understanding on earth materials behaviors. In this study, the results from different modeling approaches are compared to few earth walls sound transmission loss data publicly available. The modeling tools including simplified formulas or more elaborated softwares present different levels of complexities and assumptions sometimes highlighting the specificities of this material. Depending on the construction techniques used (earth blocks, light earth, earth panels etc ...), different assumptions can be appropriate. Extrapolations of performances on several walls for both sound transmission loss and sound absorption provide information regarding tendencies and potentially achievable performances for future earth construction projects. Nevertheless, the comparison between the modeling approaches allows gaining insight on the limits of some approaches and the necessary critical analysis before a possible generalization to other earth walls.

Impact of air gap thickness on acoustic metamaterials with internal simple expansion chamber structure

The air gap thickness plays a significant role in enhancing sound absorption. This study explores the role of air gap thickness in enhancing sound absorption in 3D-printed acoustic metamaterials (AMMs) featuring a Simple Expansion Chamber (SEC) structure. Nine samples (B1 to B9) were designed and then 3D-printed with varying SEC lengths (2-18 mm) maintaining an area expansion ratio of 16. An air gap (5-30 mm) was introduced between each sample and the rigid backing. The Sound Absorption Coefficient (SAC) was measured using the Impedance Tube Apparatus. Results showed a notable increase in peak SAC from 0.57 to 0.96 with a 5 mm air gap for a sample with a 2 mm long SEC. However, this effect did not replicate across other samples, suggesting the peak SAC was unaltered by further variation of the air gap. Additionally, the peak absorption frequency exhibited an exponential reduction with increasing air gap thickness. This research illuminates the nuanced relationship between air gap thickness and the acoustic performance of 3D-printed materials, offering insights for optimizing sound absorption in diverse applications. Keywords: Air gap thickness, Simple Expansion Chamber, Area expansion ratio, Sound Absorption Coefficient, Sound Transmission Loss

Estimation of random incidence sound transmission loss of panels based on normal incidence sound transmission loss measurements

The performance of building sound insulation is an important parameter for evaluating the quality of construction projects. So far, the sound insulation performance testing of building walls requires the installation of large-dimensioned wall samples in specialized sound insulation laboratories, which is time-consuming and costly. If small-dimensioned components of the wall panels can be applied and quickly tested at the project site, it could significantly improve efficiency and reduce costs. To address this issue, this paper investigates a method for estimating the random incidence sound transmission loss of wall panels based on the measurement of the normal incidence sound transmission loss of corresponding small samples. Initially, the sound transmission loss of single-layer and composite structures is theoretically analyzed based on the theories of Equivalent Single Layer method and Layer-Wise method. Subsequently, the random incidence sound transmission loss is estimated by the normal incidence sound transmission loss. The methodology for estimating the random incidence sound transmission loss of wall panels proposed in this study offers both a practical approach and theoretical support for the rapid assessment of the sound insulation performance of building walls. This study is of great significance for improving the quality of construction projects and inspection efficiency.

Accounting of imperfect interfaces in multilayer modeling

Multilayer systems are widely used in industry for their performances as compared with homogeneous materials. The layers are usually glued together but the bonding may be imperfect (air bubble, detachment). The interfaces are generally modeled as sliding or bonded. These two behaviors can be significantly different and the actual behavior is generally somewhere in between. The vibroacoustic behavior of the structure is thus modified by the imperfect interface. Several models have been implemented to describe the dynamic behavior of multilayer systems with imperfect interfaces. In this paper, we propose an analytical model of imperfect interfaces based on the Transfer Matrix Method (TMM). Two computations are performed in parallel (one with a perfectly bonded interface and the other with a sliding interface). Instead of mixing impedances or admittances matrices, a mixing law on the state variables is applied, resulting in an equivalent global matrix which can be coupled in any multi-layer. The model is compared in terms of the sound absorption or sound transmission loss with the existing ones and is applied on different classical multilayer systems with multi-layered solid plates and a multilayer composed of a porous layer with a heavy layer which is widely used in the automotive industry.

Noise control in ducts using Herschel-Quincke acoustic metamaterial array

Acoustic resonators, like Helmholtz, side branch tubes, expansion chambers, Herschel-Quincke, and so on, are usually used to control duct noise in acoustic systems. Noise attenuation in local resonators occurs due to impedance mismatch of incident, transmitted, and reflected waves induced by the addition of resonators. The frequency band and the attenuation level generated by the resonator depend on its geometry and behave as a narrow-band filter. However, a much larger frequency band, called bandgap, is obtained if they are appropriately distributed (periodically or not) along the duct length. This paper investigates acoustic noise attenuation for an acoustic metamaterial using a Herschel-Quincke (HQ) resonator. It is modeled by the transfer matrix method and the finite element method. Also, a study about the arrangements of the HQ resonators (serial and parallel) is conducted to verify their accuracy and efficiency related to bandwidth enlargement and sound transmission loss. Simulated results are obtained for different examples and compared to those from the literature.

A study on road noise reduction using recycled material

Recently, there has been a shift in the paradigm of the automobile industry from internal combustion engines to electric vehicles. An NVH research has also evolved rapidly from studying noise and vibration in conventional internal combustion engines to exploring methods for reducing road noise. A road noise during vehicle operation can be amplified by the resonance of the cavity inside chassis. As a countermeasure, so, filler materials have been applied inside the chassis to reduce resonance, However, fillers applied to existing cavities, such as pads and polyurethane foam, generally focus on blocking resonance inside the cavity rather than reducing vehicle road noise. In this study, a filler has been developed to fill the cavities of vehicles; Filler resources are from residual scrap gathered in manufacturing shop floor of automobile interior and exterior parts. As a result, fillers introduced in the study offer not only excellent acoustic performance but also are recyclable. The effect in terms of acoustic performance of recycled fillers was confirmed by conducting tests, sound transmission loss in reverberation and anechoic chambers. Also, actual vehicle driving tests were conducted on a proving ground.

A study on the improvement of sound insulation performance of aluminum chassis

In recent years, the automobile industry has rapidly shifted towards electric vehicles, with an increasing number of vehicles using aluminum instead of steel for a significant portion or the entire or part of the vehicle body. Aluminum body panels reduce sound insulation performance compared to steel panels because they depend on surface density. This situation causes uncomfortable sound environment for vehicle passengers. To address the sound insulation issues associated with lightweight panels, it is necessary to research how to optimize the thickness and weight of interior and exterior car components in order to enhance sound insulation performance. In this study, the acoustic characteristics were initially confirmed by measuring indoor noise while driving a vehicle constructed from two different materials: steel and aluminum, under the same conditions as other parts. In addition, to select a sound absorbing material suitable for the characteristics of the vehicle body material, steel and aluminum panels were installed between the reverberation chamber and the anechoic chamber, respectively, and sound transmission loss was evaluated by combining the various thickness and weight. Based on these results, we decided on a proposed specification (aluminum panel with weight/thickness added) above the basic specification (steel panel with existing sound insulation parts).

Optimization of graded and partial metamaterial treatments for enhanced broadband sound insulation in partition panels

Resonant metamaterials can achieve unprecedented vibroacoustic attenuation in lightweight partition panels through small resonators attached on a subwavelength scale. Conventional metamaterial treatments are effective only in a narrow band and typically cover the entire panel surface, posing important challenges for industrialization due to manufacturing and cost constraints. To address these issues, in this study we propose graded and partial metamaterial treatments, which are selectively distributed where they are the most effective, avoiding unnecessary coverage, and include a spatially varying resonance frequency for broadband effectiveness. The related design challenges are tackled by developing a numerical methodology that simultaneously optimizes the distribution and the grading of the treatment. The optimization objective is to maximize the broadband diffuse sound transmission loss (STL) of the metamaterial panel, while constraining the maximum fraction of treated area. Leveraging effective medium modelling, the approach efficiently incorporates local changes in the effective mass density of the metamaterial panel to represent partial treatments. Different design cases are considered for different target bands and panel dimensions. Results demonstrate that graded partial metamaterial treatments can achieve broadband improvements with a limited treated area, effectively suppressing the STL dips due to the modes of the host panel and to coincidence effects.

Broadband low-frequency noise attenuation with parallel U-shaped membrane resonators

The attenuation of noise at low frequencies has been the subject of extensive research in the last two decades. Employing membrane-type acoustic metamaterials (MAMs) is one of the most promising solutions due to their lightweight structure and ease of fabrication. This study fabricated a metamaterial with four U-shaped cavities supporting a thin elastic nitrile rubber membrane. We print the U-shaped cavities using fused deposition modeling (3D printing). The sound absorption coefficient (SAC) is calculated using numerical simulations. For SAC computation, an electro-acoustic model is developed that constructs an equivalent electrical circuit based on an impedance analogy. Furthermore, the findings of the numerical and analytical analyses are supported by the actual data acquired from the experiments. We have determined that the average sound transmission loss (STL) is 29.84 dB, and the average SAC exceeds 82% within the frequency range of 500 Hz to 1000 Hz. Due to its lightweight, subwavelength thickness, ability to attenuate broadband noise, and very simple fabrication, this metamaterial is suitable for use in aircraft, automotive industries, and room/building acoustics.

Development of vibration isolation system for halls and report on sound insulation and vibration experiment

Rubber and polyurethane have been widely used as vibration isolation materials in halls and studios. In this project, we have developed a floor-mounted coil spring vibration isolator with improved vibration isolation and sound insulation characteristics, while keeping the installation height equivalent or lower than rubber and polyurethane. In this paper, we evaluate the vibration isolation performance of the developed device by measuring vibration transmission loss, and confirm that the device can suppress surging, which is a weak point of metal springs, while achieving a low natural frequency. The sound transmission loss measurement also confirmed that the floating slab incorporating the device has sufficient sound insulation performance, and these results indicate that the device can be applied to acoustic spaces such as concert halls, where quietness is required.

Wall-applicable metamaterial: mitigating resonance dip for enhanced sound transmission loss

In recent years, there has been a significant advancement in thermal insulation technology, allowing for the implementation of energy-efficient solutions in existing buildings while preserving their architectural and cultural significance. This recognition has led to the widespread adoption of External Thermal Insulation Composite Systems (ETICS). However, this brings the common challenge in acoustic engineering, the resonance dip, into the spotlight. When the frequency of an incident sound wave coincides with a mass-spring-mass resonance of the thermally insulated wall, the sound insulation drop occurs, possibly resulting in a reduction in transmission losses. This study introduces a wall-applicable design of Plate-type Acoustic Metamaterial (PAM) to effectively enhance sound transmission loss. We investigated in development a transfer matrix method combined with the effective mass density of PAM to model the ideal metamaterial wall and further compared it with the results based on mass-law methods. Design strategies to construct the ideal bandgap were developed by analyzing dispersion curves.

Design of lightweight aircraft trim panel with a high sound transmission loss

The trim panels of aircraft cabin are frequently made up of multilayer materials including a honeycomb, typically in the form of sandwich panels, which have the advantage of being lightweight and space-saving, but lack good acoustic insulation. It has been shown that, for 10% more mass, weights arranged in a fractal pattern in the honeycomb affect the vibratory pattern of a sandwich panel by creating multiple reflections of the bending waves on the inclusions. They thus generate a phenomenon of focusing vibratory waves providing localized modes while attenuating the structure's acoustic radiation. The objectives of this paper are to describe: first, the vibro-acoustic model of an overloaded homogenized bending panel (to mimic a trim panel with a fractal pattern); second, the physical phenomena associated to this concept; third, the impact on the modal behavior and the vibratory and acoustic responses.

A vibroacoustic model for low-frequency sound transmission loss in circular membranes with added strip mass

Noise control strategies involve physical barriers in the transmission path. Conventional barriers conform to the mass law which states an inverse relation between sound transmission, barrier mass, and incident frequency. Practical scenarios for low-frequency noise attenuation require prohibitively thick barriers making them infeasible. Mass-loaded membranes exhibit marked differences in their dynamic nature from that of bare membranes as they do not necessarily conform to the mass law. Being thin, compact, and lightweight makes them geometrically ideal for most applications. The paper presents an analytical model based on the Newtonian approach for the vibroacoustic behavior of a pre-stretched elastic circular membrane with rigidly attached strip mass under normal incidence. The point collocation approach distributes the effect of the strip mass on the membrane as a collection of discretized concentrated loads along the interfacial boundary resulting in a summation that is easier to solve. The peak and dip frequencies of sound transmission are determined for circular membranes with eccentric and central strip mass. The present study evaluates strip mass-loaded membranes' capability to attenuate noise at low frequencies as an unconventional physical barrier.

Sound transmission through an unbaffled plate

Commonly occurring complex structures can often be modelled as simple finite plates mounted without baffles. Thus, the objective here is to study the sound transmission loss of a simply-supported rectangular finite unbaffled plate impinged upon by an acoustic plane wave. The total pressure jump across the plate surface is expressed as the sum of the pressure jump due to pure diffraction, where the plate normal displacement is zero and the pressure jump due to plate radiation. These pressure jumps are computed using the definition of the free-space Green's function in the wavenumber domain and the Euler equation. Then, the coupled equation of motion is solved for the plate displacement. The method of stationary phase is used to derive the closed-form expression for the improper double integral that appears in the far-field acoustic pressure. Finally, the far-field transmitted power is numerically computed and the sound transmission loss of the unbaffled plate is compared with that of the baffled one.

Sound transmission loss of constrained layer damping composites featuring elastic plates embedded with acoustic black hole

This paper preliminarily investigates the influence of an elastic layer with an acoustic black hole (ABH) on the airborne sound insulation of a constrained layer damping (CLD) composite plate. ABHs represent a continuous impedance reduction in a local area of the structure, achieved by reducing material thickness. They efficiently dampen structure-borne sound in plate-like structures, resulting in excellent airborne sound attenuation performance. In the CLD composite structure, a damping layer is inserted between the elastic layers. This configuration achieves noise reduction and provides excellent airborne sound attenuation performance. By incorporating an ABH on one side of the elastic layer within the constrained damping composite structure and utilizing the damping layer as the terminal damping material for ABH, we achieve a lightweight structure with outstanding airborne sound insulation performance. Using finite element numerical calculations, we assess the airborne sound transmission loss of ABHs embedded within the CLD plate. Our analysis compares their transmission losses within the frequency range of 10 to 4000 Hz under normal-incidence and random incidence acoustic field excitations. While the ABH does not enhance the overall sound isolation performance of the CLD plate, it significantly improves the sound isolation dip at the resonance frequency.

Investigation of light earth sound transmission loss at the wall scale

A fine understanding of the acoustic dissipation in walls made from biobased concrete is needed to predict and design properly their performances and to create comfortable spaces. This understanding is one of the objective of LOB+HIE (LOW BETON / HIGH EARTH) project, funded by Ademe, and focusing on the use of light earth (a biobased concrete resulting from the mix between clay and hemp shiv) to meet acoustic requirements in a public worksite. Acoustic data are already available in the literature at the material scale for this kind of solution. The aim of the present work is to provide experimental reference data at the wall scale for sound transmission on full-scale vertical walls, and then to extend it to other configurations by numerical simulations. The work is organised in three parts, including the transmission loss measurements on two different walls, the modelling of the walls with numerical simulations using a transfer matrix method approach, and finally the extension by simulation to other wall configurations, with a view to producing design aids for use by building professionals.