Transmission Loss Research Articles

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.

Sound insulation analysis of sound insulation materials laminated with membrane vibration type acoustic metamaterial

In recent years, research on various acoustic metamaterials has progressed. We fabricated PP into a honeycomb structure and pasted films on the top and bottom to create an acoustic metamaterial that has a sound-absorbing effect due to membrane vibration. Furthermore, a small hole was made in the center of the film part to add sound absorption effect through Helmholtz resonance. Considering its application to automobile trim, felt and rubber layers were added and laminated on a steel plate. This structure was modeled using the finite element method and the transmission loss was calculated. We will report on comparisons with measurement results and calculation results for various lamination patterns.

Design and analysis of acoustic metamaterial sound insulator for noise reduction at multiple frequencies

In this paper, a design of an acoustic metamaterial sound insulator based on porous material with embedded multiple Helmholtz resonators is proposed for noise reduction at multiple frequencies. The periodic unit cell of the metamaterial is made of the porous layer with embedded four, nine, and sixteen different Helmholtz resonators that are arranged in parallel. The cavities of the resonators have the same volume while the parameters of the necks are varied. The sound absorption coefficient and the transmission loss of the metamaterial obtained using finite element method show four, nine and sixteen resonant peaks that correspond respectively to the number of Helmholtz resonators within the unit cell. When the parameters of the necks and the cavities are identical resulting in identical resonators, the transmission loss and the sound absorption coefficient show only one resonant peak. The proposed metamaterial can attenuate the noise at four, nine, and sixteen different frequencies by choosing the number of resonators that constitute the unit cell and adjusting the parameters of the necks. It can be used in many engineering applications such as aerospace for noise attenuation at multiple frequencies.

Double-tuning the inlet and outlet extension lengths of a family of straight-flow expansion chamber mufflers for broadband attenuation

This work presents a generalized end-correction expression for double-tuning the extensions of the concentric inlet and concentric/offset outlet ports of a family of straight-flow circular and elliptical expansion chamber mufflers as a function of geometrical parameters, which include the wall-thickness, ratio of chamber-to-port diameter, aspect-ratio, and the offset distance along the major-axis. To this end, first, the transmission loss (TL) performance of configurations with non-tuned extensions given by half and quarter chamber lengths at the inlet and outlet, respectively, was determined using 3-D FEA, which accounted for the multi-dimensional acoustic field at area-discontinuities. The effective lengths of the quarter-wave resonators at the inlet and outlet and the corresponding end-corrections were computed from the first and second resonance frequency peaks, respectively. The 3-D FEA showed that by considering geometric length equal to the effective inlet and outlet length, three-fourths of the troughs in the TL spectrum were nullified, thereby delivering a broadband attenuation. The 3-D effects were incorporated into the 1-D model through end-correction lengths. Parametric studies were conducted to examine the variation of end-corrections with different geometrical parameters, while the accuracy of the generalized end-correction expression was demonstrated by comparison against previous results.

Experimental assessment of isolation performance of a building above metro lines: a case study with dual isolation levels

This paper presents an assessment of the acoustic performance of a five-story residential building constructed directly above a metro line in Prague. The building has been isolated at two levels: first, on top of the distribution beams above the metro tunnel and secondly, at the ground floor. The assessment of the building's performance has been conducted by a direct measurement of ground borne noise radiated inside the rooms and evaluating the vibration transmission loss through the isolation system. The transmission loss assessment was carried out using two different test setups. Firstly, during train pass-byes, ground borne vibrations were simultaneously measured at various locations above and below the vibration cut. Secondly, at the same measurement points, the vibration levels were measured using hammer impacts. Finally, the transmissibility obtained from both test setups was compared and discussed. Additionally, ground borne noise levels were measured inside rooms on different floors above the vibration cut. To identify noise events exclusively associated with train pass-byes, data obtained from synchronized vibration measurements at the center of a room inside an apartment were used. Finally, the measured noise levels were compared with the limit values of noise exposure recommended by the Czech standards.

Effect of scatterer shape, opening angle and periodicity on sound attenuation by local resonating sonic crystal

Noise Control has become critical and demanding task for acoustic engineers due to continuous Technological developments in construction, transportation, and industries. These developments have raised the level of low frequency noise in densely populated areas. Unfortunately, there cannot be sufficient noise reduction with traditional acoustic materials. However, new research has shown that the manipulation of sound waves and drastic reduction in sound transmission can be possible using periodic structure known as Sonic Crystals (SnC). This study describes the eigenfrequency analysis to investigate the effect of different shapes, periodicities, and opening angles of scatterers in a Local resonating Sonic Crystal (LRSnC). The Block-Floquet boundary condition is applied to the unit cell of the LRSnC, using the Finite Element Method to study the complex acoustic wave interactions throughout the structure. An acoustic module has been applied to find the band gap and to simulate transmission loss spectra in COMSOL Multiphysics. Results show that shape, opening angle and periodicity effect the band gap and transmission loss significantly. Based on these results, it is possible to develop advanced SnC with a wider band gap and higher sound attenuation in the lower sound frequency range by carefully considering these factors.

Preliminary study on active control of double-glazing with pressure compensation using an in-cavity microphone

Enhancing the sound insulation in double-glazed units involves actively controlling inter-glazing cavity pressure to decouple each glass. The optimal strategy to maximize transmission loss requires loudspeakers in the cavity to generate anti-noise and error microphones in the reception room, outside of the cavity. However, this configuration is not suitable for a standalone product where out-of-cavity microphones are not allowed. This study presents an original method to experimentally learn the control of one loudspeaker with one error microphone, both inside the cavity, in order to maximize acoustic isolation in a low-frequency band. The method requires a compensation FIR filter, experimentally identified during a learning phase when an optimal command is applied. Experiments were conducted using an active symmetrical double-glazed unit (30.4 x 30.6 cm, 6 mm thick, 60 mm apart cavities) in a "waveguide" setup. Results show an average transmission loss gain from 50 to 550 Hz of 10 dB with in-cavity microphone compensation. It nearly reaches optimal performance, 11 dB reduction in transmitted waves, compared to 7 dB for the uncompensated in-cavity microphone. This encouraging preliminary result leads to further developments to extend the method to multiple loudspeakers and multiple microphones.

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.

Evaluating the measurement of the velocity level difference for building constructions which combine Type A and Type B elements

As part of an ongoing research project in the new four-room flanking facility at the National Research Council Canada, the velocity level differences between cross-laminated timber floors/ceilings and lightweight timber framed walls have been measured according to the standard, ISO 10848-1. The apparent sound reduction indices between the pairs of rooms have also been measured according to ISO 10848-1. The measured apparent sound reduction indices are compared to those predicted according to the standard, ISO 12354-1 based on the measured velocity level differences, the resonant transmission losses and the dimensions of the elements of the flanking paths between the room pairs.

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.

Optimization of a metal additively manufactured hydraulic silencer with array resonators

As mobile machines shift to electrification, hydraulic circuit noise, previously masked by combustion engine noise, has emerged as a significant concern. To address acoustic comfort issues, this paper investigates an innovative solution for high-performance noise reduction in hydraulic circuits. The proposed silencer design incorporates an optimized array of Helmholtz resonators, strategically arranged to broaden the narrow band characteristics of single resonators. The chosen configuration effectively reduces pressure pulsations across a broad mid-frequency spectrum. While this design is well-established in air medium, its application in a hydraulic oil medium is pioneering. Fabricated through metal additive manufacturing, the silencer undergoes testing on a dedicated bench. Transmission Loss measurements validate both the concept and the optimization methodology.

Low-frequency broadband acoustic ventilation meta-barrier based on consecutive multiple Fano resonances

Acoustic metamaterials utilizing Fano resonance provide the possibility of simultaneous airborne sound insulation and high-performance ventilation. However, the ultra-sharp line shape results in a narrow working frequency range, which is still insufficient for practical applications. In this work, we propose a low-frequency acoustic meta-barrier supporting consecutive multiple Fano resonances (CMFRs) with broadband (~75%) capability, efficient ventilation, and ultra-thin thickness, etc. The barrier features a binary metastructure, incorporating a chiral space coiling tunnel and a hollow pipe in a planar arrangement. This configuration offers discrete and continuous states within the composed Fano system, corresponding to the tunnel and the pipe, respectively. By means of superposition of multi-order Fano resonance modes, the destructive interference between the discrete and continuum states keeps effective in a broad frequency range, while simultaneously achieving high air permeability. Numerical results of transmission loss show significant attenuation of incident energy in the range of 479-1032 Hz. Our work lays the groundwork for next generation of Fano-based metamaterial applications, with far-reaching implications for noise control and related fields.

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.

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.

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.

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.

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.