Improvement Of Transmission Loss Research Articles

Improving low-frequency sound transmission loss of double panels with a plate-type acoustic metamaterial

The sound insulation performance of double-leaf partitions is crucially affected by the mass-air-mass resonance phenomenon, whose frequency reduction can shift the high sound insulation to lower frequencies. In this work, an alternative strategy for exceeding the low limit of the mass-air-mass resonance frequency of conventional double panels is proposed in terms of the effective surface mass density. An equivalent spring-mass model based on the effective medium theory is implemented to intuitively understand the characteristics. A simple double-panel metamaterial is designed by replacing one of the panels with a single-plate metamaterial attached strip masses to decrease the structural complexity. The extended semi-analytical method is applied to predict efficiently the sound transmission loss in the low-frequency range, which is verified by the coupled vibro-acoustic finite element method with excellent agreement. The results demonstrate that the proposed double-panel metamaterial can reduce the minimum of the resonant frequency without changing the areal mass density and panel spacing. The sound insulation can be significantly improved between the reduced mass-air-mass resonance and the anti-resonance frequencies. To provide a better understanding of the sound transmission loss improvement, parametric studies are conducted. The design strategy is validated through experiments conducted in an impedance tube, and the results corroborate the findings. This study is instrumental in designing double-panel partitions that require sound insulation in specific low-frequency regions.

High-Contrast Low-Loss Antenna: A Novel Antenna for Efficient Into-Body Radiation

We present a biocompatible high-contrast low-loss antenna (HCLA) designed for efficient into-body radiation for applications as diverse as medical telemetry, sensing, and imaging. The HCLA is wearable with a compact size of 2.62 cm3 and operates across the 1 to 5 GHz bandwidth. The quasi-bowtie antenna is loaded with a high-contrast (i.e., alternating layers of high and low permittivity materials) and low-loss dielectric to improve directivity and gain into the biological tissues. Measurement results at 2.4 GHz are in good agreement with simulations and show 5.72 dB improvement in transmission loss over the most efficient into-body radiator reported in the past. At the high end of the frequency bandwidth, simulation results for two antennas placed across each other with tissue in between show ~12.5 dB improvement in transmission loss. The HCLA is fabricated with stable, low-loss materials that allow for repeatability and consistency in the fabrication process, thus, addressing limitations of the current state-of-the-art. It is also made from biocompatible materials that enable it to be placed directly on the skin for real-world implementation. In this paper, we discuss the operation principle and design of the HCLA, its transmission performance, radiation patterns, and specific absorption rate.

Efficient Modelling of Acoustic Metamaterials for the Performance Enhancement of an Automotive Silencer

Significant potential for acoustic metamaterials to provide a breakthrough in sound attenuation has been unlocked in recent times due to advancements in additive manufacturing techniques. These materials allow the targeting of specific frequencies for sound attenuation. To date, acoustic metamaterials have not been demonstrated in a commercial automotive silencer for performance enhancement. A significant obstacle to the practical use of acoustic metamaterials is the need for low cost and efficient modelling strategies in the design phase. This study investigates the effect of acoustic metamaterials within a representative automotive silencer. The acoustic metamaterial design is achieved using a combination of analytical and finite element models, validated by experiment. The acoustic metamaterial is then compared with commonly used techniques in the silencer industry to gauge the effectiveness of the acoustic metamaterials. COMSOL simulations were used to validate the developed test rig and were compared to experimental results which were obtained using the two-load transmission loss test method. Through this testing method, the implementation of a labyrinthine metamaterial cylinder proved to be a significant improvement in transmission loss within the silencer, with an increase in transmission loss of 40 dB at 1500 Hz. The research has successfully shown that acoustic metamaterials can be used in practical settings, such as an automotive silencer, to improve the overall sound attenuating performance. The described analytical model demonstrates the potential for industrially relevant low cost design tools.

Improvement of transmission loss of bellows through thickness improvement and structural modification

The bellows of the vehicle are vulnerable to noise because of the low transmission loss among the components. Therefore, in this study, we modified the thickness and the structure of the bellows to improve transmission loss. Based on the impedance tube test, the transmission loss of the silicon rubber specimen – the main material of the bellows – was analyzed; the results confirmed low transmission loss in the low-frequency region. An analysis of the natural vibration model of the simple model indicated that in the low-frequency region of the bellows, a number of vibrations occurred because of the vibration of the outer and inner components. Accordingly, to improve transmission loss, the improvement introduced by varying the thickness was analyzed, and the results confirmed that the noise performance improved by more than 3 dB for a thickness of 3.5 mm in the double-layer structure. In addition, the transmission loss improved in the low-frequency region after acoustic-structure coupling analysis was performed based on a simple model. To compare the actual performance between the existing and improved bellows, a noise comparison test was performed. The test results confirm that the existing noise reduction index improved by 3 dB from 30 to 33 dB when the thickness was increased to 3.5 mm; in the frequency domain, the highest noise performance was improved with an improvement of 5.6 dB at 160 Hz.

Performance enhancement of an automotive silencer using acoustic metamaterial baffles

The potential for acoustic metamaterials to provide a breakthrough in sound attenuation has been unlocked in recent times due to advancements in additive manufacturing techniques. These materials allow the targeting of specific frequencies for sound attenuation. To date acoustic metamaterials have not been experimentally tested in an automotive silencer for performance enhancement. This study investigates the effect of acoustic metamaterials within a representative automotive silencer. These results are then compared with commonly used techniques in the silencer industry to gauge the effectiveness of the acoustic metamaterials. COMSOL simulations were used to validate the developed test rig and were compared to experimental results which were obtained using the two-load transmission loss test method. Through this testing method, the implementation of the acoustic labyrinthine DENORMS cylinder proved to be the most significant improvement in transmission loss within the silencer, with an increase in transmission loss of 40 dB at 1500 Hz. It was also found that the traditional hard wall internal baffle silencer performance can be improved by replacing it with a DENORMS internal baffle. Acoustic metamaterials have potential as a novel method for automotive companies to attenuate sound of specific frequencies.

Transmission Enhancement Methods for Low-Emissivity Glass at 5G mmWave Band

This letter presents transmission enhancement methods for low-emissivity (low-E) windows at millimeter wavelength (mmWave) frequencies. These methods are required for installation of effective indoor customer premises equipment for mmWave fixed wireless access (FWA) services. Low-pass filtering frequency selective surfaces are patterned with low-E coatings by laser, and the effects are verified theoretically and experimentally. Antireflection (AR) dielectrics are additionally utilized and found to reduce transmission loss in various types of window regardless of frequency and incident angle. The results of the current study achieve lower losses for various types of window and incident angles. A transmission loss improvement is also achieved in a real FWA scenario, which can be applied for the coverage extension of FWA services.

Transmission loss of plates with multiple embedded acoustic black holes using statistical modal energy distribution analysis

Acoustic black holes (ABHs) in beams and plates have been extensively studied as a passive method for vibration attenuation and noise reduction. However, most research to date has focused on analyzing the behavior of a single ABH structural element, using numerical or semi-analytical deterministic approaches. If ABHs are to be exploited for practical industrial applications, there is a need to characterize their performance in complex built-up structures and to describe them with statistical methods in the mid-high frequency range. This paper presents a first step towards this goal by employing statistical modal energy distribution analysis (SmEdA) to evaluate the transmission loss of ABH panels separating two air cavities. SmEdA splits vibroacoustic systems into subsystems and establishes power balance equations between the modes belonging to different subsystems. This avoids the energy equipartition assumption of traditional statistical energy analysis (SEA) and extends it to low modal overlap systems. In this work, the benefits of embedding ABHs on plates for noise reduction between cavities are predicted with SmEdA. The role played by the size, shape and number of ABH indentations on the plate are inspected, as well as the influence of other parameters like the truncation thickness, ABH order and damping. The effects of added mass and stiffness of the damping layer are investigated and it is observed that, with proper design, ABH plates can exhibit substantial transmission loss improvement in the vicinity of the critical frequency of uniform plates. Both resonant and non-resonant transmission are considered in the analysis.

Sound transmission through micro-perforated double-walled cylindrical shells lined with porous material

Sound transmission through a double-walled cylindrical shell with porous cores and the inner shell micro-perforated is studied analytically in this paper. The Donnell's thin shell theory is used to govern shell motions and the Biot's theory is applied to describe sound propagation in the porous layer. Boundary condition at the perforated shell is presented by the mean particle velocity model. When excited by an oblique plane wave with external mean flow, transmission losses through typical configurations are numerically computed. Results show that perforating the inner shell of different sandwich configurations would produce improvement of transmission loss below the ring frequency. In further analysis regarding the perforated configuration with two airgaps in annular space, it retains superiority against the corresponding non-perforated one in the low frequency range despite alteration of the incidence angle, the azimuthal angle or the external flow. In parametrical studies, increase of either the aperture diameter or the perforation ratio is validated to enhance low-frequency sound insulation of this configuration through the imaginary part of the impedance constant.

Broadband low-frequency sound transmission loss improvement of double walls with Helmholtz resonators

Helmholtz resonators are commonly used as narrowband sound absorbers in room acoustics applications. Previous research has shown that Helmholtz resonators can also be used to improve the sound transmission loss of double walls. The focus of this paper is the broadband improvement of the transmission loss of double walls in the low frequency region by tuning the Helmholtz resonators inside the cavity to frequencies lower or higher than the mass-air-mass resonance frequency of the double wall. A new analytical model using the effective material parameters (bulk modulus and density) of a fluid volume containing Helmholtz resonators is developed to describe the vibro-acoustic behavior of double walls with Helmholtz resonators. Using this model it can be shown that by tuning the Helmholtz resonators properly, the mass-air-mass resonance frequency of the unmodified double wall can be shifted significantly, leading to an improvement of the transmission loss of the double wall roughly between the mass-air-mass resonance frequency and the resonance frequency of the resonators. This improvement, however, comes along with a decrease of the transmission loss at high frequencies due to a decoupling of the Helmholtz resonators which is also covered by the proposed analytical model. Parametric studies are performed to identify relevant design parameters to optimize the transmission loss improvement by the Helmholtz resonators. Finally, experimental results of different double wall designs with integrated Helmholtz resonators are presented to validate the proposed analytical model and demonstrate the effectiveness under diffuse field incidence.

Optimal resonance-free third-order high-pass filters based on minimization of the total cost of the filters using Crow Search Algorithm

The most common damped filters (DFs) are the second-order, third-order, C-type, and double-tuned filters. Other DFs such as the first-order and band-pass filters exist, but their high operating losses considerably diminish their usage. In this paper, firstly, for the third-order damped filter with equal and unequal capacitors, the relations among their circuit parameters are derived. Secondly, the optimal design problem of the two third-order high-pass filters is formulated by regarding these expressions to minimize the filter cost taking into account both the investment and operating expenses. The total and individual harmonic distortion indices, power factor and the harmonic voltage amplification ratios which measure the filter’s resonance damping capability, are considered as constraints. A recent metaheuristic optimization technique based on the intelligent behavior of crows, known as the Crow Search Algorithm (CSA), is employed for the solution of the formulated design problem. Further, a comparative analysis of the two designs of the third-order high-pass filters and a third-order C-type filter is presented. The results reveal that all the proposed filters guarantee no electrical resonance hazards while maintaining the allowable limits for the various performance indices of the system, load, and filter. Besides, the comparative analysis validates that the C-type filter provides higher power factor, system efficiency and transmission loss improvement than the other two filters, and that the proposed filters achieve almost the same voltage and current harmonic mitigation levels. The solution of the cost minimization problem reveals that the C-type filter and the third-order high-pass filter with equal capacitors have the worst and best resonance damping capabilities respectively, under the worst case conditions. Additionally, the filters with the lowest and highest cost are found as the third-order filter with unequal capacitors and the C-type one, respectively. Besides, the CSA is compared to the genetic algorithm (GA), and particle swarm optimization (PSO) techniques and the results show the fast convergence capability and the effectiveness of the proposed algorithm in solving the problem of optimal design of third-order resonance-free passive filters in distribution networks.

Quieting a rib-framed honeycomb core sandwich panel for a rotorcraft roof.

A rotorcraft roof composite sandwich panel has been redesigned to optimize sound power transmission loss (TL) and minimize structure-borne sound for frequencies between 1 and 4 kHz where gear meshing noise from the transmission has the most impact on speech intelligibility. The roof section, framed by a grid of ribs, was originally constructed of a single honeycomb core/composite facesheet sandwich panel. The original panel has acoustic coincidence frequencies near 600 Hz, leading to poor TL across the frequency range of 1 to 4 kHz. To quiet the panel, the cross section was split into two thinner sandwich subpanels separated by an air gap. The air gap was sized to target the fundamental mass-spring-mass resonance of the panel system to less than 500 Hz, well below the frequency range of interest. The panels were designed to withstand structural loading from normal rotorcraft operation, as well as 'man-on-the-roof' static loads experienced during maintenance operations. Thin layers of viscoelastomer were included in the facesheet ply layups, increasing panel damping loss factors from about 0.01 to 0.05. Transmission loss measurements show the optimized panel provides 6-11 dB of acoustic transmission loss improvement, and 6-15 dB of structure-borne sound reduction at critical rotorcraft transmission tonal frequencies. Analytic panel TL theory simulates the measured performance within 3 dB over most frequencies. Detailed finite element (FE)/boundary element (BE) modeling simulates TL slightly more accurately, within 2 dB for frequencies up to 4 kHz, and also simulates structure-borne sound well, generally within 3 dB.

Investigation on sound transmission through thick-wall cylindrical shells using 3D- theory of elasticity in the presence of external and mean air-gap flow

This paper studies the effects of an external mean flow and an internal air-gap mean flow on sound transmission through a double-wall thick cylindrical shell. Due to the major influence of some effective terms such as membrane, bending, transverse shearing and rotational inertia on thick-walled shell, three-dimensional theory of elasticity is used to obtain the governing equations of motion. Therefore, Newton’s second law is utilized to develop the equilibrium equations for an infinitesimal element in cylindrical coordinates. Then, the equations of motion related to the circular hollow cylinders are solved using Helmholtz potentials for arbitrary values of physical and geometrical parameters. In addition, by coupling of both inner and outer shells, a modal transfer matrix is created. This modal matrix stands for the global dynamic equilibrium of the double-wall cylinder. Moreover, the sound transmission Loss of the double-wall cylinder excited by an acoustic oblique plane wave with two angles of incident (i.e. elevation and azimuth angles) is predicted. Due to lack of studies in the field of sound transmission through the thick-walled shell, the results obtained in this study are compared with those from other researchers for a thin cylindrical shell. These results indicate an excellent agreement in comparison with each other. Furthermore, the results reveal that with thickening of the shell, critical and coincidence frequencies are getting closer to ring frequency. Moreover, the effects of external and air-gap flows on TL behave in similar way whereas the Mach number is positive. In addition, an improvement of transmission loss can be found whereas the Mach number is negative; particularly this enhancement is more specified for the external flow. Finally, the results indicate that where both external and air-gap fluids simultaneously flow in opposite directions the TL is significantly enhanced. However, for the case where these two fluids flow in the same direction the TL is decreased.

Improvement of transmission loss of a double panel by using active control with a virtual modal mass

In this article, modal feedback control is proposed to reduce the sound transmission through finite double panels using lead zirconate titanate ceramic sensors and actuators bonded to the structure. Active control allows adding virtual modal damping and mass to the structure by the use of modal velocities and accelerations. In a first step, the equations describing the structure, the actuators, the acoustic excitation, and the acoustic radiation are detailed. Next, the state space formulation of the smart structure is presented. In a second step, the implementation of active control is illustrated through the use of numerical examples. Finally, simulations are performed using two actuators, allowing five modes to be controlled. The transmission loss factors of the controlled and uncontrolled structure are shown as a function of the required command voltage. These results are also compared with those achieved using other vibroacoustic control techniques.

Improvement of Transmission Loss Using Active Control with Virtual Modal Mass

This paper deals with an alternative modal active control approach to reduce sound transmission through a structure excited by an acoustic wave. Active control makes it possible to conserve lightness while improving acoustic performances. “Modal mass damping control” is proposed for light and small structures having slight modal overlap. The aim of this control is to modify the modal distribution of high radiation efficiency modes with active modal virtual mass and active modal damping. The active virtual mass effects lower eigen frequencies to less audible frequency range while reducing vibration amplitudes in a broad frequency range. An application of this concept is presented in a simple smart structure. It is harmonically excited on large bandwidth by a normal acoustic plane wave. Results obtained by active modal virtual mass and damping control are compared to other modal control approaches.

New results in active and passive control of sound transmission through double wall structures

Abstract This paper presents an overview of recent results concerning active and passive control of sound transmission through double wall structures. First, the analytical simulations are presented for the active control of sound transmission through double wall structures. These are based on a coupling structural-acoustic modal model using piezoelectric materials and loudspeaker/microphones as actuator/sensors. The numerical results show the potential for employing piezoelectric sensor/actuators to improve sound transmission loss. In addition, some useful conclusions are obtained. When, for example, a volume velocity sensor is applied to a radiating plate, sound transmission loss will improve significantly, no matter what type of actuators (i.e. loudspeakers or PZT actuators on either plate) are used. With a loudspeaker/microphone configuration should be avoided for same thickness double wall structures, etc. Furthermore, current research activities on the control of sound transmission through double wall structures also include an arrangement of Helmholtz resonators (HRs) placed within the wall cavity for the passive control of sound transmission through a double wall structure. The current research goal is to find optimal parameters of HRs (damping ratio, numbers, natural frequency, etc.) in order to maximize the improvement of the transmission loss over a specified frequency range. An analytical model of fully coupled structural-acoustic-HRs within a double wall structure is established. Results indicate that tuning the HRs to the mass–air–mass resonance frequency does not guarantee the best possible improvement of the sound transmission loss. Using the frequency averaged sound transmission loss as the cost function to optimize the natural frequency of the HRs is a more effective method to find the HRs tuning frequency for maximizing the improvement of the transmission loss. The aim of this paper is to illustrate the potential of the active control approach using smart piezoelectric materials and different control actuators (i.e. loudspeakers in the cavity, PZT actuators applied to one of the plates) to improve the transmission loss through double wall structures. Additionally, new results on tuning and placement of HRs are presented. Both of these are based upon a refined model of the systems and offer good physical insight into the active and passive control of sound transmission through double wall structures.

Measurement of sound transmission losses of honeycomb partitions with added gas layers

Air, nitrogen, argon and helium barrier layers were mounted on honeycomb sandwich panels and the sound transmission loss (TL) measured. All the gases were maintained at 1 atm pressure during the sound transmission loss measurements. The TL was measured in a small-scale facility with a reverberant source room and an anechoic receiver room. A sound intensity probe was used to measure transmitted sound. Except helium, none of the other gases showed any significant transmission loss improvement due to insufficient impedance mismatch with the air medium. However, the sound transmission loss of honeycomb sandwich panels was substantially improved by the addition of a helium barrier layer on the incident side. The improved TL was attributed to the significant impedance mismatch between air and helium, which resulted in reduction of incident acoustical energy at the interface of the two media. Increases in the sound transmission losses by 6-8 dB were obtained in the range of 2-10 kHz. The helium layer also increased transmission loss at lower and middle frequencies. The addition of a helium barrier layer did not alter the coincidence frequency of the floor panel.

The acoustical benefits of increased thickness of building insulation on sound attenuation in residential construction

Controlled field tests were conducted on three houses that have identical floor plans, orientation, doors, and windows, but have different amounts of building insulation. Steady-state transmission loss tests were conducted to determine the sound insulation of the exterior shells as a function of increased thickness of building insulation. The results show a gradual improvement in transmission loss. In addition, flyover tests were conducted with a jet aircraft at altitudes of 1000, 500, and 250 feet. Sound levels were measured both inside and outside of each home. The results of this series of tests were compared to determine the acoustical benefits provided by increased thicknesses of insulation in residential construction. Test results and comparisons of the two test procedures are presented.

Uses of Glass-Fiber Insulation for Sound Control in Residential Construction

Glass-fiber insulation in board or blanket form is used in several applications for airborne and impact noise isolation in residential construction. These include (1) sound-absorbing cavity fill for improving the airborne transmission loss of walls and floors, (2) vibration-isolating underlayment in floors and single-stud walls, and (3) finished acoustical materials for absorption of reverberant room noise. The improvement in transmission loss of a wood-stud drywall due to a 3-in. thickness of glass-fiber cavity fill ranges typically from 2 to 10 STC, depending on the degree of structural isolation between the two sides. The improvement is found to increase with thickness of cavity fill, but is virtually independent of density or flow resistance over a wide range. In steel-stud drywalls, glass-fiber board applied between the gypsum board and the stud acts both as a vibration isolator and sound absorber. Improvements of 10 STC are typical. In hard-surfaced wood joist floors, a combination of glass-fiber-board underlayment, blanket cavity fill, and a resiliently supported ceiling yields an over-all improvement of 16 in Impact Noise Rating.

The influence of absorbent linings on the transmission loss of double-leaf partitions

Under certain circumstances the insertion of a porous blanket into the cavity of a double-leaf partition can produce a pronounced increase in transmission loss, particularly when the leaves have a low critical frequency and a broad coincidence plateau. In the measurements reported, improvements of between 7 and 10 dB have been obtained, depending on the size of the cavity. The authors show that a similar improvement in transmission loss is obtainable, even with quite small quantities of absorbent, if placed either round the reveals or in other limited regions in the cavity.

Transmission of Reverberant Sound through Double Walls

The transmission of reverberant sound through a double wall, which consists of two identical single walls coupled by an airspace, is investigated both theoretically and experimentally. A theory is developed which gives good agreement with experiment. In order to compute the transmission loss of a double wall, it is necessary to know the impedance Zw of the single wall. Zw was determined from experiments conducted on the single wall and includes the effects of mass, dissipation, and flexural motion. The treatment shows that it is impossible to get a large improvement in transmission loss for a double wall relative to a single wall under reverberant sound field conditions if the single wall is considered to have only mass reactance. In addition, the customary normal incidence theory is totally inadequate in explaining the behavior of a double wall in a reverberant sound field. For double walls having air-coupling only, very shallow airspaces can produce appreciable increases in transmission loss over a single wall. An absorbent material when inserted in the airspace produces large improvements only when the mass of the walls is relatively light and has but little effect for heavy walls. Honeycomb or other non-absorbent cellular structures having no cell walls in a direction normal to the wall faces do not result in an increase in transmission loss. Air-coupled walls having no solid sound conducting paths between individual septa are extremely effective sound insulators as compared to conventional double wall constructions. The theory indicates that a large improvement in the transmission loss of a double wall can be obtained by using as components single walls with high internal dissipation.