Transmission Loss Levels Research Articles

Experimental investigation on the mechanical and acoustic performance of hemp/kenaf hybrid composites: Influence of different stacking sequences

AbstractThis study investigated the effect of various stacking sequences on the mechanical and acoustic characteristics of hemp and kenaf fibers reinforced hybrid composites. The mechanical study was done by using universal testing machine and izod imact tester. Further, acoustic study was done by using impendence tube method. Hemp/kenaf/kenaf/hemp hybrid laminates outperformed the other composites in terms of tensile strength, flexural strength and impact strength. In the mono reinforcements, hemp/epoxy composites demonstrated superior mechanical characteristics than the epoxy matrix and kenaf/epoxy composites. However, hybrid composites have superior mechanical behavior compared to pure composites. From the acoustic results revealed that the pure hemp and kenaf/hemp/hemp/kenaf hybrid composites exhibitted highest sound transmission loss level at lowest and highest frequency region. The bestmechanical characteristics of hybrid composites will be suggested for use in automobile door panels. Also, the optimal sound transmission coefficent values signifying their appropriateness for use in applications that encompass many frequency bands.Highlights Hybridization of kenaf/hemp fibers Improved mechanical properties over the single fiber composites Better acoustic properties

Basin scale propagation modeling of MLS signals from the Kauai Beacon source

Long distance underwater acoustic propagation is of interest for a variety of applications including underwater navigation, yet modeling such propagation is challenging due to the large degree of environmental uncertainty. In this presentation, the propagation of 75Hz center frequency maximum length sequence (MLS) signals emitted from a submerged source near Kauai and received at various sites throughout the Pacific Ocean Basin is modeled with the Bellhop ray tracing [1] and Monterey-Newport Parabolic Equation [2] models. Sites such as the Monterey Accelerated Research System (MARS) observatory and the Comprehensive Nuclear Test Ban Organization (CTBTO) hydroacoustic monitoring station near Wake Island are included in the discussion. The range-dependent environment is based on historical profiles from the World Ocean Atlas database and reanalysis data from the hybrid coordinate ocean model (HYCOM) [3]. Bathymetry profiles along the axis of propagation are interpolated from the General Bathymetric Chart of the Oceans (GEBCO) 2021 database [4]. Simulated broadband channel impulse responses transmission loss levels in the modeled environment are compared to selected receptions of the signal at the hydrophone sites. Additionally, fluctuations of the modeled transmission loss in the presence of an internal wave model [5] are discussed.

Porous liner coated inlet duct: a novel approach to attenuate automotive turbocharger inlet flow-induced sound propagation

Automotive turbochargers generate high-frequency noise, especially at their inlet air duct, which has become a long-standing problem for both passengers and pedestrians and hence is required to be mitigated by provisions such as the use of absorptive materials and novel inlet duct designs. This paper proposes utilizing a thin porous material coating liner on the inlet duct of the turbocharger compressor in order to absorb the incident excitation and hence, broadband reduction of the flow-induced sound wave propagation. To this end, first, a sound-absorbing inlet duct model is designed based on efficient structural parameters. Then, three different types of absorptive materials are used as coating liners to analyze the impact of varying material properties on the acoustic absorption performance of the system. Meanwhile, a sensitivity analysis is carried out to evaluate the influence of coating layer thickness on the system’s flow-induced sound insulation performance. The research involves applying a finite element approach to conducting acoustic pressure and poroacoustics studies over the desired frequency range of human hearing (1–20 kHz). In this regard, sound transmission loss (TL) and sound pressure level (SPL) are investigated for the models with and without liners. The results indicate that sound-absorbing materials can be employed to significantly diminish flow-induced sound levels in the turbocharger inlet duct. According to the results, absorptive liners would considerably increase the maximum TL and lessen the sharp drop in TL that appeared in the model without liners at around 4321 Hz. In addition, the average TL has increased due to the replacement of valleys with peaks. The sensitivity analyses demonstrated that denser absorptive materials and larger liner thicknesses are associated with higher TL levels and better sound attenuation response. By adding 15 mm of rock wool liner to the designed inlet duct, the average TL and root mean square of TL were increased up to and respectively. The proposed design procedure of this study unlocks a fundamental approach that can be practically applied to many promising fields, such as aerospace and automotive engineering.

Constrained layer damping concept used on isolated gym floors. Better than Concrete?

As the global fitness trends continues in a post-pandemic world, intra-building disruptions from high-energy fitness create serious acoustical challenges. Solutions from simple roll-out mats and rubber tiles to concrete floating floors supported by spring jack-up isolators are used to achieve acoustical separation for gyms. Understanding the sources of sound and vibration produced in fitness environments and finding effective solutions was a journey that CDM Stravitec began in the 1970s and this area of the acoustics industry has been intensively researched as part of the development of our range of lightweight floating floors. As part of this research, we extensively investigated how the concept of constrained layer damping (CLD) could be employed to mitigate vibration radiation through structures. In the application of fitness flooring, the CLD functions to restrict the physical movement between the rigid board layers decreases the area of influence from the impact, reduces the energy radiation through the composite panel system to its support and subsequently reduces the energy transfer into the structure. Our research has also shown that elastomeric supports balance low-resonant frequency performance to prevent energy build-up at the driving frequencies and can achieve high levels of transmission loss which are comparable to concrete based solutions.

Estimation of acoustic transmission loss for carbon honey comb panel with melamine foam

In this paper, the acoustic noise reduction of aluminum honeycomb/carbon face sheet sandwich panel with Melamine Foam (MF) lining of variable thicknesses (25 mm & 50 mm) is numerically estimated and then experimentally verified using impedance tube based on transfer matrix method. The presence of carbon face sheets and MF lining can have a significant impact on the Transmission Loss (TL). The impedance tube can measure the TL for frequency range of 64 Hz–6.2 kHz. A conventional, two-microphone impedance tube, is connected to a sample holder downstream of the first microphone pair and a section downstream of the sample holder that accommodates a second pair of microphones. The Finite Element Analysis and experimental results are compared and found in good agreements. The TL levels for each layer of material is estimated prior to have TL for combined structure. Furthermore, the optimized insulation foam thickness of 50 mm is obtained based on required acoustic parameters.

Mechanical, acoustic and vibration performance of intra‐ply Kevlar/PALF epoxy hybrid composites: Effects of different weaving patterns

AbstractThe influence of different weaving patterns on tensile, acoustic, and vibration behavior of intra‐ply Kevlar and pineapple leaf fiber (PALF) hybrid woven fabric reinforced epoxy matrix composites was investigated. Intra‐ply hybrid Kevlar/PALF woven fabrics were analyzed by three weaves: plain, twill, and basket. Also, epoxy matrix, pure Kevlar, and PALF woven fabric composites were analyzed for comparison purposes. Results revealed that the basket‐type hybrid composites exhibited a higher tensile strength of 77 MPa compared to plain (63 MPa) and twill‐type (73 MPa) hybrid composites. Plain and twill weave hybrid composites presented higher sound transmission loss levels of 28.9 and 30.5 dB, respectively, at higher frequency levels. Free vibration analysis showed that the PALF composites had higher damping and a lower natural frequency whereas the pure Kevlar and hybrid composites (KP1, KP2, and KP3) exhibited lower damping and higher natural frequencies.

Optimization and analysis of the acoustic and resistance performance of the plenum chamber via sample entropy and large eddy simulation

The noise of the plenum chamber plays an important role in the acoustic performance evaluation of ventilation and air condition (VAC) systems. The large eddy simulation (LES) model and Ffowcs Williams & Hawkings (FW–H) equation were used to obtain the transient flow and aerodynamic noise, respectively, and a continuous and uniform impedance boundary condition was introduced to calculate the transmission loss (TL) of the perforated board plenum chamber. The aerodynamic noise, TL, resistance and velocity uniformity distribution characteristics of the plenum chamber were predicted by numerical simulation. A full-scale test bed was built to verify the flow field prediction results, and the acoustic prediction results were verified by analog experiments. In addition, based on the sample entropy theory, the pressure fluctuation on the inner surface of the plenum chamber was evaluated. The results show that the perforated board changes the vortex structure in the plenum chamber and reduces the local resistance and the pressure fluctuation on the inner surface wall, and the local resistance coefficient can be reduced by up to 34%. However, the coefficient of velocity uniformity in the plenum chamber decreases by 4%. Furthermore, the perforated board can significantly reduce the TL and peak sound pressure level (SPL) of the plenum chamber. At the receiving point, the overall SPL can be reduced by up to 2.5 dB.

Identification of uncertainty levels of acoustic properties of biocomposites under different mounting conditions in impedance tube tests

Impedance tube method is widely used to measure acoustic properties of materials. Although this method yields reliable acoustic properties for soft textured materials, uncertainty levels of measured acoustic properties for hard materials, including biocomposites, can be quite large, mainly due to uncertain mounting conditions. Here, the effects of mounting conditions on the acoustic properties of biocomposites in an impedance tube are investigated. First, nominally identical biocomposite samples with a diameter equal to the inner diameter of impedance tube are manufactured and their acoustic properties are determined. As hard materials practically cause fitting problems in the impedance tube, the diameters of samples are reduced, as in practice, by small amounts and acoustic properties of modified samples are determined. Furthermore, in order to match the diameters of samples to the inner diameter of impedance tube, different materials such as tape, petroleum jelly and cotton are applied around samples to close the air gap between the samples and the tube's inner wall. All the results are compared, and the uncertainty levels caused by different mounting conditions on the acoustic properties of biocomposites are identified. The results show that the transmission loss (TL) measurements are dramatically affected by the mounting conditions while the sound absorption conditions are less sensitive to the mounting conditions. The deviations in the measured TL levels are highest for the samples with tape and wax (10–15 dB). On the other hand, the deviations in the measured sound absorption coefficients are highest for the samples with cotton and tape (1–2%).

Acoustic radiation and transmission loss of FG-Graphene composite plate under nonuniform edge loading

The influence of nonuniform edge loads on the acoustic response of a functionally graded graphene reinforced composite plate is investigated analytically. The energy method is implemented to calculate the buckling load (Pcr). An analytical method based on Reddy's third-order shear deformation theorem is used to obtain the vibration response, and acoustic response is obtained using Rayleigh Integral. The nature of edge load variation on buckling and vibro-acoustic response is significant. Free vibration mode shape changes with an increase in edge load and consequently affects the resonant amplitude of responses also especially for the plates with a higher aspect ratio. Volume fraction and dispersion pattern of graphene nano-platelets also influences the resonance amplitudes. Plate with FG−GRCC dispersion pattern has improved buckling and vibro-acoustic response behavior. Similarly, change in sound transmission loss level is significant in the stiffness region compared to the damping and mass dominated region.

Acoustic Characteristics of Composite Structures Used in Train

This paper presents a study focused on sandwich structures as well-known train construction materials that are composed of two thin and rigid face sheets and a thick, low-density core material. For trains, the wheel-rail inter-face is the main source of noise, and the wheel-rail roughness, especially in the presence of rail corrugation, is the main excitation source transmitted to the interior and area for passengers. The purpose of the study is to optimize the acoustic properties of a composite sandwich panel used for train floors and walls. Sound absorption coefficient (?), noise reduction coefficient (NRC) and Transmission loss (TL) evaluations have been implemented and experimentally validated on a typical sandwich material used for trains. The opportunity to use a different material can be concretely calculated and modified for critical frequency ranges. Sound transmission loss levels of the structural components as the floor and wall of the train body, which are required of producers and cus-tomers, were tested in acoustic laboratory and acoustic devices according to ASTM and ISO standards. It is demonstrated that, for honeycomb and cork sandwich panels, acoustic response is not sensitive to cell size. For foam core sandwich panels, it is observed that different compositions with thin layers are effective in the fre-quency range of 50 - 1000 Hz.

Modeling acoustic wave propagation and reverberation in an ice covered environment using finite element analysis

A three-dimensional, longitudinally invariant finite element model of acoustic propagation and reverberation in an ice-covered shallow water waveguide has been developed. The ice is modeled as both an elastic medium and a pressure release surface. Transmission loss levels are calculated and compared for both assumptions of ice. Using Fourier synthesis, the time-harmonic acoustic pressure results are transformed into the time domain, and reverberation levels are then compared for both models. Finally, using a fully three-dimensional version of the finite element model, compressional-to-shear wave conversion at the elastic ice and water interface is characterized to inform propagation mechanisms of acoustic waves in Arctic ice sheets. [Work supported by ONR, Ocean Acoustics and the Robert W. Young Award for Undergraduate Student Research in Acoustics.]

A Study of the Effects on Transmission Loss of Modelled Water Column Variations within an Area off the East Australian Coast

The present accuracy of ocean and atmospheric models permits the description of water column features, and sea surface wind stress, at a resolution which enables employment in underwater acoustic transmission applications. In an investigation of aspects of linking modelled ocean data with range-dependent acoustic transmission models, a Parabolic Equation (PE) transmission code was used with data generated by the BLUElink suite of ocean and atmospheric models for a deep-water region off the east Australian coast for a particular summer period. A typical presence of warm and cold core eddies was observed to accompany a highly variable acoustic environment. Variations in expected range to particular levels of Transmission Loss were found to be mainly related to changes in the depth of the mixed surface layer, but also due to changes in the sound speed gradient in the thermocline. The study also made a brief consideration of the likely impact of wind speed variation over the region, and the effects of modelled ocean currents on acoustic transmission.

Identification of the Dynamic Characteristics of Luffa Fiber Reinforced Bio-Composite Plates

Luffa cylindrica plant fiber is a new biodegradable engineering material. However, the dynamic behaviors of these new green materials or their composites should be explored to consider them for practical applications. The dynamic characteristics including modal behavior and the elastic and sound isolation properties of luffa-based bio-composite plates were explored in this study. Structural frequency response function measurements were conducted using a few luffa bio-composite plates to identify their modal behavior. The modal frequencies and loss factors of the luffa bio-composite plates were identified by analyzing the frequency response function measurements using a few modal analysis methods such as half-power, circle-fit, and line-fit. The same luffa bio-composite structures were modelled using a finite element formulation with damping capability, and the elastic moduli of the composite plates were identified. In addition, the transmission loss levels of the same luffa composite samples were measured using the impedance tube method. The results showed that luffa composite structures have considerably high stiffness (elasticity modulus: 2.5 GPa), damping levels (loss factor: 2.6%), and transmission loss level (25 to 30 dB for a 1 cm thickness), and their mechanical properties are promising as an alternative disposable material for noise and vibration control engineering applications.

Feedback control of an active acoustic metamaterial

Acoustic metamaterials have recently been of significant interest due to their potential ability to exhibit behavior not found in naturally occurring materials. This extends to the realization of acoustic cloaks, but perhaps of greater industrial impact is their ability to achieve high levels of noise control performance. In particular, previous research has demonstrated the high levels of transmission loss that can be achieved by an array of locally resonant elements. However, these passive metamaterials are inherently limited in performance due to both losses and their static nature. Therefore, there has been an increasing interest in active acoustic metamaterials, which can allow both increased performance and adaptability. Recent work has investigated the integration of active elements into a passive resonator-based metamaterial, and it has been demonstrated using a feedforward control architecture that significant increases in the level and bandwidth of transmission loss are achievable. However, in many practical noise control applications, it is not possible to obtain a time advanced reference signal that is required for a feedforward control implementation. Therefore, this paper will explore the design of a feedback control architecture that is applicable to the active resonator based acoustic metamaterial and demonstrate the potential performance of such a system.

Sound insulation analysis and optimization of anti-symmetrical carbon fiber reinforced polymer composite materials

Due to the advantages of high strength and low density, carbon fiber reinforced polymer (CFRP) composite material has become more and more attractive given the increasing demand of lightweight design requirement in the automobile industry. However, as density reduces, the sound insulation ability of CFRP panels is usually inferior compared with traditional metal materials such as steel and aluminum, which may lead to deteriorated noise insulation performance inside the vehicle cabin when the CFRP material is used as body panels. For the sake of investigating the sound insulation ability of CFRP, two optimization methods are proposed, and the optimal ply angles that lead to the largest sound transmission loss in two different frequency ranges are obtained accordingly. On the basis of optimization, the sound insulation capability of CFRP and two metal materials are thoroughly compared. It is found that, when optimally designed, the CFRP panel can achieve a transmission loss level comparable to its metal counterparts at low frequencies (e.g., <300Hz) with substantially less weight.

Spatial variation of ocean ambient noise spectrum in offshore China

Ocean ambient noise always presents spatial difference because of the diversity of the noise source and the propagation condition. This paper analyzes the noise spectrum level (NL) of two stations measured in offshore China, and finds that the difference of the two NL is more than 10 dB with different variation for per octave. Through analyze the NL and the transmission loss level (TL) from the noise source to the measuring station, a correlation is found between the noise level difference (NLD) and transmission loss difference (TLD), with a correlation coefficient of 0.78 at the band of 50 Hz ~500 Hz. This is a strong proof to connect the spatial variation of NL with the varied propagation channel, and also suggests that at this band the ambient noise is closely related to the ship noise sources. In addition, the difference of the ship noise source level coincides with the number of vessels before and after the fishing moratorium period in China, which further explains the difference between the NL of two experimental stations are highly related to ships.

SEA and contribution analysis for interior noise of a high speed train

This paper presents a detailed Statistical Energy Analysis (SEA) and contribution analysis of the interior noise of a high-speed train through extensive simulations and measurements. The SEA model was developed based on the actual geometrical parameters of a benchmark high-speed coach. Sound transmission loss levels of the structural components of the car body, which are required in the SEA model, were tested in a dedicated acoustic laboratory following international standard ISO 140-3:1995. Modal densities of these structural components were derived from measured frequency response functions using the modal counting method. Damping loss factors were obtained using the half-power bandwidth method and the vibration attenuation method. By considering the relationship between sound radiation and power transmission, coupling loss factors between structures and cavities were estimated. Source inputs to the SEA model were derived from field experiment data. Interior noise due to those sources was predicted using the SEA model and the prediction was generally in good agreement with measurement. Contribution analysis was then performed using this validated model through parametric study, and this analysis was further examined experimentally. In conclusion, for the coach that was investigated in this paper, the key factors for interior noise are sidewall vibration, bogie area noise, and floor sound transmission loss. Based on this and other engineering considerations, an interior noise control strategy can be defined.

Investigation of the acoustic properties of bio luffa fiber and composite materials

Considering the adverse effects of petroleum-based materials on nature, finding and developing new materials as alternatives to these chemical materials become a necessity in practice. On the other hand, these new materials need characterization to be considered and effectively used in practical applications. The acoustic properties of luffa bio fiber and composite materials are investigated in this study. First, the preparation of various luffa test samples and the method for acoustic characterization of the luffa samples is presented. Then, the acoustic absorption properties of both luffa fiber and composite materials are identified using the impedance tube method. After that, the transmission loss levels of the same luffa samples are determined. All the results are evaluated and the acoustic performances of luffa materials are highlighted.

An Investigations of Sound Absorbance Properties for Weft Knitted Spacer Fabrics

Nonwoven fabrics have been used for many years in different technical applications; they have particularly been used as isolation materials in vehicles in order to reduce the noise heard within them, and they. They have achieved great popularity due to low production cost and good absorbance. However, the fabrics produced by making use of the nonwoven technique have some disadvantages including low resistance, low abrasion strength, poor aesthetic appearance and thickness. In order to eliminate these disadvantages, recent studies have reported that knitted fabrics could be an alternative to nonwoven fabrics. Various studies have focused on the impact on sound absorbance that the thickness and surface structure of knitted fabrics have. In this study, a number of knitted spacer fabrics, which had five different connection angles, were manufactured by using a plain knitting machine. The sound transmission loss levels of the developed fabrics were tested and analysed by Bruel and Kjaer tube instruments. At the end of the examinations, the sound absorbance behaviors at different frequencies were demonstrated in graphics based on the type of knitting. It was determined in the study that three factors have a major impact on the sound absorbance behaviour; thickness of fabric, micro porosity between fabric surfaces and yarn linear density in the interconnection of the fabrics.

Waveguides containing frame elements with electrically controlled characteristics of permitted and forbidden bands

The feasibility of waveguide structures that have both forbidden and permitted transmission bands and are based on a diaphragm and frame coupling elements placed on both of its sides is shown experimentally and numerically using the finite element method. It is found out that the dynamic range of controlling the transmission coefficient by means of an n-i-p-i-n-structure placed in the gap between the post and the frame elements attains 23.5 dB, the transmission loss level not exceeding 1.5 dB.