Transmission Loss Peak Research Articles

Research on the application of second-order HR in attenuating tire cavity resonance noise

Tire cavity resonance noise (TCRN) is well-known as one of the significant sources causing the interior noise at low-frequency band. Previous research shows that Helmholtz resonator (HR) array in an automotive wheel may obviously attenuate TCRN. To further improve the effect, the second-order HRs in the HR array are introduced here. Firstly, the theoretical model of second-order HR is established, and the results are verified by finite element method. The results show that the second-order HR with the same cavity volume may produce two transmission loss peaks, and a higher peak than that of the original first-order HR in the specified frequency band. Then, the optimization is performed to obtain a higher effect of attenuating TCRN at the original design frequency and a certain effect of attenuating road noise in the specified lower-frequency band. The results indicate that the desired effect may be achieved, and the higher resonance frequency can easily fall in the frequency band below 600Hz although hard to fall in the frequency band of TCRN. So the second-order HR is helpful to improve attenuating road noise including TCRN.

Phage-modified dual-peak long-period fiber grating biosensor for ultrasensitive, rapid and specific detection of pathogen strains

Herein, we present an ultrasensitive, rapid and specific phage-modified dual-peak long-period fiber grating (Phage-DP-LPFG) biosensor for detecting foodborne pathogens. Compared to the single-peak resonance LPFG with a grating period of 360 μm, the DP-LPFG with a grating period of 163 μm offers enhanced sensitivity to refractive index changes caused by bacterial adsorption. By directionally immobilising phages on the DP-LPFG, the sensor achieves high specificity for target bacterial strains. Upon capturing the target strain, the short-wavelength near-infrared transmission loss peak undergoes a blue shift, whereas the long-wavelength transmission loss peak exhibits a red shift. The shift in spacing between the two peaks (Δλ) increases with higher bacterial capture, enabling the quantification of target strains based on Δλ changes. Using Escherichia coli O157:H7 (E. coli O157:H7) as a model, the Phage-DP-LPFG biosensor demonstrates a linear detection range of 25–108 CFU/mL with a low limit of detection of 8 CFU/mL in 10 min under optimal conditions. The biosensor was also tested on food samples such as fish and milk, showing no notable differences from standard methods but with reduced detection time. It was further extended to the detection of another foodborne pathogen strain Hafnia paralvei (MCCC 1K06097) (H. paralvei (MCCC 1K06097)) with satisfactory results. This study offers a novel approach for the rapid, sensitive and specific detection of trace pathogenic strains in foods.

Ultra-thin ventilated metasurface pipeline coating for broadband noise reduction✰

This paper proposes a method of designing a metasurface muffling coating for pipeline ventilation and noise reduction, which realises low-frequency broadband noise reduction by laying ultra-thin labyrinth-channel acoustic absorbing metasurface structures on the inner wall surface of the pipeline. With the advantage of relatively loose size constraints in the length direction of the fluid conveying pipeline, the thickness dimensions required for low-frequency broadband sound absorption are transferred to the length direction, which significantly increases the effective ventilation area, and the ventilatable area is as high as 55.5 % of the overall structural section area. Since sound waves can be gradually attenuated in the pipeline as the fluid medium flows along the length of the pipeline rather than needing to be attenuated at the same time as in a flat plate acoustic structure, the performance requirements of the acoustic structure are substantially reduced, enabling a significant increase in the operating bandwidth. We have tried the arrangement method of coating layers with gradient thickness, which shows that the combination of different thicknesses of metasurface coating layers can broaden the muffling frequency band and improve the transmission loss of the muffling structure. The design is equally suitable for the noise reduction of liquid-filled piping systems, and the peak transmission loss can be as high as 54.6 dB, which represents a pleasing combination of low-frequency muffling and lightweight design, as well as excellent ventilation capability, and embraces a promising future for application.

A tunable acoustic absorber using reconfigurable dielectric elastomer actuated petals

Dielectric elastomer actuator (DEA)-based unimorphs that actively bend in one direction, can mimic the blooming motion of flower petals. Here we explore an application of such reconfigurable DEA to create tunable acoustic absorber capable of adapting to fluctuations in dominant noise frequency. The DEA-unimorphs consist of alternate layers of dielectric elastomers and compliant electrodes bonded to a Mylar sheet and were micro-slotted to form triangular petal-like structures that bend upon voltage activation. When arranged in an array, the micro-slotted dielectric elastomer bending actuators (MSDEBA) can open like flower petals, actively reconfiguring their open-ratio. Integrated with a base resonator comprising a micro-slotted panel (MSP) and a parallelly arranged varying-depth (VD) back-cavity, the MSDEBA forms a tunable acoustic absorber effective in the low-mid acoustic frequency range at inactive state. Meanwhile, upon voltage activation, it increased the absorber’s open-ratio and tuned the absorber to target a higher frequency. A 5 kV activation reconfigured the MSDEBA to shift its transmission loss peak by 72.74% (i.e., from 697 Hz to 1204 Hz). This acoustic spectrum tuning capability doubled the 15 dB absorption bandwidth of these absorbers from a bandwidth of ~435 Hz to 820 Hz. Such absorbers have the potential to tune the absorption spectrum to match the noise frequency in real-time to ensure optimal acoustic attenuation.

An acoustic design project for final year engineering students using resonant arrays

This paper reviews the creation of a design course for mechanical engineering students at the University of Auckland. The primary objective of the course is to introduce students to the field of acoustics, teaching basic theoretical and experimental acoustics principles, together with product design and fabrication methods, in an engaging and educational manner. During the course, students were assigned the task of developing a noise reduction duct jacket that reduced sound propagation whilst allowing free airflow, all within specified dimensions. Students were required to create designs that attenuated a predefined sound spectrum, with both narrow-band and wide-band components. Over six weeks students developed their designs using mathematical modelling and fabricated their prototypes. The performance of their suppressors was assessed through sound pressure level and impedance tube transmission loss measurements. Students then modified their designs to maximise performance before final submission. Four-person teams produced a diverse range of implementations. The most successful designs achieved impressive performance with peak transmission loss up to 70dB, 2kHz bandwidths of 10 + dB transmission loss and a delta dBA reduction of 21.5 dB. Student feedback indicates a high level of satisfaction with the course, highlighting its effectiveness in imparting key knowledge and skills in acoustics engineering.

Numerical design of Helmholtz resonators with multiple necks for multi-tonal noise control

The reduction of multi-tonal noise at multiple frequencies simultaneously is a challenge in many industrial fields. Different solutions such as metamaterials consisting of periodic Helmholtz resonators embedded into a porous layer have been studied in the literature. Generally, a classical resonator made of a cavity connected to a neck provides only one resonant transmission loss peak. In this study, a design of acoustic metamaterials is proposed numerically using the finite element method for multi-tonal noise reduction. The resonator is made of multiple necks extended into the cavity and is periodically distributed within a porous material. The cylindrical global cavity of the resonator is partitioned into several sub-cavities, which are separated from one another by a rigid wall, and each sub-cavity is connected to one neck. Helmholtz resonators with 2, 3, 4, and 5 necks are presented, they exhibit multiple resonance transmission loss peaks which correspond respectively to the number of the resonator necks. The proposed acoustic metamaterial designs can be used to reduce multi-total noise at several frequencies simultaneously.

Experimental and Numerical Studies on the Hilbert Fractal Architecture as an Acoustic Metamaterial

Experiments and numerical methods were used to investigate the key parameters which affect the transmission loss behaviour of Hilbert fractal acoustic metamaterials. The tests were conducted using a four-microphone impedance tube, and the numerical simulation was performed using COMSOL Multiphysics software. Fractal order and cavity slot widths on Hilbert fractal metamaterials were investigated. 3D printing manufacturing techniques were used to make polylactic acid specimens. The COMSOL model developed utilised thermo-viscous and lossless domains with boundary layer mesh in the fractal cavities. The tests and simulations' frequency range were 0.2 kHz to 3.0 kHz, with parametric gap widths ranging from 0.5 mm to 3.0 mm. There is a reasonable agreement between the numerical models and the experimental results; the second-order Hilbert fractal had the most significant effect on transmission loss, with an experimental peak of nearly 50 dB around 1600 Hz. Moreover, multiple transmission loss peaks were observed as a function of the gap width in the five Hilbert fractal orders studied. The gap width is one of the critical parameters for optimising the performance of the Hilbert fractal as an acoustic metamaterial.

Modelling of acoustic metamaterial sound insulator using a transfer matrix method for aircraft cabin applications

The incorporation of acoustic metamaterials in the design of aircraft fuselage panels in order to reduce the low frequency noise inside the cabin is nowadays a major research subject in order to offer passengers a comfortable cabin environment. In this study, acoustic metamaterial made of Helmholtz resonators periodically embedded into a porous material is investigated using a transfer matrix approach combined with finite element calculations. This transfer matrix method is based on the two-load methods where the two loads in the numerical simulations are two different termination conditions (plane wave radiation and rigid wall) in order to retrieve the equivalent transfer matrix of the porous layer with embedded periodic Helmholtz resonators. The equivalent matrix is then coupled analytically in series with others analytical matrices to model complex multilayer metamaterials. The results of the transmission loss obtained exclusively using finite element method for single and double wall configurations are compared with the results of the proposed transfer matrix method and good agreements are obtained. It is observed that the frequency band of the transmission loss of the double wall configuration is larger than the one of the single wall configuration with higher transmission loss peak value. The investigated metamaterial can potentially be used in the design of aircraft cabin panels for low frequency noise reduction.

The acoustic performance of a dual Helmholtz resonators system in the presence of a grazing flow

Helmholtz resonators (HR) are widely used in aero-engine systems for noise reduction. By connecting a pair of HRs in series (neck-cavity-neck-cavity), a dual HRs system is formed. This study investigated the influence of neck length, cavity volume and flow Mach number on the noise attenuation performance of a dual HRs system. A three-dimensional numerical simulation was performed to calculate the transmission loss results. The transmission loss (TL) results indicated that the second neck length can influence the second resonance frequency and TL max. Changing the cavity volume significantly influences the noise attenuation ability under lower flow rate conditions compared to higher flow rate conditions. The flow Mach number had a more significant impact on the first TL peak than on the second TL peak. This study shows the relationship between the geometric parameters, grazing flow and noise attenuation performance of a dual HRs system and could provide guidance in designing suitable dual HRs for aero-engine systems.

Acoustic properties of honeycomb like sandwich acoustic metamaterials

Low frequency noise has strong penetrability and has always been an important part of environmental pollution. Limited by the law of mass, traditional lightweight porous materials are difficult to effectively control low-frequency noise. Based on the characteristics of honeycomb structure, Helmholtz resonator and acoustic metamaterials, a kind of honeycomb like sandwich acoustic metamaterials is proposed. This structure combines the lightweight characteristics of honeycomb sandwich structure and Helmholtz resonance characteristics. The finite element models of honeycomb and Helmholtz resonators element are established. The transmission loss (TL) characteristic shows that Helmholtz resonators element will be a TL peak in a narrow frequency band than honeycomb element. The effects of neck diameter, neck length and cavity volume of Helmholtz resonator on its TL are analyzed. The TL of single and multiple Helmholtz resonators are compared. Furthermore, The TL of honeycomb panel and honeycomb like sandwich acoustic metamaterials are analyzed. The results show that the different TL peaks can be introduced into honeycomb like sandwich acoustic metamaterials with different neck diameters. Each Helmholtz resonators element plays an active role in its own frequency band. The sound insulation performance of the honeycomb like sandwich acoustic metamaterials is tested by impedance tube experiment. Excellent sound insulation ability is confirmed. The reasonable design of honeycomb like sandwich acoustic metamaterials structural parameters can solve the problem of low-frequency noise of thin-layer lightweight structure.

Acoustic attenuation characteristics of the muffler phononic crystal with hybrid resonators

Aiming at achieving broadband and strong sound attenuation in the duct, this study proposes one compact and hybrid muffler phononic crystal. The two-dimensional (2D) transfer matrix method is applied to calculate the transmission loss (TL) of one unit cell validated by the numerical simulation and experiment. On this basis, the unit cell dispersion theory is employed to predict the bandgap properties of the infinite periodic mufflers. The sound reduction mechanism in the hybrid muffler is disclosed, while the effect of the period number on the acoustic performance is examined. It demonstrated that the acoustic resonant behaviors of the short-length mufflers are conducive to the continuous and superior attenuation band through the compact structure. The number of the TL domes and the boundary length of the short chamber could be evaluated by the cut-off frequency at the (0,1) radial mode and the first passing frequency for the muffler. Although each resonant-type component of the hybrid muffler is responsible for the TL peaks at different frequency ranges, via the coupling effect, they provide remarkably improved performance over a wide frequency range. The unit cell periodicity brings two types of bandgap, the locally resonant bandgap (LRG) and Bragg bandgap (BG), further enhancing the attenuation amplitude. Utilizing the different resonance mechanisms as well as the merging of the LRG and BG, the hybrid muffler phononic crystal is improved, which possesses a wide attenuation range with a tinier structure. The results develop new approaches for broadband and strong sound attenuation using the concept of the phononic crystal and offer a promising application in duct noise control.

Bistable sound insulator with an abrupt stiffness shift using magnetic-coupled dielectric elastomer actuator

Aiming at noise isolation in low frequency range, this paper presents a novel kind of membrane sound insulator featuring a bistable actuation, by combining magnets and multilayer dielectric elastomer actuator (DEA). With a critical applied voltage on DEA, it deforms and the magnets attract in terms of a bistable snapping, which leads to an abrupt stiffness shift, and consequently regulates the sound transmission loss peak frequency. An electromechanical model is established to reveal the bistable characteristics and to study the effect of voltage and structure parameters for design optimization. The sound-insulation measurement experiment verifies the tunable acoustic performance. The sound-insulation peak frequency has a maximum shift of 142, 130, and 141 Hz under voltages of 2000, 2500, and 3000 V, respectively, showing an advancing figure of merit compared with the existed acoustic metamaterial based on dielectric elastomer.

Transmission Loss analysis of a simple expansion chamber muffler with extended inlet and outlet combined with inhomogeneous micro-perforated panel (iMPP)

This paper studies the acoustic performance of a simple expansion chamber muffler combined with an inhomogeneous microperforated panel (iMPP). Three different shaped mufflers, i.e., circular, rectangular, and elliptical, with extended inlet and outlet pipes, have been studied. The thermo-acoustic module of COMSOL Multiphysics 5.5a software is used for finite element method (FEM) simulation. Then the 3D printing (stereolithography) technology is used to manufacture the samples. The transmission loss (TL) of the muffler given by the FEM simulation model is verified and measured by the Two-load method experiment, and the muffler with and without iMPP is tested. Different configurations have been tested to detect the effect of resonator absorbers based on iMPP in the expansion chambers. To observe the effectiveness of using iMPP, it is compared with other homogeneous MPPs. The results show that the TL of the low to medium frequency region increases by 25∼30 dB after adding iMPPs into the expansion chamber. Finally, the air-cavity depth behind the iMPP and the length of the expansion chamber is varied to study its effect on TL performance. The results show that the larger the cavity depth, the lower the TL peak. At the same time, with the decrease of the expansion chamber length, the TL peak gradually decreases and narrows. By carefully controlling and selecting the iMPP and expansion chamber parameters, we can obtain the required TL. Therefore, when the reactive effect of the muffler becomes invalid, iMPP can be used as an alternative further to improve the TL from low frequency to medium frequency.

Low frequency noise control in duct based on locally resonant membrane with attached resonators

In this paper, locally resonant membrane with attached resonators is introduced to duct system to achieve good noise control effect especially in low frequency range. A rigorous analytical model based on modal expansion method is developed to investigate the vibroacoustic coupling characteristic of proposed duct-membrane-resonator system. The corresponding theoretical results show a good agreement with numerical simulation. The velocity distribution, sound transmission loss curve, and the mechanism behind are discussed and explained deeply. It is found that the attached resonators on the membrane can lead to new transmission loss peak which is related to but not exactly equal to the resonance frequency of attached resonator itself. Correspondence between zero equivalent density mass and transmission loss peak is achieved at the non-antisymmetric modes. Compared to single resonator, multi-resonators system can realize more transmission loss peaks for better sound transmission effect. In principle, ultra-low frequency noise reduction and arbitrarily adjustable frequency can be achieved by changing the frequency, position, and number of exerted resonators, which provide a new approach for duct noise control.

Noise Reduction in Tractor Cabs Using Coupled Resonance Acoustic Materials

The noise level in a tractor cab directly affects the driving experience. Reducing the noise level in a tractor cab is an important means of improving the image of the product brand and protecting the physical and psychological health of employees. To reduce medium- and low-frequency noise in a tractor cab, the influence of the installation of coupled resonance acoustic materials (CRAMs) on tractor driver ear noise is investigated in this study. Theories for sound absorption by perforated plates and sound insulation by thin membranes are used to design a CRAM composed of a perforated plate, elastic membrane and additional mass. The acoustic impedance and sound absorption coefficient of the CRAM are obtained by an equivalent circuit method, and the resonance frequency of a local resonance system composed of an elastic membrane and additional mass is calculated. The effects of different hole diameters and porosities of the perforated plate on the CRAM sound absorption coefficient are compared. The results of tests performed using impedance tubes over a wide frequency range of 50 Hz~1600 Hz show that the maximum sound absorption coefficient is 0.99 and several transmission loss peaks of 15 dB~26 dB. Measurement of the acoustic response of the CRAM installed on the cab front panel shows that an ear noise reduction of approximately 1.3 dB(A) for a tractor working speed of 2200 rpm. The results show that the designed CRAM has a relatively good effect on reducing driver ear noise over a wide frequency band and is a feasible scheme for noise reduction in a tractor cab.

Research and design of broadband muffler based on second-order Helmholtz resonators

Noise is always a serious factor affecting people's quality of life. The most common sound-absorbing materials are porous materials, which work based on the principle that sound waves entering into the pores inside the material are subjected to air friction and viscous resistance, thus converting sound energy into heat. Porous materials have excellent performance of absorbing medium-frequency and high-frequency sound , but they are required to be thick enough to control the low-frequency sound waves with large wavelengths, which limits the application of porous materials in low-frequency noise control. In recent years, acoustic artificial structures have become a research hotspot, which can realize exotic effective acoustic parameters based on periodical structure or local resonance. Acoustic artificial structure provides a new material basis for noise control, in which Helmholtz resonator plays an important role because of its simple geometry. In this study, a broadband muffler is designed based on the second-order neck embedded Helmholtz resonator. In order to achieve low-frequency and broadband sound insulation with a limited number of units and structure length, the second-order resonator is chosen as a basic structure unit, which has a stronger low-frequency noise reduction capability and has one high-frequency transmission loss peak more than a conventional Helmholtz resonator. The acoustic characteristics and insulation performance of second-order resonators are analyzed through theoretical calculation, simulation calculation and experimental test. Then, based on the theoretical model and empirical rules, a broadband muffler composed of nine second-order resonators is designed by carefully adjusting the geometry parameters of each resonator. The three-dimensional printed resonators are installed on the side wall of a square standing wave tube for experimental measurement. In the experiment, the transmission loss curve of the muffler is measured by the two-load method. The result shows that the designed muffler has good sound insulation performances in a frequency range of 267–927 Hz, with the whole transmission loss above 20 dB and the maximum sound insulation up to 60 dB. The experimental result is consistent with the calculation result and simulation result. The muffler has simple structure and high practicability, which will have a wide application prospect in noise control engineering.

Tunable acoustic insulation characteristics of membrane-type acoustic metamaterials array with compact magnets

This paper presents membrane-type acoustic metamaterials array with compact magnets (MAMMs array) for acoustic insulation. The proposed MAMMs array consist of four cells. Every cell is realized by placing a membrane with a centered magnet in a magnetic field. The magnetic fields of each cell do not affect each other. To clarify the tunable sound insulation characteristics of the MAMMs array, a mode superposition model and the corresponding finite element analysis model were established to predict the transmission loss (TL) of the MAMMs array. Experiments were performed in an impedance tube, and the experimental results agree well with the predicted ones. This research demonstrates that each cell of the MAMMs array is individually adjustable and the enhanced tension of the membrane due to the increased magnetic force leads to the upward shifting of the TL peak frequency and TL peak enlargement in each cell. By optimizing different cells of the MAMMs array, the TL curve of the MAMMs array can produce multiple TL peaks, and the noise isolation of multiple frequency bands can be realized.

Dual-target-frequency-range stop-band acoustic metamaterial muffler: acoustic and CFD approach

Acoustic metamaterials can control, direct, and manipulate sound waves, hence, they attract more researchers’ attention in the field of automotive, aerospace, and architecture to reduce noise levels. Here, vehicle exhaust noise is investigated through finite element simulation. In this study, an improved muffler is proposed to affect the transmission loss. In the case of acoustic performance, we first designed the acoustic metamaterial baffles (AMBs) with locally resonant structures in order to reduce the noise level in the desired frequency range; a desired frequency range was targeted to be affected by tuned resonators. For describing AMBs, four models were designed to obtain transmission loss peaks and stop-band behavior at the dual-target-frequency range. Hence, in order to prove the effectiveness of the proposed muffler, they were compared with conventional mufflers. Results indicated that AMBs improve the muffler transmission loss by up to 89 dB. The combination of resonators and their configurations, as well as spacing of the AMBs, play a significant role in affecting the transmission loss peak. At the end, to observe and show the actual performance of the proposed acoustic metamaterial muffler, essential aspects like pressure drop and flow are obtained using computational fluid dynamics (CFD) software, and highly reasonable and realistic results are achieved.

Optimizing the acoustic properties of a meta-material using machine learning techniques

The scope of this work is to consolidate research dealing with vibroacoustics of periodic media. This investigation aims at developing and validating tools for the design of global vibroacoustic treatments based on foam cores with embedded periodic patterns, which allow passive control of acoustic paths in layered concepts. Firstly, a numerical test campaign is carried out by considering some solid (but still non-perfectly rigid) inclusions in a 3D-modeled porous structure; this causes the excitation of additional acoustic modes due to the periodic nature of the meta-core itself. Then, some design guidelines are provided in order to predict several possible sets of characteristic parameters (i.e. inclusion geometry, elastic and foam properties) that, constrained by the imposition of mass and thickness of the acoustic package, may satisfy the target functions (i.e. the frequency at which the first Transmission Loss peak appears, together with its amplitude). Results are obtained through the implementation of machine learning algorithms, which may constitute a good basis in order to perform preliminary design considerations that could be interesting for further generalizations.

Numerical investigation on low-frequency noise damping performances of Helmholtz resonators with an extended neck in presence of a grazing flow

In this work, modified designs of Helmholtz resonators with extended deflected neck are proposed, numerically evaluated and optimized aiming to achieve a better transmission loss performance over a broader frequency range. For this, 10 Helmholtz resonators with different extended neck configurations (e.g. the angle between extended neck and the y-axis) in the presence of a grazing flow are assessed. Comparison is then made between the proposed resonators and the conventional one, i.e. in the absence of an extended neck (i.e. Design A). For this, a two-dimensional linearized Navier Stokes equations-based model of a duct with the modified Helmholtz resonator implemented was developed in frequency domain. The model was first validated by comparing its numerical predictions with the experimental results available in the literature and the theoretical results. The model was then applied to evaluate the noise damping performance of the Helmholtz resonator with (1) an extended neck on the upstream side (Design B); (2) on the downstream side (Design C), (3) both upstream and downstream sides (Design D), (4) the angle between the extended neck and the y-axis, i.e. (a) 0°, (b) 30°, and (c) 45°, (d) 48.321°. In addition, the effects of the grazing flow Mach number (Ma) were evaluated. It was found that the transmission loss peaks of the Helmholtz resonator with the extended neck was maximized at Ma = 0.03 than at the other Mach numbers. Conventional resonator, i.e. Design A was observed to be associated with a lower transmission loss performance at a lower resonant frequency than those as observed on Designs B–D. Moreover, the optimum design of the proposed resonators with the extended neck is shown to be able to shift the resonant frequency by approximately 90 Hz, and maximum transmission loss could be increased by 28–30 dB. In addition, the resonators with extended necks are found to be associated with two or three transmission loss peaks, indicating that these designs have a broader effective frequency range. Finally, the neck deflection angles of 30° and 45° are shown to be involved with better transmission loss peaks than that with a deflection angle of 0°. In summary, the present study sheds light on maximizing the resonator’s noise damping performances by applying and optimizing an extended neck.