Resonant Research Articles

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.

A fundamental investigation on the sound absorption characteristics of acoustic meta-surface type baseboards

The sound of footsteps generated in corridors propagates into surrounding spaces, sometimes becoming a problem of noise pollution. Therefore, the aim of this study is to reduce footstep noise by incorporating sound absorption mechanisms into the baseboard of corridors. We focus on the acoustic metasurface that has gained significant attention in recent years in order to absorb a relatively wide range of frequencies. In this study, we proposed acoustic metasurface type baseboards by utilizing multiple resonators with different resonance frequencies. The proposed models feature parallel arrangement of multiple resonators with straight or inclined slits. The absorption performance of these models is evaluated through theoretical analysis, numerical simulations, and experiments. Results suggest that acoustic metasurface type baseboards is found to be able to absorb the dominant frequencies in the sound of footsteps. In addition, increasing the number of resonators and incorporating slanted perforations in the neck contribute to improved absorption performance. Furthermore, the possibility of thinning the models through slanted perforations is also suggested.

28/38 GHz compact rhombic-shaped antenna for future generations of mobile communications

ABSTRACT It is mandatory for the modern antennas to be designed to operate in the millimetre-wave (mm-wave) range of the electromagnetic spectrum for future generations of mobile communication requires multiband operation of a single antenna. The present work proposes a dual-band antenna to operate at 28 and 38 GHz . The design stages of the proposed antenna and examples of the parametric study performed to arrive at the final antenna design are demonstrated and discussed in detail. The proposed antenna is constructed as a main patch with parasitic element that is capacitively coupled to the main patch through a narrow slot. The main patch is a rhombic-in-shape and excited through a microstrip live with inset feed. This rhombic shaped patch antenna is originally designed to operate at 28 GHz . To achieve the dual-band operation, the geometry of the main patch is modified and is loaded by a parasitic element. The antenna is fabricated for experimental assessment through measurement of the antenna impedance, radiation patterns, maximum gain and radiation efficiency. The proper operation at 28 and 38 GHz is confirmed by experimental verification of the simulation results. Both the simulation and measurements show that the proposed antenna has 50 Ω -matched impedance over the frequency bands 27.7 − 28.4 GHz and 37.7 − 38.4 GHz . The proposed antenna has been designed to enhance the communications system performance when used in the mobile networks through noise rejection by limiting the operation within relatively narrow bands around the resonant frequencies. The values of the maximum gain obtained at 28 and 38 GHz are 6.98 and 8.8 dBi , respectively. The radiation efficiencies of the proposed antenna are 89 % and 87 % , respectively, and the total efficiency is also calculated.

Optimization of an active device for global control of low-frequency wall reflections in a semi-anechoic room

An active device has been designed and built at LMA to control low-frequency reflections on the absorbing walls of a semi-anechoic room: the sound pressure reflected by the walls in the presence of an unknown source is estimated by linear filtering of the total sound pressure near the walls, then neutralized using acoustic sources placed on the walls. Here, we present 2D numerical simulations, involving monopole sources and series of damped analytic modes, which have enabled us to optimize the active device, in particular to (i) deal with resonance frequencies at which cancelling the diffracted pressure near the walls is not sufficient to cancel it inside the room, (ii) minimize the number of the total pressure measurement points required to estimate the pressure diffracted by the walls at each location, and (iii) select the strategy for minimizing the virtual signals corresponding to the diffracted pressure. Simulations show that with the optimized device, the control remains efficient up to frequencies corresponding to one wall source per wavelength and a little less than two pressure sensors per wavelength. Experiments are underway to compare measurements with simulations.

Estimation of mass and moisture content of flat square lumber in a stack during air-drying process by the vibration method with additional mass

A vibration test to measure the mass of a specimen without weighing it using the difference between the resonance frequency with an additional mass and that without it (vibration method with additional mass, VAM) was applied to Japanese cedar flat square lumber with dimensions of 210 mm × 135 mm × 3000 mm both with and without a pith in the process of air-drying. The longitudinal and bending vibration tests were performed on the specimens with and without an additional mass. The accuracy of VAM that was expressed by the estimated mass by VAM to the measured mass (MVAM/M0) was sufficiently high throughout the air-drying process. The accuracy of VAM for the longitudinal vibration was higher than that for the bending vibration. The moisture content based on the estimated mass by VAM was in the expected range.

Experimental encircling of an exceptional point in a two degrees of freedom system: overview and application to noise control

When a resonator is added to an acoustic cavity or a vibrating structure, a new resonance frequency, or pole, is created in the system. When dissipation is present, this may breakdown when some parameters are finely tuned. This phenomenon, typical of non-Hermitian dynamics is called an exceptional point in the parametric space where two (or more) eigenvalues, as well as their associated eigenvectors, coalesce. In the recent years, EPs have gained interest in physics because of the counter-intuitive concepts associated with them. In the fields of acoustics and vibration, understanding EP may lead to a better comprehension of modal energy exchanges and decay. This work aims at exploring the key concepts related to EPs by revisiting the well-known coupled pendulums problem in the presence of damping. First, the free response of the experimental system is investigated after tuning it on an EP. Then an encircling is performed by varying the parameters through time. Our experimental results illustrate in a pedagogical manner nearly-optimal dissipation, energy exchange and chirality effects which have been recently evidenced in physics.

Extending the frequency range of ISO 717-2 - a look at options and issues

In 2019, the Norwegian Standard NS 8175, which defines requirements for sound insulation in Norwegian buildings, was revised to include low frequency spectrum adaption terms for new dwellings. This change affects the applicability of measurements of weighted improvement of impact sound insulation, delta Lw, as the spectrum for calculating delta Lw, given in ISO 717-2, is limited downwards to 100 Hz. For a floating floor construction, this means that floors with a resonance frequency well below 100 Hz, and correspondingly a relatively high delta Lw, will be overrated in the sense that the measured delta Lw will be unaffected by the poor low frequency performance of the floating floor. To properly address this increased focus on low frequencies, an extended range spectrum is needed. In the work leading to this paper, a selection of extended range values for the reference heavyweight floor have been suggested, starting with a linear extrapolation of the values given in ISO 717-2. Data from laboratory measurements of delta Lw for floating floors have then been used to investigate the suitability of the suggested values. The results are summarised and discussed in this paper.

Towards enhanced metamaterials via shape memory alloy-embedded nonlinear resonators

Standard acoustic metamaterials create an attenuation around specific frequency bands. Finite structures, with distinct boundary conditions and a limited number of unit cells, present less attenuation and narrower bands than those predicted by idealized infinite systems. A critical engineering goal is enhancing performance to motivate practical applications. To that aim, this work incorporates shape memory alloys into a quasi-zero stiffness resonator design, allowing the tuning of each cell's internal resonance. Such a feature enables the system reconfiguration to achieve different purposes, from following a disturbance frequency fluctuation to realizing graded nonlinear metamaterials. While the former can work under an adaptive framework, the latter can result in time-invariant locally resonant metamaterials with wideband vibration attenuation. The proposed design combines a frame that axially compresses an SMA beam in the vicinity of its buckling. The critical buckling force changes by thermally activating the SMA, affecting the residual stiffness, hence the unit cell resonant frequency. The unit cell concept is developed analytically and via finite elements, then built and validated experimentally. The promising results indicate the viability of the proposed concept. Preliminary numerical simulations and experiments show the advantages of the proposed smart resonator over a linear time-invariant counterpart.

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.

Design and evaluation of an acoustic liner applied to a UAV ducted rotor in hover

Unmanned aerial vehicles (UAVs) are currently being used for reconnaissance missions, tactical surveillance, and infrastructure inspection. These missions and operations could be performed near inhabited areas, possibly leading to noise complaints. For most UAVs, the rotor blades are the dominant source of noise. While some UAV concepts are equipped with rotor ducts for aerodynamic and safety reasons, few studies focus on the acoustic benefit of rotor ducts with integrated acoustic liners, similar to implementations on turbofan aircraft. The liner concept used in this study, called a Long Elastic Open Neck Acoustic Resonator (LEONAR), achieves good low frequency performance in a limited available space. The LEONAR concept extends the holes of the facesheet perforations into the resonator cavities with hollow tubes. As a result, the columns of air in the facesheet holes are prolongated without increasing facesheet thickness, thereby shifting the resonant frequencies of the liner lower. The objectives of this paper are to describe the simulation of noise radiated from a rotor in a duct with a LEONAR liner, the design of a liner integrated along the duct inner surface to mitigate radiated noise between 1000-5000 Hz, and impedance tube tests of liner samples to validate the predicted impedances.

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.

Validation of an eigenvalue-based inverse method for estimating locally-reacting surface impedance

Accurate boundary condition descriptions are essential for creating accurate computational models for room acoustics. These descriptions are often defined in terms of absorption coefficients or surface impedances. However, obtaining complex-valued, frequency-dependent values for an acoustically absorbing material sample at low frequencies is a challenging task. In fact, the current measurement standard ISO 354:2003 does not provide a method for measuring absorption coefficients below 100 Hz. In this paper, a recently proposed eigenvalue-based inverse method for estimating locally-reacting impedance at measurement system resonance frequencies is validated. The validation uses data from three samples measured in an impedance tube. The method is also used to estimate complex-valued, frequency-dependent impedances at frequencies below 100 Hz in a measured reverberant room and in a simulated reverberation chamber.

Impact of surrounding environment on impinging supersonic jets resonances and their hysteretical behavior

Supersonic jets impinging on flat surface produce intense acoustic tones. Recent measurement campaigns have indicated that the surrounding environment geometry or its acoustic treatment can drastically change the observed resonance frequencies and amplitudes at identical flow regimes. For example, at supersonic speeds, a hard walled external nozzle shape leads to the existence of two main types of resonances: antisymmetric resonances (with frequencies close to the free jet screech) and axisymmetric resonances. Frequencies associated to those different resonances are very distinct. We have observed that Applying a simple non-reflective material to the outer surfaces of the nozzle suppresses the axisymmetric resonances and that antisymmetric resonances seems to be contained in the frequency range of existence of trapped acoustics modes, known to be responsible for screech resonances. Additionally, when no acoustic treatment is applied on the nozzle wall, the experiments show that the switch between different resonance regimes (symmetric - antisymmetric), induced by changes in Mach number, has an hysteretical behavior. To the authors' knowledge, this has been rarely discussed in the literature and adds to the actual complexity of the aeroacoustic behavior of such jet flows, especially when modeling is considered.

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.

Deep subwavelength sound absorption by a buckling plate resonator

Conventional sound-absorbing materials face great difficulties in achieving deep subwavelength broadband sound absorption due to the causal constraint on the minimum thickness. A buckling plate resonator is proposed to alleviate the causal constraint on the thickness of sound-absorbing materials. The resonator consists of a back cavity sealed by an elastic circular thin plate. When the plate is subjected to a uniform in-plane compressive force up to its critical load, it buckles and behaves as a negative stiffness spring. The positive stiffness of the back cavity is counteracted by the negative stiffness of the buckling plate, resulting in resonance absorption at the deep subwavelength scale. The sound absorption performance of the buckling plate resonator under different in-plane loads was measured in an impedance tube. Quasi-perfect sound absorption was observed in a buckling plate resonator with a thickness less than 1% of the wavelength corresponding to the resonance frequency. This work provides a solution for the design of ultra-thin broadband sound absorbers.

Comparison of traditional and deep learning optimisation for the design of acoustic metamaterials

This paper presents a comparative analysis of traditional and Deep learning (DL) optimisation approaches for AMM design. A case study comparing a generalised pattern search (GPS) optimisation to a previously trained inverse design deep neural network (IDDNN) of an acoustic meta-absorber using various metrics is considered. For benchmarking, a baseline design is realised with arbitrarily chosen geometric parameters in a CAD software. This design is 3D printed and experimentally tested. The peak absorption and corresponding resonant frequency obtained from experimental absorption, along with other geometric parameters, are used as inputs to IDDNN and GPS. Results show that IDDNN provides superior inference speed, requiring 7.24e-3 [s] to generate predictions on a single input instance, compared to GPS which took about 30.5464 [s]. While GPS exhibits slightly closer alignment to experimental curve based on Kullback-Leibler divergence (2.79e-3) compared to IDDNN (3.31e-3), IDDNN still achieves a slightly higher accuracy of 92%, compared to 90% for GPS. It is emphasised that trade-offs may be necessary in practice to determine the most suitable design approach for a given application. Traditional optimisation offers ease of design flexibility, allowing for on-the-fly model improvement and modification. In contrast, DL may require re-training, leading to delays in process workflow

Analysis of nonminimum phase components of noise control filters in active noise control

In the active noise control (ANC), it is known that the optimal filter becomes a noncausal filter due to the effect of the nonminimum phase components of the secondary path. We have investigated how to make the causal optimal filter by replacing the nonminimum phase components with the frequency components of the primary path. Through the computer simulations, we have confirmed that this filter maintained some noise reduction performance and can be used as a quasi-optimal filter. In this paper, as another approach, we attempt to identify noncausal components using the pre-trained adaptive noise control filter. The nonminimum phase components of the secondary path exist in the frequency band below the lowest resonance frequency of the secondary loudspeaker. Based on this fact, we design a noise control filter by using band-limited white noise only with the low frequency components below the lowest resonance frequency in advance. Then, we design another adaptive noise control filter by using the band-limited white noise with high frequency components above the lowest resonance frequency. Finally, we combine these filters in the frequency domain. Through the computer simulations, we show the difficulty to handle the nonminimum phase components in the ANC system.

Comparative assessment of the stability of buccal shelf mini-screws with and without pre-drilling- a split-mouth, randomized controlled trial

ObjectivesTo examine and compare the stability of buccal shelf mini-screws using self-drilling and pre-drilling implant placement techniques.MethodologyIt was a split-mouth, randomized controlled trial comprising of 7 patients, each receiving two buccal shelf bone screws. The screws were placed using a self-drilling protocol in one quadrant and a pre-drilling protocol in the opposing quadrant decided via coin toss randomization. Stability was examined at the time of placement and 1,2, 3 and 4 months thereafter, using the Resonance Frequency Analysis method with the Osstell Beacon TM device. The Implant Stability Quotient (ISQ) obtained was then compared and assessed between both quadrants. Friedman’s Two-Way Analysis of Variance and the Wilcoxon signed rank test were utilized for the intergroup comparison. A statistically significant result was defined as one with a p-value of less than 0.05.ResultsA statistically significant difference between the mean ISQ reading in the pre-drilling and self-drilling group was observed, indicating higher stability of bone screws placed with the pre-drilling protocol. The primary stability of the buccal shelf screws decreased after placement, but the secondary stability remained stable.ConclusionBuccal shelf bone screws placed with a pre-drilling protocol depicted greater primary and secondary stability as compared to the self-drilling protocol, as depicted by the ISQ readings obtained. Resonance Frequency Analysis can be used as a valuable tool to assess the stability of buccal shelf bone screws.Clinical relevanceThe use of buccal shelf screws has increased tremendously over the past few years due to their myriad applications and have now become an essential part of an orthodontist’s armamentarium. It is therefore essential for clinicians to be well-informed about all aspects of their use including insertion techniques. The results of this trial indicate that the pre-drilling protocol provides better stability and therefore treatment outcomes.

Characterization of pressure oscillations and combustion noise of hydrogen reactivity-controlled compression ignition (RCCI)

Knock and combustion noise are the main reasons limiting the increase in hydrogen (H2) replacement rate in H2 reactivity-controlled compression ignition (RCCI). This study aims to identify the knock-dominated mechanism and elucidate the characteristics of knock and combustion noise of H2 RCCI operated at a broad range of parameters through the combined analysis of the heat release profile and pressure oscillations. The results indicated that under inhomogeneous heat release conditions, the onset of pressure oscillations is mainly attributed to the local fast heat release at the initial combustion stage, but for the conditions with relatively homogeneous and high-reactivity mixture, the pressure oscillations are primarily excited by the end-gas autoignition. Compared with the single injection strategy, the double injection strategy with the first and second start of injection of −60 and −15°CA ATDC (SOI1/2 = −60/-15°CA ATDC) can effectively mitigate the pressure oscillations and shorten the propagation period of pressure waves. However, as the SOI2 is advanced to −30°CA ATDC, the pressure oscillations dramatically increase, because the enhanced fuel reactivity of premixed H2 exacerbates the end-gas autoignition. The results of frequency analysis of in-cylinder pressures show that with the increase in inhomogeneous combustion, the first resonance frequency reduces. As the intake pressure (Pin) is raised from 1.2 to 1.6 bar, the first resonance energy increases, since the elevated in-cylinder temperature associated with the piston compression during the compression stroke enhances the propagation and development of pressure waves. The high-order resonance is closely related to the autoignition of end gas with higher fuel reactivity. Overall, the combustion noise shows a good linear correlation with peak heat release rate for all the points investigated in this work, but the linearity of H2/polyoxymethylene dimethyl ethers RCCI is better than that of H2/diesel RCCI. Moreover, with the increase in Pin, the engine noise associated with the piston compression increases.

Development and Field Test of Integrated Electronics Piezoelectric Accelerometer Based on Lead-Free Piezoelectric Ceramic for Centrifugal Pump Monitoring.

In this study, an Integrated Electronics Piezoelectric (IEPE)-type accelerometer based on an environmentally friendly lead-free piezoceramic was fabricated, and its field applicability was verified using a cooling pump owned by the Korea Atomic Energy Research Institute (KAERI). As an environmentally friendly piezoelectric material, 0.96(K,Na)NbO3-0.03(Bi,Na,K,Li)ZrO3-0.01BiScO3 (0.96KNN-0.03BNKLZ-0.01BS) piezoceramic with an optimized piezoelectric charge constant (d33) was introduced. It was manufactured in a ring shape using a solid-state reaction method for application to a compression mode accelerometer. The fabricated ceramic ring has a high piezoelectric constant d33 of ~373 pC/N and a Curie temperature TC of ~330 °C. It was found that the electrical and physical characteristics of the 0.96KNN-0.03BNKLZ-0.01BS piezoceramic were comparable to those of a Pb(Zr,Ti)O3 (PZT) ring ceramic. As a result of a vibration test of the IEPE accelerometer fabricated using the lead-free piezoelectric ceramic, the resonant frequency fr = 20.0 kHz and voltage sensitivity Sv = 101.1 mV/g were confirmed. The fabricated IEPE accelerometer sensor showed an excellent performance equivalent to or superior to that of a commercial IEPE accelerometer sensor based on PZT for general industrial use. A field test was carried out to verify the applicability of the fabricated sensor in an actual industrial environment. The test was conducted by simultaneously installing the developed sensor and a commercial PZT-based sensor in the ball bearing housing location of a centrifugal pump. The centrifugal pump was operated at 1180 RPM, and the generated vibration signals were collected and analyzed. The test results confirmed that the developed eco-friendly lead-free sensor has comparable vibration measurement capability to that of commercial PZT-based sensors.