Back Cavity Research Articles

Modeling of the tensioned membrane backed by a variable-shape cavity and research on acoustic characteristics

The drum-type muffler comprises a rigid back cavity and a tensioned membrane, which utilizes the radiation sound field of a membrane and the superposition effect of the enclosed sound field to achieve noise reduction. However, most previous studies have been focused on idealized models, with little research on the sound radiation power from a membrane concerning a variable-shape back cavity. This article aims to establish a membrane vibration-acoustic model and investigate the impact of irregular shape and pre-tension conditions on the radiated sound energy. The microunits on the surface of the membrane and the pressure of the sound field are represented as cosine Fourier series. The variable-shape cavity is converted into a normal one by employing the coordinate transformation method. The analytical model of the coupling structure composed by a membrane and the enclosed sound fields is developed based on the energy principle. The acoustic and vibration characteristics of the membrane are solved based on the Rayleigh integral equation. This study also considers the difference in sound insulation performance between the thin plate and the membrane. The analytical results are compared with Finite Element Analysis (FEM) to verify the correctness of the theoretical model. In addition, based on the LMS Simcenter measurement system, experiments are designed to predict the sound pressure on both sides of the membrane. The test results closely correspond with the theoretical calculation results, which confirms the practicality of the theoretical approach.

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.

Random incidence absorption characteristics and parameterized design of micro-perforated panel absorbers with parallel-arranged backing cavities at different depths

Micro-perforated panel absorbers (MPAs) are recognized as efficient and environmentally friendly materials for sound absorption, prompting significant research efforts to expand their frequency ranges. This study investigates the acoustic performance of MPAs with parallel-arranged backing cavities at different depths (PCD) under random incidence conditions. Initially, an analytical model is proposed to predict the acoustic performance of PCD-MPA. The model exhibits significant consistency with finite element simulations and experimental measurements, validating its reliability in calculating absorption coefficients within the primary frequency band of concern (500~4000 Hz) in architectural acoustics applications. Subsequently, a thorough discussion of the key parameters, including the depth sequence and perforation parameters, which influence the random incidence absorption characteristics of PCD-MPA is presented based on the proposed model. These discussions lead to the development of an enhanced parameterized design method aimed at improving the absorption performance of PCD-MPA, enabling precise modulation of parameter combinations to meet specific application requirements and design nearly perfect absorbers.

Suppression of Azimuthal Combustion Instability Using Perforated Liner Under Symmetry Breaking

Symmetry breaking occurring in annular combustors shows different stability patterns due to the formation of nondegenerate modes with different frequencies. This also brings about more profound complexity for control methodology. This paper presents a three-dimensional analytical model to investigate how to control nondegenerate azimuthal modes by modifying the wall boundary conditions of a multiburner annular combustor. The stability of the system can be evaluated by solving the eigenvalue of the dispersion relation equation derived in this study. Results show that changes in the parameters of the perforated liner cause different performance in nondegenerate modes due to frequency splitting. Meanwhile, the asymmetric perforated liner alters the original asymmetry of the annular combustor, inducing a shift in the nature of nondegenerate modes when significant symmetry breaking exists. Furthermore, variations of pressure amplitude in the backing cavity lead to diverse acoustic energy dissipation and even distinct stability patterns under two types of azimuthally nonuniformly perforated liner. Overall, this paper presents a new methodology for suppressing nondegenerate azimuthal modes under symmetry breaking by utilizing a perforated liner in an annular combustor. Additionally, a clear understanding of the evolution behavior of nondegenerate modes is also provided by studying their stability behavior and energy dissipation.

Wideband Millimeter Wave Antenna with Cavity Backed Slotted Patch and Magneto-Electric Dipole

Wideband Millimeter Wave Antenna with Cavity Backed Slotted Patch and Magneto-Electric Dipole

Dual-Band Antenna Array Fed by Ridge Gap Waveguide with Dual-Periodic Interdigital-Pin Bed of Nails.

A dual-band (K-/Ka-band) antenna array is presented. An ultra-wideband antenna element in the shape of a double-ridged waveguide is used as a radiation slot, and a novel dual-periodic ridge gap waveguide (RGW) with an interdigital-pin bed of nails (serving as a filter) is used to realize dual-band operation. By periodically arranging the pins of two different heights in two dimensions, the proposed RGW with interdigital-pin bed of nails is able to realize and flexibly adjust two passbands. The widely used GW-based back cavity boosts the realized gain and simplifies the feed network design. A 4 × 4 prototype array was designed, fabricated, and measured. The results show that the array has two operating bands at 24.5-26.4 GHz and 30.3-31.5 GHz, and the realized gain can reach 19.2 dBi and 20.4 dBi, respectively. Meanwhile, there is a very significant gain attenuation at stopband.

The utility model relates to an L-band slot antenna conformal with a submarine cable

In order to better receive Beidou satellite signals on ships, miniaturized antennas are becoming more and more important. To solve the problem of antenna miniaturization, this paper proposes a novel back cavity slot antenna (CBSA) in conformation with submarine cable. The slot antenna works in the L-band of BDS, adopts parasitic patch structure, and uses HFSS electromagnetic simulation software for simulation and analysis. Results The maximum electric field intensity between the gaps on the surface of the antenna decreased from 854.31V/m to 681.88V/m, and the average gain of the antenna increased by 0.15dB.

Frequency band optimization of cavity-type metamaterials by acoustic split-frequency multiplexing

This paper reports an experimental study on a cavity-type metamaterial for broadband sound absorption to break the one-to-one correspondence between back cavity and absorption peak. An acoustic valve composed of a thin plate and a concentrated mass was proposed to pass and block specific frequency sound waves. The results indicate that under the action of the acoustic valve, the unit cell of the metamaterial can obtain more absorption peaks than the number of physical back cavities. Finite element simulation shows that this mechanism can be attributed to acoustic split-frequency multiplexing in the back cavity. Specific parameters for the unit cell are optimized based on genetic algorithms. The consistent finite element simulation and experimental results indicate that the metamaterial with the acoustic valve can achieve efficient absorption performance at 150–550 Hz with a thickness-to-wavelength ratio of 1/28. This study provides valuable design strategies for cavity-type metamaterials to achieve broadband sound absorption.

Characterization and analysis of granular activated carbon stacks subject to the edge-constraint effect

The objective of the current work was to identify the conditions under which the acoustical properties of unconsolidated stacks of granular activated carbon (GAC) can be accurately characterized based on impedance tube measurements. GAC particles are of interest since they have proven beneficial in various low-frequency applications: e.g., micro-speaker backing cavities. However, since the particles tend to stick to the sample holder wall, a time-consuming 2-D numerical model is often required to capture their acoustical physics. Here, by identifying a configuration in which the edge-constraint effect was minimized while still maintaining distinct frame resonance features, the GAC stack properties could be successfully characterized by using a more efficient 1-D model. The inferred parameters were validated by comparing predictions with measurements of stacks with different depths, diameters, and excitation levels. The proposed framework can be followed to accurately characterize GAC stack properties, thus allowing optimizations of sound packages including similar materials.

Integrated analytical, experimental, and numerical study of vibrational response in clamped circular plates with pressurised air cavities

This paper presents a comprehensive investigation, comprised of analytical, experimental and numerical approaches, into the interaction between the sound field produced within a pressurised air-filled back cavity and the vibration of a thin clamped circular plate subjected to its pressure. Previous investigations into the interaction of sound fields with vibrating boundaries have primarily been restricted to non-pressurised cavities. We extend the application of the weak modal coupling method to derive a solution for this unexplored physical problem by utilising classic plate theory, Fourier-Bessel series formulation and the uncoupled solutions for a pressurised cylindrical sound field and a thin clamped circular plate with uniform radial tension. We describe the vibration response of the coupled system in terms of the uncoupled cavity and plate modes. Using a set of geometric and physical properties for the system, the resonance frequencies and response functions are calculated and the coupling between the sound field and the plate is investigated as a function of cavity pressure, providing novel fundamental insights into the physical system. We construct an experimental rig to embody the conditions of the analytical investigation for the purpose of validation. Finite element analysis (FEA), employing modal analysis and transient dynamic analysis, is used to further validate and extend the analytical insights while more accurately mirroring the experimental conditions. The response functions, mode shapes, resonance frequencies and their dependence on the cavity pressure were experimentally measured and numerically simulated, with direct comparisons made with the analytical model. A high degree of accuracy for the analytical model is validated along with its ability to describe the underlying physical phenomena. The validated analytical model is then leveraged to perform fundamental explorations into the modal contributions as a function of cavity pressure and a parametric sensitivity analysis on the effects of plate radii and plate thickness on the coupled system resonance frequencies.

Numbering-up and stability strategies of bubbles in yield stress fluids in asymmetric parallelized microchannel

Preparation of foams within microchannels is a clean and capable of precisely controlled synthesis. In this work, high throughput production of bubbles in Carbopol gels (0.05 wt%, 0.1 wt%, 0.2 wt%) in asymmetric parallel channels is investigated. Bubbles in Carbopol gels are strongly unstable, and the asymmetry of the microchannel makes bubble instability more pronounced. A “slug-slug” flow pattern is observed for only 0.05 wt% Carbopol and microchannels with back cavities, and the bubble train is highly unstable. The strategy proposed in this article solves the challenge of numbering-up of bubbles produced in YSFs by strategically reducing the gas pressure, and the stability of bubble train and activation rate of microchannel are improved, which is of great engineering significance for the high-throughput production of high-quality foam materials.

Centrifugal pump performance improvement using back cavity filling

Centrifugal pumps consume majority of energy used by the electric motor driven systems among all end users of the industrial sector. Only in European region, a 1% improvement in efficiency (η) of the centrifugal pump is capable enough to reduce CO2 emissions by at least 572 tons per day. The prime motive of this research is to assess the applicability of the novel back cavity filling (BCF) in reducing the disk friction losses of centrifugal pumps without changing the pump's original design. By adding a solid ring to the bearing housing's back-cover plate, the BCF was applied to a medium specific speed pump (Ns = 54 rpm), resulting in a 1 mm back axial clearance from 13 mm. Numerical simulations and experiments were carried out to investigate the effect of BCF on pump performance under two conditions: with and without BCF at best efficient point (BEP) and various partload conditions for 1450 rpm (rated) as well as 1000 rpm. Pump performance characteristics were enhanced by BCF at both rpms for the whole flow rate range. At 1450 rpm, BCF resulted in an average 0.74% increment in total head, 2.08% reduction in input torque, and 2.70% increment in overall efficiency over the flow rate range. While an average 2.04% improvement in total head, 1.33% reduction in input torque, a 3.30% enhancement in overall efficiency were obtained at 1000 rpm. Performance parameter enhancement was higher at lowest part load compared to the BEP on rated rpm.

Miniaturized wide‐band microstrip grid array antenna loaded with slow‐wave transmission line

AbstractA miniaturized microstrip grid array antenna (MGAA) in X‐band is proposed in the paper. The antenna consists of a radiation grid array, a feed network and a metal back cavity. The 2 × 2 planar grid array is constructed by using four MGAAs as subarrays. MGAA does not require complex feeders in contrast to microstrip patch arrays. Furthermore, a curved slow‐wave transmission line (SWTL) is adopted as the long side of grid. The SWTL not only reduces the physical size of the grid while keeping its electrical size unchanged, but also improves the impedance matching and greatly broaden the bandwidth of the antenna. A fabricated prototype was measured in the microwave chamber. The results indicate that the impedance matching band of |S11| < −10 dB is from 9 to 10.5 GHz (15.4%), and the maximum gain is 19.2 dBi. Meanwhile, the projected area of the antenna is reduced by about 40% by loading the SWTL.

Mechanically robust poly(urethane-imide) aerogels with excellent thermal insulation and sound absorption by ambient drying

Polyimide (PI) aerogels are the great candidates for sound absorption, thermal insulation and mechanical cushioning materials due to their low density, excellent mechanical properties and good thermal stability. However, the high shrinkage and hydrophilicity of PI aerogels, as well as the time-consuming and complexity of the process directly limit their wide application in many fields. Herein, a facile one-step method for the synthesis of porous monolithic poly(urethane-imide) (PUI) aerogels was successfully developed. Hydrothermal synthesis and ambient drying methods were adopted in the construction of monolithic chemically cross-linked PUI aerogels, which avoids the complex imidization process. PUI aerogels were homogeneously produced by a chemical cross-linking copolymerization reaction between polyurethane (PU) oligomers, Benzene-1,2,4,5-tetracarboxylic dianhydride (PMDA) and 1, 3, 5-tris (6-isocyanatohexyl)-1, 3, 5-triazine-2, 4, 6(1H, 3H, 5H)-trione without any catalysts. Interestingly, the introduction of linear flexible PU oligomers chain segments effectively prevented the shrinkage of the network backbone and enhanced the hydrophobicity of PUI aerogels. The obtained PUI aerogels have low density (0.196 g/cm3), low shrinkage (9.30%), high porosity (79.54%) and hydrophobicity (water contact angle of 129.2°). Under high pressure, PUI aerogel was compressed but not break and the compression Young's modulus as high as 3.638 MPa. Moreover, the thermal conductivity of PUI aerogels was only 59.92 mW/m/K at 150 °C. When the back cavity depth was 5 mm, the highest sound absorption coefficient of PUI aerogel was up to 0.7219. This work provides a new method for the development of lightweight, high-mechanical-strength polymer aerogels, and the prepared aerogels have a potential application in the field of multifunctional integrated sound absorption materials.

Random incidence sound absorption of micro-perforated panel absorbers backed with parallel-arranged cavities of different depths

This study investigates sound absorption performance of a kind of wideband micro-perforated panel (MPP) absorbers with parallel-arranged backing cavities at different depths in a diffuse field. An analytical prediction model is presented for evaluating the random incidence absorption coefficient of this type of absorbers, and it exhibits excellent agreement with finite element model and experimental measurements. Then, numerical cases were established using this model to investigate the absorption characteristics of this type of absorbers under random incidence, considering the influence of cavity depths and MPP parameters. Results show that the cavity depth and MPP perforation diameter in this absorber are primary influencing parameters controlling the overall sound absorption performance. Cavities at different depths introduce multiple resonant absorption peaks, while the perforation diameter determines matching conditions between the MPP and air impedance, which simultaneously broaden the absorption bandwidth of this MPP absorber. Although individually adjusting the panel thickness and perforation rate has a smaller impact compared to the perforation diameter, the interaction between these two parameters has significant influences across the entire absorption frequency band.

Miniaturized and highly sensitive fiber-optic Fabry–Perot sensor for mHz infrasound detection

Infrasound detection is important in natural disasters monitoring, military defense, underwater acoustic detection, and other domains. Fiber-optic Fabry–Perot (FP) acoustic sensors have the advantages of small structure size, long-distance detection, immunity to electromagnetic interference, and so on. The size of an FP sensor depends on the transducer diaphragm size and the back cavity volume. However, a small transducer diaphragm size means a low sensitivity. Moreover, a small back cavity volume will increase the low cut-off frequency of the sensor. Hence, it is difficult for fiber-optic FP infrasound sensors to simultaneously achieve miniaturization, high sensitivity, and extremely low detectable frequency. In this work, we proposed and demonstrated a miniaturized and highly sensitive fiber-optic FP sensor for mHz infrasound detection by exploiting a Cr-Ag-Au composite acoustic-optic transducer diaphragm and a MEMS technique-based spiral micro-flow hole. The use of the spiral micro-flow hole as the connecting hole greatly reduced the volume of the sensor and decreased the low-frequency limit, while the back cavity volume was not increased. Combined with the Cr-Ag-Au composite diaphragm, a detection sensitivity of −123.19 dB re 1 rad/μPa at 5 Hz and a minimum detectable pressure (MDP) of 1.2 mPa/Hz1/2 at 5 Hz were achieved. The low detectable frequency can reach 0.01 Hz and the flat response range was 0.01–2500 Hz with a sensitivity fluctuation of ±1.5 dB. Moreover, the size of the designed sensor was only 12 mm×Φ12.7 mm. These excellent characteristics make the sensor have great practical application prospects.

Investigating the Impact of Back Cavity Filling and Axial Clearance on the Flow Physics and Performance of a Pump as Turbine

Investigating the Impact of Back Cavity Filling and Axial Clearance on the Flow Physics and Performance of a Pump as Turbine

Exploring the acoustic potential of 3D printed micro-perforated panels: A comparative analysis

In the present study, the sound absorption performance of inhomogeneous Micro-Perforated Panels (MPPs) with multiple cavities is investigated. Two models, a three-cavity system and a four-cavity system, are proposed and a numerical study is performed using MATLAB. The models are validated through experimental analysis in an impedance tube. The study meticulously varies the geometrical parameters, including pore diameter, thickness of the MPP, perforation ratio, and back-cavity length. It is found that MPPs with a greater number of sub-cavities have a better sound absorption coefficient than two-cavity systems. The results suggest that the back air cavity is predominantly responsible for multiple peaks, ensuring wideband sound absorption. It is also found that smaller perforation ratios for sub-cavities with larger pore diameters improve sound absorption performance in the lower frequency region. The study indicates that a pore diameter of less than 0.5 mm should be used for better sound absorption above the range of 800–850 Hz, and back cavity length has greater control than pore diameter between 850 Hz and 2000 Hz to make the curve smooth with less fluctuation. The findings have significant implications for the design of MPPs for real-world applications.

Further investigation of the vibration characteristics and sound radiation of Balinese gamelan gongs

Balinese gamelan gongs are percussion instruments of special interest because of their unique shape and sound. Unlike a Chinese tam-tam, the gongs are thick and deep, with a protruding dome in the center and long edges that sharply wrap around the circumference of the gong. When struck, the larger gongs are designed to produce a strong beating pattern. Recent work in both the analytical modeling and high-resolution measurements of the gong has advanced our understanding of the physics of this suite of instruments. However, there is still much to be understood about the influence of the shape of the gong and what boundary conditions to expect from an instrument of the dimensions described. This paper will present measurements and discussion of the nature of both the gong’s boundary conditions and its back cavity. Both will be used to support the continued development of theoretical models of the gong.

A fluid filled MEMS hydrophone

MEMS hydrophones are of interest to provide a small form factor acoustic sensing capability in water. The majority of previous work on MEMS hydrophones (e.g., Bernstein 1997, Moon, 2010, Gu, 2018) use air backed diaphragms in order to provide increased sensitivity. However, this limits the operating depth of the device due to a reduction in the diaphragm burst pressure. In this work we investigate an architecture that has the potential to increase the operating depth by filling the backing cavity with a heavy fluid. We have designed architectures with multiple acoustic ports (Moon, 2010) and coupled piezoelectric diaphragms, in a bid to maintain sufficient sensitivity while incorporating the filling fluid. The sensing elements are Parylene coated, repackaged Vesper VM1000 aluminum nitride MEMS microphones. It was reported (Travaglione, 2018) that such a device can sense underwater sound, but characterization was limited. In our prototypes, the microphone die are co-packaged onto modified printed circuit boards which include the acoustic coupling elements and required preamplifiers. We report on computational predictions of sensitivity and resolution, and provide initial test results as part of our ongoing investigation. Support from OUSD (R&E) Innovation and Modernization Office.