The vibroacoustic response and sound absorption performance of multilayer, microperforated rib-stiffened plates

The vibroacoustic response and sound absorption performance of a structure composed of multilayer plates and one rigid back wall are theoretically analyzed. In this structure, all plates are two-dimensional, microperforated, and periodically rib-stiffened. To investigate such a structural system, semianalytical models of one-layer and multilayer plate structures considering the vibration effects are first developed. Then approaches of the space harmonic method and Fourier transforms are applied to a one-layer plate, and finally the cascade connection method is utilized for a multilayer plate structure. Based on fundamental acoustic formulas, the vibroacoustic responses of microperforated stiffened plates are expressed as functions of a series of harmonic amplitudes of plate displacement, which are then solved by employing the numerical truncation method. Applying the inverse Fourier transform, wave propagation, and linear addition properties, the equations of the sound pressures and absorption coefficients for the one-layer and multilayer stiffened plates in physical space are finally derived. Using numerical examples, the effects of the most important physical parameters-for example, the perforation ratio of the plate, sound incident angles, and periodical rib spacing-on sound absorption performance are examined. Numerical results indicate that the sound absorption performance of the studied structure is effectively enhanced by the flexural vibration of the plate in water. Finally, the proposed approaches are validated by comparing the results of stiffened plates of the present work with solutions from previous studies.

Effect of Plastocene Embedment in the Microperforated Plate on Its AcousticPerformance

Noise pollution is one of the contemporary environmental pollution, which seriously damages people’s green andhealthy life. In order to further improve the low frequency sound absorption performance of microperforatedpanel (MPP), a new plastocene coupled microperforated plate (PCMPP) is proposed. The acoustic propertiesof PCMPP with different apertures and perforation ratio were measured by transfer function method and compared with that of conventional MPPs. It is found that when the aperture was 0.8 mm, the peak value of soundabsorption coefficient of PCMPP decreased by 150 Hz compared with MPP. In a certain range, PCMPP with larger apertures showed a greater influence on sound absorption property in low frequency. In addition, higher perforation ratio led to a greater PCMPP bandwidth of sound absorption. On the other hand, the effect of PCMPPwith aperture of 0.2 mm on the performance of MPP was reduced, which could be compensated by increasing theperforation ratio. Furthermore, we found that the effect of aperture, perforation ratio and cavity on the soundabsorption performance of PCMPP was consistent with that of ideal rigid MPP. The step cooling curve showedthat the plastocene began to soften at about 50℃, representing a great potential for a non-high temperaturework environment.

Experimental and numerical investigations on acoustic damping of monoclinic crystalline wideband sound absorbing structures

In order to overcome the limitations of traditional microperforated plate with narrow sound absorption bandwidth and a single structure,two multi-cavity composite sound-absorbing materials were designed based on the shape of monoclinic crystals:uniaxial oblique structure(UOS)and biaxial oblique structure(BOS).Through finite element simulation and experimental research,the theoretical models of UOS and BOS were verified,and their sound absorption mechanisms were revealed.At the same time,the influence of multi-cavity composites on sound absorption performance was analyzed based on the theoretical model,and the influence of structural parameters on sound absorption performance was discussed.The research results show that,in the range of 100-2000 Hz,UOS has three sound absorption peaks and BOS has five sound absorption peaks.The frequency range of the half-absorption bandwidth(α>0.5)of UOS and BOS increases by 242% and 229%,respectively.Compared with traditional microperforated sound-absorbing structures,the series and parallel hybrid methods significantly increase the sound-absorbing bandwidth of the sound-absorbing structure.This research has guiding significance for noise control and has broad application prospects in the fields of transportation,construction,and mechanical design.

Enhancement of low-frequency sound absorption of microperforated panels by adding a mechanical impedance

In order to solve the bad low frequency sound absorption of the Micro-Perforated panel （MPP） absorber, mechanical impedance was introduced in the back of the MPP absorber to form a composite structure. According to the same particle vibration velocity on both sides of a plate, the mechanical impedance plate transfer matrix could be obtained. The units of the mechanical impedance, cavity and MPP were connected in series with the use of the transfer matrix method, thus creating the composite structure＇s theoretical calculation model. The qual- ity factor affecting absorption bandwidth was analyzed. Bandwidth is inversely proportional to the mechanical impedance plate mass. During the experiments, when at close to 400 Hz, the composite structure reached an absorption peak with a coefficient of above 0.8. Experimen- tal results concurred with theoretical calculations. Mechanical resonance is added based on the traditional MPP resonance sound absorption mechanism. Through this, the performance of low frequency sound absorption can be improved without increasing the thickness of the structure. The frequency band can be broadened by reducing the mechanical impedance plate mass and controlling its boundary-damping coefficient.

Statistical absorption coefficient of microperforated absorbers

Special attention is paid to the characteristics of microperforated panel absorbers (MPA) in a sa-called random or diffuse sound field. Preliminary analysis based on the theory of “normal model” are given. Measurements for different kinds of MPA in the reveberation chamber have been carried out and useful results have been obtained. For the theoretical predictions, PC-software has been developed. The further stuides in the near future are proposed.

Sound absorption characteristics of microperforated absorbers for random incidence

Based on previous work on 'Statistical absorption coefficient of microperforated absorbers', in which it was shown that theoretical results agree well with experiments on the absorption characteristics of microperforated absorbers (MPA) for random incidence. Further work was carried out in this investigation of the statistical absorption coefficients of MPA in random fields by computation, in order to find the best. structure of MPA. It is established that ordinarily the absorption curves of MPA for random incidence and that for normal incidence are quite alike, only that the absorption coefficients are more or less reduced and the whole curve is shifted to higher frequencies without any change of shape. But when the perforate constant k = d ωρ0/4η where d is the diameters of perforations in mm and f0, the absorbers resonance frequency is reduced below 2, say, secondary absorption bands start to play more important role. Pretty soon, they merge with the main absorption band and form a long tail of the latter,extending the absorption far into high frequencies, raising the resulting absorption band to three, four or more octaves. The behavior of the secondary absorption bands is discussed.

An investigation on the acoustic characteristics of microperforated muffler

A mathematical model for insertion loss in a micro-perforated muffler with the effects of temperature gradient, gas flow speed and structure parameter is obtained by the mode matching technique. By dividing the microperforated tube into N segments, and assuming the flow speed and temperature are constant in each element, sound pressure and volume velocity of each segment can be readily acquired. The transmission matrix of microperforated muffler can be established based on the continuity condition of adjacent elements on the boundary. The numerical results of the present theory are calculated and the experiment is performed in Shanghai-495A engine. The experimental results of microperforated muffler show that they are in good agreement with the theoretical results.

Sound absorbing properties of spiral metasurfaces inspired by micro-perforated plates

As a kind of classical low-frequency sound-absorbing material,the microperforated plate(MPP)has been widely used.Here,we inspired by the sound absorption mechanism of the MPP,a spiral metasurface(SM)is designed and the analytical solution of acoustic impedance and sound absorption coefficient are obtained.The relationship between the sound absorption properties of the MPP and the SM with their own structures is systematically studied,and the analytical solutions are used to optimise the structure.It is concluded that the MPP and the SM of the same thickness achieve effective absorption in the frequency range between 390-900 Hz and 1920-4266 Hz,with a total thickness less than 1/6 of the wavelength.Meanwhile,the numerical calculation shows that the MPP and SM can match well with the background medium in the effective rang.Our study provides new insights into the design methods of sound-absorbing materials and is potentially suitable for many acoustic engineering applications.

High Temperature Effect on Absorption Coefficient of M-MPPs and Sandwich Structures Coupled with MPPs

This paper addresses the effect of high temperature on absorption performance of sandwich material coupled with microperforated panels (MPPs) in multiple configurations using a finite element model (FEM) over a frequency range from 10 to 3000 Hz. The structure is backed with a rigid wall which can either be Aluminium or Al-Alloy used in aeronautic or automobile. The wave propagation in porous media is addressed using Johnson Champoux Allard model (JCA). The FEM model developed using COMSOL Multiphysics software makes it possible to predict the acoustic absorption coefficient in multilayer microperforated panels (M-MPPs) and sandwich structure. It is shown that, when structures made by MPPs or sandwich materials are submitted to high temperature, the absorption performance of the structure is strongly modified in terms of amplitude and width of the bandgap. For application in sever environment (noise reduction in engines aircrafts), Temperature is one of the parameters that will most influence the absorption performance of the structure. However, for application in the temperature domain smaller than 50?C (automotive applications for example), the effect of temperature is not significant on absorption performance of the structure.

Appendicitis-mimicking presentation in fishbone induced microperforation of the distal duodenum:A case report

BACKGROUND Upper gastrointestinal fishbone microperforations are rare and not commonly reported in medical literature.Despite the increasing use of computer tomography(CT)imaging and the employment of the Alvardo criteria,misdiagnosis of acute appendicitis can still occur.We report the rare case of an elderly Chinese gentleman who had a fish-bone induced microperforation of the duodenum that closely mimicked the symptoms of acute appendicitis.CASE SUMMARY This 79-year-old man presented with migratory lower abdominal pain that localized at his periumbilical region and right lower quadrant.He had associated pyrexia,general malaise and was noted to have an elevated white cell count.CT investigations initially revealed a distended appendix which was resected laparoscopically but showed no obvious signs of gross inflammation.The patient then deteriorated clinically and had increased oxygen requirements immediately after the surgery.This prompted further investigations.A further review of his CT scan revealed a fine fishbone microperforation in the distal duodenum associated with retroperitoneal abscess formation and seepage extending into the right lower quadrant.He was then started on broad spectrum intravenous antibiotics and subsequently underwent a laparotomy 12 h later to manage the obscure aetiology and to drain the abscess.The post-operative course was uneventful and he was discharged 11 d later including a 2-d stay in the intensive care unit.CONCLUSION This case offers an insight into a potential mimic of acute appendicitis and the diagnostic difficulties experienced in such presentations.

Microperforate (Pinhole) hymen and infertility;a rare case report

Inroduction Female genital tract anomalies may have a devastating impact on the potential for sexual activity and fertility. Microperforate (pinhole) hymen is one of them. Microperforate hymen can lead to infertility, primary amenorrhea, acute or chronic pelvic pain, abnormal vaginal bleeding, dysuria, pollakiuria or a foul-smelling vaginal discharge. Aim We aim to report a patient who had intercourse problems and infertility complaints later found to have a microper-forate hymen. Discussion Pinhole opening of hymen may permit passage of fluids. Consequently spontaneous pregnancy can occur rarely. A few reports showed pregnancies with intact hymen without penetration of penis. But in general patients have fertility problems. Conclusion The results of the surgery are similar in all techniques and the particular surgical centre will have its own preference of which technique it prefers since after diagnosis definitive treatment is surgical and effective. Failure to manage these patients correctly may have long-term sequelae for their psychological, sexual and reproductive health.

Sound Reduction Control in Acoustic Enclosure with Air Ventilation

An acoustic enclosure system with both interior sound reduction and air ventilation is designed and demonstrated.The system consists of a rectangular enclosed space coupled with ventilated metamaterials and microperforated panels(MPPs).By modeling the ventilated metamaterial as an impedance boundary condition,an analytic model is developed to characterize the frequency response of interior acoustic fields and evaluate the sound reduction performance of MPP structures.Numerical simulations are conducted to validate the accuracy of the theoretical model.It is found that the resonance response of the enclosure system can be suppressed by proper arrangement of the MPPs.Even with open area for airflow,the system still possesses good sound isolation originating from the low-transmission behavior of the ventilated metamaterial.The proposed model system may find potential applications in noise control engineering.

Microperforation of the human nail plate by radiation of erbium lasers

The nail plate forms a barrier that limits the effectiveness of drug delivery in the treatment of nail diseases and prevents the outflow of fluid in the case of subungual hematoma formation. Microperforation of the nail plate through laser radiation can increase the effectiveness of drug delivery and ensure the possibility of blood outflow. This study detected and identified the type and threshold of effects that arise from exposing the nail plate to Yb,Er： Glass （λ = 1.54 μm） and Er：YLF （λ= 2.81 μm） laser radiation. The rate and efficiency of nail plate ablation by the radiation of these lasers were studied. The effect of the storage time of a freshly extracted nail plate in open air on its ablation rate by Er：YLF （ λ=2.81 μm） laser radiation was also investigated. The impact of the Yb,Er：Glass and Er：YLF laser pulses on the nail plate caused bleaching, carbonization, ablation with microcrater formation, and microperforation. The laser energy densities WE （thresholds） required for these effects were determined. The maximum ablation rate for Yb,Er：Glass laser radiation was 8 μm/pulse at WE = 91 ±2 J/cm2, whereas that for Er：YLF laser radiation was 12 μm/ pulse at WE= 10.5＋0.5 J/cm2. The maximum ablation efficiency for Yb,Er：Glass laser radiation was 0.1 μm/mJ at Ws = 10.5±0.5 J/cm2, whereas that for Er：YLF laser radiation was 4.6 μm/mJ at WE = 5.3±0.3 J/cm2. The laser ablation rate depends on the storage time and conditions of the freshly extracted nail plate. For example, when exposed to Er：YLF laser radiation, the laser ablation rate decreased by half from the initial maximum value in 96 h of air storage and returned to the initial value after 1 h of storage in distilled water.