Analysis of the sound transmission loss of panels with resonant systems on the basis of equivalent representations

Effects of increasing the sound transmission loss of panels with the help of resonant systems on the basis of equivalent representations are analyzed. Emphasis is placed on the least-studied resonant systems, the inertial bodies of which simultaneously interact with media on each side of the panels. A universal expression for the sound transmission loss of panels with an arbitrary system of resonant elements with one degree of freedom is presented. It includes the parameters common to all types of resonators (the total mass, compressibility, quality factor, and characteristic frequencies). The expression can be directly used to compare the efficiency of different types of resonant systems mounted on a panel and to determine their optimum parameters.     

基于局域共振单元实现声子晶体低频多通道滤波

从理论上提出一种由局域共振单元组成的声子晶体低频多通道滤波模型.在二维三组元局域共振声子晶体中引入不同填充率的共振单元构成波导结构,通过有限元法计算出其能带结构、透射曲线和透射场图.结果显示:这种设计能够在低频带隙范围内不同填充率散射体的共振频率附近产生新的分立模,且这些分立模能够使相应的声波在声子晶体中沿波导方向传播;这些分立模只与相应的共振单元相关,抗干扰能力强.所得结果为低频多通道滤波器的设计提供了一种新的理论依据.     

Microwave transmission through a single subwavelength annular aperture in a metal plate

The resonant transmission of a small annular aperture, with a diameter much smaller than the radiation wavelength, in a thin metal plate is studied at microwave frequencies. It transpires that such an annular aperture supports several resonant guided modes, including those that are not quantized in the azimuthal direction. Such modes have resonant frequencies that are largely independent of the diameter of the annular aperture, thus being supported by annular apertures that tend to zero radius. The transmittance of such a structure at microwave frequencies is detailed and compared with the predictions of a finite element method model.     

Low frequency resonant dispersion of sound in bubble media

Presented is a theory of a new type of resonant dispersion of sound in a gas-liquid bubble media based on the use of effective dynamic density. We show that because of spheroidal-translational oscillations of the bubbles the dynamic density of the gas-liquid media has a resonance dependence on the frequency, which is manifested in wave process as a low-frequency resonant dispersion of sound. This dispersion is significantly different from the known high-frequency resonant dispersion, which is due to resonance in the volume oscillations of the bubbles. The results of the experiments confirming the existence of the resonant dispersion of sound at a frequency equal to half the natural frequency of the spheroidal oscillations of bubbles are provided.     

开孔对封闭空间共振特性的影响

研究了开孔对封闭空间声场的影响。通过将孔内振动空气等效为点源,用模态展开法建立了开孔封闭空间的声场模型,计算了开孔封闭空间高阶共振频率和在共振频率激励下的声压分布。结果显示:开孔等效于孔处声质量减小,一般使得开孔封闭空间的共振频率增加;但当孔位于某模态节点时,由于该阶模态与任一模态在开孔处未发生耦合,该模态共振频率不变;由于在开孔区域对应于激励频率的模态声压和其余各阶模态声压之和的相位相反,高阶共振频率激励下靠近小孔位置的声压减小。因此,开孔对封闭空间声场有影响,其影响程度与开孔位置和开孔尺寸有关。      

Optimization of locally resonant acoustic metamaterials on underwater sound absorption characteristics

The study of acoustic metamaterials, also known as locally resonant sonic materials, has recently focused on the topic of underwater sound absorption. The high absorption occurs only within a narrow frequency band around the locally resonant frequency. Nevertheless, this problem can be addressed through a combination of several acoustic metamaterial layers that have different resonant frequencies. In this paper, an optimization scheme, a genetic and a general nonlinear constrained algorithm, is utilized to enhance the low-frequency underwater sound absorption of an acoustic metamaterial slab with several layers. Both the physical and structural parameters of the acoustic metamaterial slab are optimized to enlarge the absorption band. In addition, the sound absorption mechanism of the acoustic metamaterial slab is also analyzed. The result shows that each layer is found to oscillate as a nearly independent unit at its corresponding resonant frequency. The theoretical and experimental results both demonstrate that the optimized metamaterial slab can achieve a broadband (800–2500 Hz) absorption of underwater sound, which is a helpful guidance on the design of anechoic coatings.     

Low frequency impact sound transmission in dwellings through homogeneous concrete floors and floating floors

An experimentally validated analytical model has been developed in order to investigate the effect on impact sound transmission at low frequencies of location of the impact, type of floor, edge conditions, floor and room dimensions, position of the receiver and room absorption. The model was developed in order to allow rapid repeated calculations necessary for a parametric survey, described in a companion paper. The analytical model uses natural mode analysis to predict the sound field generated in rectangular rooms by point sound sources and the point excitation of homogeneous rectangular plates with different edge conditions. A floor-room model of the sound field generated in a room by a vibrating floor also has been derived. Laboratory and in situ measurements confirm that the models can be used to estimate impact sound transmission at low frequencies. The approach applies to homogeneous simply supported base plates of uniform thickness with homogenous floating floors, which again were experimentally validated in the laboratory and in situ.     

环境因素对海-气界面低频异常声透射的影响研究

针对海中声源在海-气界面低频异常声透射问题, 根据两层媒质声传输模型, 分析了大气声速和密度与气压、气温、湿度及海水中声速和密度与海温、盐度间的关系, 研究了低频声透射和传输受温度、气压、盐度、湿度等因素的影响, 分析了各因素对声透射和传输的影响程度. 结果表明: 1) 声透射到大气中的声功率与气温、湿度负相关, 与海温、盐度、气压正相关; 2) 单极子与水平偶极子声源辐射到海中的声功率与海温、盐度负相关, 而垂直偶极子声源辐射到海中的声功率与海温、盐度正相关; 3) 声透射指向性与海温正相关, 与气温负相关; 4) 低频声透射受温度影响最大, 其次是盐度, 受气压和湿度影响较小, 垂直偶极子声源的声透射受温度影响大于水平偶极子和单极子声源.     

Measurements of the acoustic target strengths of fish in dorsal aspect,including swimbladder resonance

The need for measurements of the acoustic target strength of fish is discussed. The phenomenon of swimbladder resonance of small deep ocean fish is well known and is a useful means of estimating their sizes. For larger commercial fish in shallower seas the resonant frequency is much lower and resonance is very difficult to observe in the field. A method of observing and measuring the swimbladder resonance of a captive live fish in controlled conditions is described, and results on several gadoids are given. Reasons for the observed resonant frequencies being higher than predicted are given; the damping of resonance is high, which is expected. Application of these results to acoustic sizing at sea appears remote. They are relevant, however, to studies of low-frequency sound propagation, and the experimental technique is offered as a useful tool in physiological studies involving swim-bladder function.     

可听声频段的声学超材料

制作了一维周期排列的亥姆霍兹共振器超材料,在空气环境下测试了其在可听声频段声学透射行为.实验结果表明,在2.1—3.5 kHz附近该材料具有透射衰减的吸收峰,利用声传输线理论（ATLM）计算的透射率和实验结果一致,同时由计算的等效阻抗分析可知,实验中出现的吸收峰是由HRs共振的回波反射引起的.另外,实验测试的样品中透射信号分布进一步验证了材料的共振效应,也就是会出现与外加激励反相响应.基于前述的共振模型计算出该材料的等效弹性模量为负. 关键词： 亥姆霍兹共振器 声传输线 吸收峰 等效弹性模量     

The effect of an air flow on a single side branch helmholtz resonator in a circular duct

A single side branch Helmholtz resonator has been tested in a circular duct in which both sound and an air flow could be passed in the same direction. The sound input was broad band and the transfer function across the resonator was determined by using two microphones and a cross-correlation analysis technique in which the broad band sound input was cross-correlated with each microphone signal in turn. The fundamental resonant frequency, as obtained from the transfer function of the resonator, was found to increase with increasing flow velocities in the duct. It was possible for the entire mass end correction of the orifice of the resonator to be eliminated by the flow. The higher natural frequencies of the resonator were not affected as much by the air flow.     

Coupling effects of layers on spin transport in ZnSe/Zn 1 - x Mn x Se heterostructures

By use of the scattering matrix method, we investigate the coupling effects of layers on spin-polarized transport through semimagnetic semiconductor heterostructures with triple paramagnetic layers. Due to the coupling between double non-magnetic layers or among triple paramagnetic layers, spin tunneling exhibits interesting and complex features, which are determined by the structural configuration, the external fields as well as the spin orientations. It is shown that for electrons with either spin orientation tunneling through the symmetric or asymmetric heterostructures with triple paramagnetic layers, transmission resonances can approach the optimum under several biases. Moreover, for asymmetric structures, the resonant enhancement can occur under both several positive and negative biases. The spin-dependent resonant enhancement is also clearly reflected in the current density. In addition, for spin electrons traversing the multilayer heterostructure, the resonant splitting occurs in the transmission, which shows rich variations with the bias. These interesting results may be helpful to the development of spintronic devices. Received 28 April 2001     

Theoretical and experimental study on active sound transmission control based on single structural mode actuation using point force actuators

This study deals with the feedforward active control of sound transmission through a simply supported rectangular panel using vibration actuators. The control effect largely depends on the excitation method, including the number and locations of actuators. In order to obtain a large control effect at low frequencies over a wide frequency, an active transmission control method based on single structural mode actuation is proposed. Then, with the goal of examining the feasibility of the proposed method, the (1, 3) mode is selected as the target mode and a modal actuation method in combination with six point force actuators is considered. Assuming that a single input single output feedforward control is used, sound transmission in the case minimizing the transmitted sound power is calculated for some actuation methods. Simulation results showed that the (1, 3) modal actuation is globally effective at reducing the sound transmission by more than 10?dB in the low-frequency range for both normal and oblique incidences. Finally, experimental results also showed that a large reduction could be achieved in the low-frequency range, which proves the validity and feasibility of the proposed method.     

Sound radiation into air by a point source moving underwater

Based on the discovery that the majority of radiated energy of a stationary sound source in shallow water is into the air at infrasonic frequencies, the sound transmission into air from a point source moving underwater is investigated in this letter. It is found that a moving sound source can radiate more acoustic energy into the air than a stationary one and the amount of energy radiated into the air increases with the speed of the moving source. Simulations show that the sound transmission into air is dominated by the inhomogeneous waves generated by the moving source.     

局域共振复合单元声子晶体结构的低频带隙特性研究

本文提出了一种新型局域共振复合单元声子晶体结构, 并结合有限元方法对结构的带隙机理及低频共振带隙特性进行了分析和研究. 共振带隙产生的频率位置由所对应的局域共振模态的固有频率决定, 并且带隙宽度与局域共振模态的品质因子及其与基体之间的耦合作用强度有关. 采用局域共振复合单元结构可以实现声子晶体的多重共振, 在低频范围能打开多条共振带隙, 但受到共振单元排列方式的的影响. 由于纵向和横向局域共振模态的简并, 复合单元结构能在200 Hz以下的低频范围打开超过60%宽度的共振带隙, 最低带隙频率低至18 Hz. 这为声子晶体结构获得低频、超低频带隙提供了一种有效的方法. 关键词： 局域共振 低频带隙 复合单元 声子晶体     

Sound absorption of microperforated panel mounted with helmholtz resonators

Microperforated panel (MPP) absorbers have been widely used in noise reduction and are regarded as a promising alternative to the traditional porous materials. However, the absorption bandwidth of a single-layer MPP is insufficient to compete with the porous materials. In order to improve the sound absorption ability of the single-layer MPP, MPP mounted with Helmholtz resonators (MPPHR) is introduced. Based on the MPP, Helmholtz resonators theory and electro-acoustical equivalent circuit principle, sound absorption properties of MPPHR are studied. Simulation and experimental results show that MPPHR have two peak frequencies and one anti-resonant frequency. The low-frequency peak is dependent on the Helmholtz resonators, while the high frequency peak is close to the peak of the single-layer MPP. The low-frequency sound absorption peaks move to low frequency with the neck length and the volume of Helmholtz resonators increasing. The high-frequency sound absorption peaks move to high frequency with the volume of Helmholtz resonators cavity increasing. Multiple Helmholtz resonator parallel MPP structure can provide more sound absorption than single MPPHR at low frequency range due to the introduction of more additional sound absorption peaks.     

Transmission of low-frequency sound through the water-to-air interface

L.M. Brekhovskikh revealed and studied the important role played by inhomogeneous waves emitted by a point source when they pass through an interface with a medium in which the velocity of sound is lower, for example, from water to air. This paper studies the energy characteristics of sound emitted into air by an underwater point source. The energy transfer due to inhomogeneous waves is shown to cause the phenomenon of anomalous transparency of the interface for low-frequency sound. The anomalous transparency manifests itself in that the energy flux through the interface increases with decreasing frequency of sound and, at sufficiently low frequencies, almost all of the acoustic energy produced by the underwater source is emitted into air. Conversely, at high frequencies, when the contribution of the inhomogeneous waves becomes negligible, the water-to-air interface is similar to a perfectly reflecting surface and almost all of the acoustic energy produced by the source is emitted into water. The anomalous transparency phenomenon changes the conventional opinion on the possibility of acoustic coupling between points in water and air and on the role played by physical processes evolving in the water column in generating atmospheric acoustic noise.     

A note on a circular panel sound absorber with an elastic boundary condition

Panel sound absorbers are typically used to absorb low-frequency noise in concert halls, auditoriums, recording studios, and other architectural applications. These systems are composed of flexible panels mounted over an air space that can be either partly or completely filled with a porous material. In this paper, a theoretical model is derived for predicting the sound-absorption coefficient of a cylindrical low-frequency absorber made of a circular plate. The theory takes into account the mass, bending stiffness, damping loss and the elastic boundary condition of the circular plate. The effects of the stiffness of an air-back cavity and of partially adding a porous material into the cavity are also considered. It is observed that the low-frequency resonances of such a system are dependent upon the clamping condition, the width of the air-back cavity, and mechanical properties of the plate. There is good agreement between theoretical and experimental results.     

Development of an optimised, standard-compliant procedure to calculate sound transmission loss: numerical measurements

Availability of low-frequency characteristics of sound insulating elements is required in order to achieve efficient control of noise sources and reduced level of annoyance in the low-frequency range. Previous work by the author has addressed the problem of designing an enhanced calculation environment for the estimation of sound Transmission Loss (TL). In this work, numerical prediction of TL of sound insulating structures is performed using a procedure, which is in compliance with the ISO recommendations for acoustic measurements. The room-structure-room finite element representation, employed to solve sound propagation and sound-structure interaction problems, as well as the dynamic coupling of and the sound energy propagation through successive air-structure layers are investigated. Several cases of single-layered plain structures of common sound insulating materials such as steel, glass and aluminium with various thickness values are modelled and the calculated TL is compared with published experimental results. It is shown that although the detailed dynamic response of the structures is not accurately predicted due to uncertain parameters, such as the test-specimens dimension and vaguely known boundary conditions, the octave band averaged TL is sufficiently predicted for the majority of the tested materials. Extension of the method to multi-layered structures is attempted and discrepancies at low frequencies are depicted. Finally, the effect of poor mode distribution of the measurement rooms upon the estimated TL is examined in focus. Comparison is performed between TL values calculated with typical and intensely modified transmission rooms. The low-frequency improvement on measurements, when the second ones are used, is demonstrated.     

Prediction of the spatial characteristics of higher mode sound transmission through a periodic slit screen

Periodic slit screens are often installed in rectangular apertures either to mask the opening or to reduce transmitted noise. This investigation considers the higher-order scattered and transmitted sound from a point source through a periodic screen mounted in a rigid baffle. In particular, this paper considers the spatial characteristics, or directivity, of the scattered field for a particular higher order mode. Uncoupled higher-order mode analysis is used to estimate the scattered and transmitted sound fields for the selected mode of the aperture without the presence of the screen. Transmission coefficients for the screens are then calculated using the well established equivalent mass layer effect and applied to the calculated higher-order mode scattered and transmitted sound pressures. Using an anechoic chamber, measurements were made over a small arc of the scattered sound field through a range of screens of different aspect ratios but the same porosity. The screens consisted of equally spaced open slits and solid laths and the number of slits was reduced to change the aspect ratio. In each case the point source was positioned on the impinging side of the aperture so as to drive one particular scattered higher-order aperture mode at or near cut-on. Comparison of the measured and predicted sound pressures indicate that good estimates of the sound field can be obtained through the approach of applying simple corrections for the presence of the screens to the estimates for the open aperture established using uncoupled calculations for the higher-order modes. It is also shown that the results for a specific mode excited at one particular frequency are applicable at other excitation frequencies provided that the correct non-dimensionalisation is applied.