On the sound absorption performance of a felt sound absorber

In this work, we have developed an analytical model of a multilayer porous material based on the transfer matrix method to predict the absorption behavior at plane wave incidence. The aim of this study is to modify/tune the sound absorption coefficient of a felt to obtain an improved absorbing performance in the mid frequency range without increasing its weight. To achieve this target, the developed model has been used to find the best combination of each layer type and thickness. The analytical results were validated by test results.     

On the molecular sound absorption in fluid mixtures

Measurements of sound absorption in binary mixtures of gases in which two relaxation processes can occur due to the specific heats of the molecular vibrations, have hitherto been evaluated by using a theory based mainly on gaskinetic considerations. The theory presented here, uses the thermodynamics of irreversible processes as developed byMeixner for sound absorption in fluid systems due to interior reactions of arbitrary kind and number. This treatment permits a concise derivation of the final formulas which are also valid for mixtures of real gases and fluids. According to which set of independent thermodynamical variables for the derivation is used, the final formulas seem to differ considerably. It is pointed out that these differences arise from introducing two types of relaxation times and may be eliminated by means of the relationship between these relaxation times.     

On the longitudinal magnetic susceptibility of fractal ferrodielectrics

The longitudinal component of the magnetic susceptibility tensor χ _zz of a fractal ferrodielectric with dimension d _F = 3 − ɛ, where ɛ > 0, is calculated. The magnon dispersion in this case is shown to be a strongly anisotropic function of the parameter ɛ which, in turn, leads to nontrivial frequency dependences of χ _zz .     

Pressure influence on the ferromagnetic resonance in ferrodielectrics

The pressure influence on the ferromagnetic resonance (FMR) is considered. Magnetostatic mode coupling caused by sample compression is taken into account by means of a perturbation theory. Two simple models are considered for strong coupling when perturbation theory works badly.     

On site validation of sound absorption measurements of occupied pews

Laboratory measurements of sound absorption by audiences are known to be scarcely reliable when applied to actual rooms as a consequence of several problems, among which the different area of the “sample” and the different distribution of the reflected sound may play important roles. When dealing with worship places, characterized by a variable degree of occupation and much lower absorption due to unoccupied seats, things become more complicated as absorption seems to be proportional to the number of occupants rather than to the area they cover (as normally accepted in performing spaces). The combination of these variables has been investigated by taking advantage of laboratory measurements and analysing their application to six churches, where on site measurements of reverberation time were carried out with and without occupation. The results are discussed both in terms of simple prediction formulae (Sabine, Eyring, and Arau-Purchades) and of computer simulations, showing that laboratory measurements may be reliably used in computer simulations (at least in the frequency range from 500 Hz on). At low frequencies greater attention must be paid as the absorption coefficients need to be corrected as a function of the actual distribution of the sound field in the room.     

On the use of perforations to improve the sound absorption of porous materials

The concept of meso-perforations in appropriately chosen porous media can help enhance their sound absorption performance. The meso-perforated materials are also referred to as “double porosity materials” since they are made up of two interconnected networks of pores of different characteristic size. Several theoretical, numerical and experimental works have been accomplished on the subject by the authors. The purpose of this paper is to give a synthetic review of these works and establish practical design rules to develop optimized noise control solutions based on this concept. The paper presents two complementary models to deal with this kind of materials: an analytical model based on homogenization techniques and a numerical model relying on a finite element discretization of the domains. The limits of these models are discussed. The choice of the design parameters is then been investigated in order to provide practical design rules. This choice relies on a criterion which is evaluated from the knowledge of the resistivity, porosity and tortuosity of the micropous medium, and the calculation of a geometrical parameter defined from the chosen mesoscopic structure. Experimental and numerical results regarding the influence of the mesopore profile along the thickness performed in a appropriately chosen substrate microporous medium are presented. The agreement between the models and the experiments is satisfactory. Results show that significant enhancements of the absorption properties can be obtained over a selected frequency band by adjusting the mesopore profile. It is also shown that interesting absorbing properties can be obtained when coating a double porosity medium with an impervious screen.     

The absorption of sound in emulsions

The absorption of sound in homogeneous dilute emulsions due to the viscosity of the components of the medium is theoretically investigated. Deformation (capillary) oscillations of the emulsion droplets are taken into account. The viscous wavelength is assumed to be small compared to the droplet size, and the latter is assumed to be small compared to the wavelength of sound. Resonance phenomena related to capillary oscillations are considered. The resonance contribution to the attenuation coefficient of a plane sound wave propagating in the emulsion is analyzed.     

Interaction of spin waves with dislocations in ferrodielectrics

It is demonstrated that the presence of dislocations in a ferromagnetic leads to localization of spin waves on it. The dispersion equation is derived and investigated, and the dependence of the localized wave amplitude on the distance to a dislocation is determined. The frequency interval separating the localized fluctuations from the volume ones is determined.     

Numerical analyses of the sound absorption of three-dimensional MPP space sound absorbers

Because microperforated panels (MPPs) can provide wide-band sound absorption without fibrous and porous materials, they are recognized as next-generation absorption materials. The fundamental absorbing mechanism is Helmholtz-resonance absorption due to the perforations and air-back cavity. Consequently, MPPs are usually placed in front of rigid-back walls. However, one of the authors has proposed MPP space sound absorbers without backing structures. Among these space absorbers, cylindrical MPP space absorbers and rectangular MPP space absorbers are advantageous due to their design flexibility and easy-to-use properties. Although their performances have been investigated experimentally, it is necessary to predict their absorption characteristics to develop improved shapes and efficient designs. Herein their absorption characteristics are numerically predicted using the two-dimensional boundary element method, and the applicability of a numerical method as a design tool to sufficiently predict the performance of MPP space absorbers is discussed.     

伪随机扩散体的吸声性能及其应用

本文介绍伪随机扩散体吸声性能的应用，提出减少扩散体结构厚度的实用设计，并应用于剧场观众厅以减少混响时间．     

Dispersion and absorption of sound in nematics

The unified hydrodynamic theory for systems with a liquid crystalline phase is generalized to include a frequency dependence of the elastic and dissipative parameters of the system. Application is made to nematics for which the frequency dependent anisotropies of the velocity and absorption of sound are calculated. It is shown that the relaxation approximation for the dispersion leads to reasonable results.     

Universal sound absorption in low-temperature amorphous solids

A general elastohydrodynamic theory is developed based on the phenomenological assumption of a sharp decrease of shear relaxation time at large wave vectors k>k(xi), where k(xi) is of order of inverse of several interatomic distances a. This theory describes the low-energy excitations of glassy and amorphous solids, which contribute to anomalous linear-in-temperature specific heat and limit phonon thermal conductivity. The ratio of the wavelength of the phonon, lambda, to its mean free path, l, which is the universal property of sound absorption in glasses, is derived in this theory to be lambda/l=(2/3)(c(t)/c(l))(2)(k(xi)a)(3), where c(t) and c(l) are transverse and longitudinal sound velocities correspondingly.     

On the validity criteria of the phenomenological theory of electron absorption and amplification of sound in semiconductors

By using the kinetic equation for the electron distribution function in the acoustic wave field, the criteria are established for the validity of the phenomenological (“hydrodynamic”) theory of electron absorption and amplification of sound (acoustic waves) in the semiconductors. It is shown that the upper frequency limit of the validity of this theory lies, generally speaking, in the region of considerably lower (maybe by orders of magnitude) frequencies than it follows from the usually accepted criterion ql ? 1.     

声波的吸收与反射

测量声波幅度随传播距离变化的实验曲线,用波动学的观点分析实验数据,考虑到吸收和反射等耗散因素,计算出空气的吸收系数和换能器的反射系数.     

Notes on the absorption of sound in high magnetic fields

We study the absorption of sound by a free electron-impurity system in a quantizing magnetic field and derive a formula for the absorbed power on the basis of linear response theory. In the approximation of constant relaxation time and in the limit of high temperatures (1/τ?kT) this formula is essentially equivalent to the formulas used by Gurevichet al. and by Svirskii. Without these restricting assumptions we obtain general conditions for the appearance of quantum oscillations. We discuss the influence of spin flip scattering on the shape of the 0⁺-absorption line. Furthermore, we show that the usual kinetic approach leads to the incorrect results given by Quinn.     

空间声场有源声吸收中的多极源结构

本文从声波基本方程导出了空间声场的有源声吸收能量公式,讨论了单极子,偶极子,三极子等多极次级源对初级声场的有源声吸收及相应的实现条件。在此基础上提出偶极次级源是较简单的次级源结构。在消声室的实验结果表明它具有较好的声吸收性能,为其应用作了初步的可行性研究。     

水下弹性微穿孔吸声结构吸声系数研究

利用模态叠加法建立了水介质微穿孔板的数学模型,基于声电类比法得到其等效电路模型。研究了弹性微穿孔板和弹性背腔对垂直入射吸声系数的影响。与空气介质中的微穿孔板不同,水下微穿孔板因结构阻抗不足,难以取得满意的吸声效果,为此提出了增强型微穿孔吸声结构,并在水介质阻抗管内对理论结果予以验证。结果表明,随着增强型弹性微穿孔板弯曲刚度的增大,其在[20,2000] Hz范围内的平均吸声系数得到提高,逐步趋近于刚性微穿孔板的结果;弹性背板使微穿孔吸声结构的吸声峰向低频移动,低频吸声效果得到提高。     

Low-frequency sound absorption and attenuation in marine medium

Original experimental data are analyzed on the low-frequency sound attenuation in the Mediterranean, Black, and Baltic Seas, Sea of Japan, and the north-western region of the Pacific Ocean. In these regions, waters significantly differ in their temperatures and salinities. The analysis is aimed at obtaining an expression for calculating the low-frequency absorption coefficient in sea water. The analysis uses the previously published data on the measured (by the temperature discontinuity method) low-frequency relaxation times associated with boron present in sea water. The dependence of the absorption on the pH value (which was revealed in the 1970s) and the experimental data on sound absorption at frequencies higher than 5–10 kHz are also taken into account. As a result of the analysis based on the assumption that low-frequency relaxation takes place, an expression is proposed that relates the low-frequency absorption to the temperature, salinity, and pH value and equally well describes the experimental frequency dependences of attenuation for the four regions at hand (except for the Baltic Sea). Increased attenuation coefficients are noticed for shallow seas and deep-water regions where waters are influenced by intense currents, strait zones, and zones of mixing waters of different origin, i.e., for the ocean areas where, in addition to the attenuation, sound scattering by inhomogeneities of the marine medium and sound energy leakage into the sea floor are significant.     

水下弹性微穿孔吸声结构吸声系数研究

利用模态叠加法建立了水介质微穿孔板的数学模型,基于声电类比法得到其等效电路模型。研究了弹性微穿孔板和弹性背腔对垂直入射吸声系数的影响。与空气介质中的微穿孔板不同,水下微穿孔板因结构阻抗不足,难以取得满意的吸声效果,为此提出了增强型微穿孔吸声结构,并在水介质阻抗管内对理论结果予以验证。结果表明,随着增强型弹性微穿孔板弯曲刚度的增大,其在[20,2000]Hz范围内的平均吸声系数得到提高,逐步趋近于刚性微穿孔板的结果,弹性背板使微穿孔吸声结构的吸声峰向低频移动,低频吸声效果得到提高。