Redistribution of energy flow in a material due to damping

The field lines of energy flow of the radiation emitted by a linear dipole in free space are straight lines, running radially outward from the source. When the dipole is embedded in a medium, the field lines are curves when the imaginary part of the relative permittivity is finite. It is shown that due to the damping in the material all radiation is emitted in directions perpendicular to the dipole axis, whereas for a dipole in free space the radiation is emitted in all directions except along the dipole axis. It is also shown that some field lines in the near field form semiloops. Energy flowing along these semiloops is absorbed by the material and does not contribute to the radiative power in the far field.     

A note on structural damping of ship hulls

Existing information on the structural damping of ships is far from satisfactory. It cannot be calculated and it can only be measured in the presence of hydrodynamic damping, whose nature and magnitude are also somewhat obscure. Yet it is very important.Symmetric responses to wave excitation can be estimated on the basis of existing hydrodynamic theories, with use of rough estimates of hull damping; our limited knowledge of structural damping is then only likely to be a handicap with heavy slender ships and/or fast ships. Much less is known about antisymmetric responses to waves, either as regards the means of estimating them or the appropriate levels of hull damping.Vibration at higher frequencies, due to excitation by machinery (notably propellers), is limited by structural damping to a much greater extent that it is by the fluid actions of the sea. Damping measurements at these frequencies therefore give more accurate estimates of hull damping. Even so, the estimation of ship vibration responses to excitation by machinery remains a matter of considerable difficulty.     

Noise due to turbulent flow past a trailing edge

A theoretical method [I] for calculating far field noise from an airfoil in an incident turbulent flow is extended to apply to the case of noise produced by turbulent flow past a trailing edge, and some minor points of the theory in reference [1] are clarified. For the trailing edge noise, the convecting surface pressure spectrum upstream of the trailing edge is taken to be the appropriate input. The noise is regarded as generated almost totally by the induced surface dipoles near the trailing edge and thus equal, but anticorrelated, noise is radiated into the regions above and below the airfoil wake, respectively. The basic assumption of the analysis, from which these concepts of appropriate input and dominance of dipole sources follow, is that the turbulence remains stationary in the statistical sense as it moves past the trailing edge. The results show that such trailing edge noise often is quite small, compared say to that produced by typical oncoming turbulence levels of one percent, but that it might be appreciable for an airfoil with a flow separation, or for a blown flap.     

Surface plasmon damping due to rough boundary scattering

The surface plasmon damping due to carriers scattering at the statistically rough (semiconductor-dialectric) interface is considered. The specular parameter and the integral relation are used as the boundary condition for a non-equilibrium part of the distribution function. There exist certain cases when the rough surface scattering of carriers is shown to play an important role in the surface plasmon damping.     

Vibration damping material as a means to reduce steel noise barrier cost

A preliminary analysis has been made of the use of vibration damping material as a means to reduce the cost of steel noise barriers in primarily highway applications. For cost-effective barriers, the sound transmission through, as well as over, a noise barrier must be considered. The through-barrier sound transmission characteristics of sample panels from a Toronto noise barrier were measured with and without damping material. It was found that a given through-barrier sound transmission performance could be achieved at less cost with the damping material than without it. Further study is recommended.     

Noise variability due to traffic spatial distribution

The inverse relation of the uncertainty of the equivalent sound pressure level to the square root of the number of vehicle pass-bys is investigated. The influence of the traffic spatial distribution is evaluated by means of a computer model, based on Monte Carlo and the Weyl–Ingard Theory. The model results are discussed and compared with experimental data. By least mean squares fitting of simulated results, a new expression is obtained for the sound pressure level uncertainty, that takes into account the receiver position, road configuration and traffic flow balance. This expression explains the different values found in the literature and yields uncertainty values that may not be covered by the ISO 1996-2:2007 recommendation, in some specific circumstances. Finally the path for building traffic noise uncertainty maps is sketched out.     

Frequency power law of material damping

The experimental studies often show that the damping in various solid materials increases with frequency over a finite bandwidth, and the increase is weak if the damping is low. A frequency power law is suggested and discussed in this paper to describe this damping behaviour with special respect to the low loss resilient materials used for sound and vibration control. The dynamic modulus as a function of frequency is determined from the loss modulus through the Kramers-Kronig dispersion relations to satisfy the causality requirement. It is proved that the dynamic modulus obeys the frequency power law of the same type as the loss modulus. In addition, it is proved that the weak frequency dependences of damping properties are inevitably associated with the low loss factor. Examples of fitting the frequency power law to experimental data on some materials of acoustical purposes are presented.     

Noise induced transitions due to external additive noise

The stationary Fokker-Planck equation in the two-dimensional case is solved locally using an antisymmetric tensor which transforms the problem into the potential case. Several examples are discussed which show that even additive external noise influences the extremal behaviour of the stochastic potential.     

Noise of a model helicopter rotor due to ingestion of isotropic turbulence

A theoretical and experimental investigation of the noise of a model helicopter rotor due to the ingestion of grid-generated, isotropic turbulence is described. Simulated forward flight and vertical ascent tests were performed with a 0·76 m diameter, articulated model rotor in the United Technologies Research Center Acoustic Research Tunnel. Far field noise spectra and directivity were measured in addition to inflow turbulence intensities, length scales and spectra. Measured inflow turbulence statistics and rotor operating parameters were employed in a theoretical procedure to predict turbulence ingestion noise spectra and directivity. This theoretical formulation represented an absolute level prediction method in that empirical or adjustable constants were not employed. From this study it is concluded that incident turbulence represents a potentially important source of rotor narrowband random (quasi-tonal) and broadband noise. With due regard to the absolute level nature of the prediction method, agreement between theory and experiment is reasonable: quasi-tonal noise at low frequency tends to be overpredicted whereas mid-frequency tonal noise and high frequency broadband noise are well predicted. This theory, therefore, provides a means to predict the contribution of turbulence ingestion noise to overall rotor noise spectra and directivity in cases where reasonable estimates of incident turbulence statistics can be made. Ingestion of main rotor wake turbulence by tail rotors in flight and ingestion of atmospheric turbulence by main and tail rotors in hover would be expected to be the most important sources of turbulence ingestion noise in full scale applications.     

Noise reduction analysis for a stiffened finite plate

This paper presents an analytical solution for the vibration and acoustic responses of a finite stiffened plate that is covered with decoupling layers and subjected to external excitation. The theory of elasticity is used for the decoupling layer, and the stiffened plate is modeled by the plate theory and Euler–Bernoulli beam equation. Equations are constructed by the boundary conditions at the plate/coating and coating/fluid interfaces. The problem can be solved by the proposed method in this paper. Test verification shows that a good correlation exists between theoretical and test results. Thus, the theoretical study in this paper is correct. Numerical results show that shear waves insignificantly affect the structural vibration level difference (VR) under low frequencies. The noise reduction of the stiffened plate covered with decoupling layers is greatly influenced by the decoupling layer loss factor. A failure region of the vibration level difference is present in the low frequency band of the decoupling layer. Furthermore, the thickness of the decoupling layer significantly affects noise reduction.     

Effects on the damping of ultrasonic transducers due to a lack of parallelism in the piezoelectric element

Using a theoretical model, we simulate the responses of piezoelectric transducers having lack of parallelism between their two faces. Verification of the model's validity is carried out by recording theoretical and experimental responses of piezoelectric transmitters and receivers with different degrees of parallelism defects. We conclude that, by using a simple theoretical model, we may forecast experimental responses of a transducer whose faces are not rigorously parallel.     

Pairing reduction due to the blocking effect

Pairing reduction in deformed nuclei due to the blocking effect is treated exactly with the particle-number-conserving method. Merely due to the blocking effect the gap parameter Δ becomes configuration dependent. The pairing reduction δ(ν_o) depends sensitively on the single-particle level distribution near the Fermi surface and on the location of the blocked level ν_o relative to the Fermi surface. The pairing reduction per blocked level near the Fermi surface is about $\text{δ(ν}{}_{\mathrm{o}}\text{) ≈}\text{1}\text{2}\text{d}$, where d is the average spacing of the Nilsson levels, and decreases rapidly with |ε_νo-λ|. With an increasing number of blocked levels near the Fermi surface the pairing reduction increases dramatically. In some special cases pairing reduction may be negative. Three anomalous even-odd shifts in band-crossing frequencies in ¹⁶¹Er, ¹⁷³W and ¹⁸¹Os are analyzed.     

具有形状记忆效应的新型智能阻尼材料及其热弹性力学性能研究

将传统金属橡胶制备工艺和形状记忆合金材料相结合, 研制了一种新型智能阻尼材料——-记忆合金金属橡胶, 并对其成型工艺、形状记忆效应、热弹性力学性能及参数影响规律进行了试验研究. 研究表明, 作为一种特殊的金属橡胶, 该材料不仅具备普通金属橡胶高阻尼、低密度、孔隙度可控等优点, 同时具备普通金属橡胶所不具备的智能材料特征: 1)良好的单程形状记忆效应, 该材料在大载荷加载后的残余塑性变形可以在升温过程中完全恢复; 2) 弹性模量和损耗因子具有温变特性, 在相变温度区间内弹性模量和损耗因子随温度近似线性变化. 由于同时具备金属橡胶和形状记忆合金两大金属类功能材料的优点, 形状记忆合金金属橡胶成为一种可应用于振动主动控制、 集良好承载能力和阻尼特性于一身、具有结构功能一体化优 势的新型智能阻尼材料.     

Noise reduction in material-handling machines

Three case studies of industrial noise control relating to two material-handling systems and a press are presented. Each one involves baseline measurements, identification of major noise sources, development and implementation of appropriate design modifications and/or acoustic treatments followed by assessment of their effectiveness. Since the noise from these machines is of an impulsive nature the baseline measurements are carried out with an impulsive SLM. For diagnostic purposes, however, the noise signals were recorded on site and analysed later on a mini-computer. The existing noise level was found to be in the range 100–110 dB(A), major peaks appearing in the 1–3 kHz band. The main sources of noise are the rattling of the components under production in the conveyer systems and their impacts in the various bins. The vibrator units also emit significant noise. The problems were remedied by reducing the drop height and chute slope. Also, a fin system, made of acoustic material, has been designed so that the falling components lose some of their kinetic energy by impacting on the fins. A noise abatement of 10–15 dB(A) was achieved at minimal cost.     

Transverse elastic-wave absorption due to reversible rotation in a triaxial magnetic material

Calculations have been performed on the energy absorption related to rotation of the spontaneous-magnetization vectors due to external shearing sign-varying stresses applied to a multidomain magnetic material, where allowance is made for the magnetic-phase concentrations. The perturbation solution for a system of six equations for the rotational moments is found together with the wave equation to derive the deviations of the magnetization vectors from the easy directions in the stress wave field. The absorption coefficients and internal friction have been derived, together with the defect of the shearing modulus, including the frequency dependence, which are determined via the magnetic, magnetoelastic and elastic constants of the crystal in relation to the orientation of the X and Y axes of the tensor for the stresses _xy with respect to the 100 easy axes. When the domain boundaries are fixed, the main contribution to this type of absorption in a ferromagnetic arises from the micro-eddy currents, while in a ferrite, it is due to the spin viscosity and the radiational friction.Kursk Polytechnical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 59–62, June, 1995.     

Noise control by barriers—Part 1: Noise reduction by a thick barrier

This paper presents a method of estimating the excess attenuation of noise by a thick barrier. In this method, the excess attenuation of noise by a thick barrier is assumed to be composed of two parts, one being the effect of a virtual thin barrier with the same height and the other the effect of thickness. A single chart for estimating this thickness effect is offered under conditions which only permit an error of a few decibels. The validity of the method presented here is verified by comparing the estimated with the measured values. Consequently, this method may be useful for the purpose of estimating the excess attenuation of a band of noise by the barrier whose thickness is larger than half a wavelength.