The effect of nearby bubbles on array gain

The coherent processing of signals from multiple hydrophones in an array offers improvements in angular resolution and signal-to-noise ratio. When the array is steered in a particular direction, the signals arriving from that direction are added in phase, and any signals arriving from other directions are not. Array gain (AG) is a measure of how much the signal arriving from the steering direction is amplified relative to signals arriving from all other directions. The subject of this paper is the manner in which the AG of an acoustic array operating in water that contains air bubbles is affected by scattering from nearby bubbles. The effects of bubbles on acoustic attenuation and dispersion are considered separately from their effects on AG. Acoustic measurements made in bubbly water using the AB Wood tank at the Institute of Sound and Vibration Research, University of Southampton, in June 2008 show that as bubble density increases, relative phase shifts in individual hydrophone signals increase and signal correlation among the hydrophones is reduced. A theory and numerical simulation linking bubble density at the hydrophone to the AG is in good agreement with the measurements up to the point where multiple scattering becomes important.     

The effect of damping on some nonlinear oscillators

Summary The nonlinear-oscillator equation is examined with an approximate technique worked out for the linear-oscillator equation. This approximate technique is valid for β/ω<1, where β is the damping term and ω the oscillation frequency. It is shown that damping in the nonlinear oscillator produces a second oscillation which is adequately described by the double periodicity of the elliptic function. If the damping term is permitted to vanish, the double periodicity again becomes a single periodicity. Since the nonlinear oscillator is described by a second-order differential equation with two independent solutions each with its own frequency, the nonlinear oscillator has two modes of decay, a ?low frequency? mode and a ?high frequency? mode relating to these solutions. It is suggested that this double mode of decay may relate to the two modes of decay of ball lightning.

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Riassunto L'equazione per l'oscillatore non lineare è esaminata con una tecnica approssimata elaborata per l'equazione per l'oscillatore lineare. Questa tecnica approssimata è valida per β/ω<1, dove β è il termine di smorzamento ed ω la frequenza di oscillazione. Si mostra che smorzamento nell'oscillatore non lineare produce una seconda oscillazione che è adeguatamente descritta dalla doppia periodicità della funzione ellittica. Se si permette che il termine di smorzamento si annulli, la doppia periodicità diventa ancora una singola periodicità. Poiché l'oscillatore non lineare è descritto da un'equazione differenziale di second'ordine con due soluzioni indipendenti, ciascuna con la sua frequenza, l'oscillatore non lineare ha due modi di decadimento, un modo a ?bassa frequenza? ed un modo ad ?alta frequenza?, che mettono in relazione queste soluzioni. Si suggerisce che questo doppio modo di decadimento possa essere in relazione con i due modi di decadimento dei fulmini globulari.

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Резюме Исследуется уравнение нелинейного осциллятора, используя приближенную технику, разработанную для уравнения линейного осциллятора. Этот приближенный подход справедлив для β/ω<1, где β есты член затуханиг и ω-частота осцилляций. Показывается, что затухание в нелинейном осцилляторе приводит к второму колебанию, которое адекватно описывается с помощью двойной периодичности эллиптической функции. Эсли член затухания стремится к нулю, то двойная периодичностВ снова переходит в единственныю периодичность. Так как нелинейный осциллятор описывается с помощью дифференциального уравнения второго порядка с двумя независимыми решениями, каждое из которых имеет собственную частоту, нелинейный осциллятор имеет две моды распада, ?низкочастотную? моду и ?высокчастотнуы? моду, связанные с этими решениями. Предролагается, что эти две моды распада могут быть связаны с двумя модами распада шаровой молнии.

     

The effect of cavity geometry on the nucleation of bubbles from cavities

The heterogeneous nucleation of gas bubbles from cavities in a surface in contact with a liquid is a widely recognized phenomenon. This process has previously been theoretically analyzed extensively for a conical crevice, although in practice a wide range of cavity geometries might be expected. The method of analysis originally presented by Atchley and Prosperetti [J. Acoust. Soc. Am. 86, 1065-1084 (1989)] for the unstable growth of a gas-liquid interface in a conical crevice is here extended to any axisymmetric cavity geometry and four such different geometries are analyzed. Although the method presented neglects gas transfer, and therefore is most directly suitable for acoustic cavitations, this method is still valuable in comparing the nucleation behavior of different cavity types. It is found that once the interface has emerged outside the cavity, its behavior is determined by the size of the cavity's opening. Given that the behavior of the interface once it is outside the cavity will also be determined by the local flow conditions, the threshold for unstable growth of the interface inside the cavity leading to its emergence is the important value and will determine differences between cavity geometries in practice, as shown in the examples presented.     

The effect of reverberation on the performance of cepstral mean subtraction in speaker verification

Speaker verification (SVR) performance is degraded under reverberation conditions. Cepstral mean subtraction (CMS) is often applied to the feature vectors in order to compensate for convolutive effects of transmission channels, which are considered to have a short-duration impulse response. The effect of reverberation on the performance of CMS applied to the feature vectors in SVR is investigated. Although CMS was found effective in reducing the effect of reverberation for short reverberation time (RT), in cases of long RT, it is shown that CMS may degrade SVR performance rather than improve it. Hence, CMS should not to be used in these cases. In addition, the effect of the room volume was tested and found less critical than the effect of long RT.     

混响场中脉冲超声空化效应的研究

本文实验研究了不同脉冲宽度的８２０ｋＨｚ超声波的空化效应，结果得出，空化致自基产量随脉冲宽度及声强呈现规律性变化，尤其观察到了“脉冲空化峰”现象，本文从液面受迫振动角度出混响场中“脉冲空化峰”现象的机制。     

The photoacoustic effect from particles and bubbles

Irradiation of colloidal suspensions with the output of a high power, pulsed laser can give rise to chemical reactions and phase transitions that are linked to the production of ultrasound. Transient grating experiments, which record both photoacoustic and photothermal processes, show the effects of the concentration of heat at the sites of colloidal particles, chemical reactions leading to the formation of stable gaseous species, and the production of steam bubbles. The efficiency of sound by these processes is, in general, determined by the rapidity of the density change that follows the absorption of the laser radiation.     

混响场中超声化学效应的研究

本文借助荧光光谱分析法,使用815kHz超声波对小尺度混响场中声化学产额进行了研究。研究发现混响场中的声化学效应有两个特点:一是声空化的阈值声强下降到约0.3W/cm²(行波场为0.7W/cm²);二是声强大于阈值时,声化学产额随声强而增加,随后(声强约为1.69—2.13w/cm²)出现陡然上升的拐点(行波场则趋向饱和)。理论分析表明,阈值下降是由于混响场的声能密度增大;声化学产额随声强变化的拐点则来自于声波辐射压力对液体表面的干扰.为此,本文从实验与理论上证明,为使声化学获得尽可能高的产额应在声化学反应器内建立混响声场。     

The effect of wind-generated bubbles on sea-surface backscattering at 940 Hz

Reliable predictions of sea-surface backscattering strength are required for sonar performance modeling. These are, however, difficult to obtain as measurements of sea-surface backscattering are not available at small grazing angles relevant to low-frequency active sonar (1-3 kHz). Accurate theoretical predictions of scattering strength require a good understanding of physical mechanisms giving rise to the scattering and the relative importance of these. In this paper, scattering from individual resonant bubbles is introduced as a potential mechanism and a scattering model is derived that incorporates the contribution from these together with that of rough surface scattering. The model results are fitted to Critical Sea Test (CST) measurements at a frequency of 940 Hz, treating the number of large bubbles, parameterized through the spectral slope of the size spectrum for bubbles whose radii exceed 1 mm, as a free parameter. This procedure illustrates that the CST data can be explained by scattering from a small number of large resonant bubbles, indicating that these provide an alternative mechanism to that of scattering from bubble clouds.     

A study on effect of sonochemistry in a reverberation field

In this paper,an ultrasound with frequency of 815 kHz was used to re-search the sonochemical yield in a small-size reverberation field by the methodof fluorescent spectrum analysis.There are two characteristics on the effect ofsonochemistry in the reverberation field:First,the cavitation threshold wasabout 0.3W/cm~2(it was 0.7W/cm~2 in travelling field);Second,when thesound intensity was larger than the threshold,the sonochemical yield increasedas the intensity increased and increased rapidly after the intensity was at1.69-2.13W/cm~2,so that there was a upturned point in the curve of the result(which would tend to saturation in the travelling field).The theoretical analysisshows that the reason of the threshold decrease is that the sound energy densitybecomes high in the reverberation field,and the upturned point results from thedisturbance of the radiation pressure on the liquid surface.Therefore,by exper-iment and theory this paper shows that a reverberation field has to be built forthe higher sonoche     

Garnet films for micron and submicron magnetic bubbles with low damping constants

Two types of films for small bubbles have been grown without making use of the rare earth ions which are the cause of increased damping of the Bloch wall motion in the usual small bubble film compositions. Firstly, Bi-containing films were grown on (111) orientated GGG for 1 μm bubbles and secondly, Mn³⁺-containing films were grown under compression on (100) orientated GGG substrates for 0.4 μm bubbles. The damping constants in these films have been obtained by measuring the FMR line width. The correlation between the damping of the FMR and of the Bloch wall motion is determined in films with bubble diameters of, 2 to 4 μm. The damping constants are a factor 10 to 100 smaller than in the existing device films for small bubbles.     

The effect of turbulence damping on acoustic wave propagation in tubes

The attenuation of sound due to the interaction between a low Mach number turbulent boundary layer and acoustic waves can be significant at low frequencies or in narrow tubes. In a recent publication by the present authors the acoustics of charge air coolers for passenger cars has been identified as an interesting application where turbulence attenuation can be of importance. Favourable low-frequency damping has been observed that could be used for control of the in-duct sound that is created by the engine gas exchange process. Analytical frequency-dependent models for the eddy viscosity that controls the momentum and thermal boundary layers are available but are restricted to thin acoustic boundary layers. For cases with cross-sections of a few millimetres a model based on thin acoustic boundary layers will not be applicable in the frequency range of interest.In the present paper a frequency-dependent axis-symmetric numerical model for interaction between turbulence and acoustic waves is proposed. A finite element scheme is used to formulate the time harmonic linearized convective equations for conservation of mass, momentum and energy into one coupled system of equations. The turbulence is introduced with a linear model for the eddy viscosity that is added to the shear viscosity. The proposed model is validated by comparison with experimental data from the literature.     

海洋混响特性研究

海洋混响是海洋环境产生的回声。对于目标探测来说,混响是一种严重的干扰;对于大规模海洋生物探测及海洋环境遥感来说,海洋混响是其应用的基础。经过近一个世纪的理论和实验研究,深海混响的产生机理相对明确,基本的特性也较为清楚。浅海环境较深海环境复杂,目前浅海混响研究大多关注混响的平均特性,对其完整特性的研究将成为未来浅海混响特性研究的重点。文章对海洋混响的研究工作进行了综述,介绍了海洋混响的基本理论和应用。     

The effect of damping and mass ratio on the stability of a tube bank

Experiments have been conducted in a wind tunnel on a parallel triangular tube array with a pitch ratio of 1·375. The aerodynamic component of damping was determined as a function of flow velocity. This damping was found to increase linearly with flow to about 50% of the stability threshold and then to decrease. Additionally, six different sets of tubes were tested to examine the effect of mass ratio on fluid-elastic stability. The results were compared with similar experiments in which damping alone was varied. It is seen that damping and mass ratio do not appear to be linearly dependent parameters which is contrary to the assumption used commonly in the stability analysis of tube banks. More importantly, however, the stability threshold is substantially less dependent on both damping and mass ratio than indicated by conventional theory.     

气垫导轨上的磁阻尼效应实验

为了克服摩擦力对运动磁体与非磁性导体相互作用产生的磁阻尼效应的影响,在铝合金气垫导轨上进行磁阻尼效应实验,推导了相互作用过程中各物理量间的解析关系,并运用自己开发的专用实验设备,获得了与理论推导一致的实验结果.     

Coherent backscattering effect from mid-frequency shallow water reverberation

The coherent backscattering (or backscattering enhancement) effect results from persistent interferences between multiply scattered waves propagating along reciprocal paths. This effect results in a doubling of the recorded backscattered intensity reflected exactly in the backward direction with respect to any other direction, after averaging intensity measurements collected over multiple emission angles. This letter presents experimental observations of the coherent backscattering effect from mid-frequency (3-4 kHz) shallow water reverberation measurements collected using a two-dimensional hydrophone array (8.4 × 1.5 m(2)). These results demonstrate the presence of coherent effects in shallow water reverberation not commonly accounted for.     

Blind single-channel suppression of late reverberation based on perceptual reverberation modeling

A blind method for suppressing late reverberation from speech and audio signals is presented. The proposed technique operates both on the spectral and on the sub-band domains employing a single input channel. At first, a preliminary rough clean signal estimation is required and for this, any standard technique may be applied; however here the estimate is obtained through spectral subtraction. Then, an auditory masking model is employed in sub-bands to extract the reverberation masking index (RMI) which identifies signal regions with perceived alterations due to late reverberation. Utilizing a selective signal processing technique only these regions are suppressed through sub-band temporal envelope filtering based on analytical expressions. Objective and subjective measures indicate that the proposed method achieves significant late reverberation suppression for both speech and music signals over a wide range of reverberation time (RT) scenarios.     

位阻效应对受阻酚杂化体系阻尼机理的影响

针对尚未解决的受阻酚结构变化与杂化体系阻尼机理间关系的问题,本文采用分子动力学模拟方法构建了三种受阻程度不同的受阻酚/聚合物杂化体系,从理论上探讨了位阻效应对阻尼机理的影响.对体系氢键相互作用、结合能、相对自由体积及扩散系数进行模拟分析表明,位阻效应对受阻酚分子内氢键相互作用有显著的弱化效果,可减少小分子团聚倾向,有利于小分子与聚合物分子间氢键相互作用的形成.但是,过高的位阻对小分子运动有阻碍作用,不利于小分子与聚合物形成强烈的氢键键合,也即不利于杂化体系阻尼性能的提高.因此,如何选择受阻程度适中的受阻酚是制备高阻尼杂化材料的一关键要素.