Hawking-like radiation does not require a trapped region

We discuss the issue of quasiparticle production by "analogue black holes" with particular attention paid to the possibility of reproducing Hawking radiation in a laboratory. By constructing simple geometric acoustic models, we obtain a somewhat unexpected result: We show that, in order to obtain a stationary and Planckian emission of quasiparticles, it is not necessary to create a trapped region in the acoustic spacetime (corresponding to a supersonic regime in the fluid flow). It is sufficient to set up a dynamically changing flow asymptotically approaching a sonic regime with sufficient rapidity in laboratory time. This result is generic to curved-space quantum field theory, the "analogue spacetimes" we consider providing a guide to physical intuition, and a possible route to laboratory experiments.     

Scaling and universality in rock fracture

We present a detailed statistical analysis of acoustic emission time series from laboratory rock fracture obtained from different experiments on different materials including acoustic emission controlled triaxial fracture and punch-through tests. In all considered cases, the waiting time distribution can be described by a unique scaling function indicating its universality. This scaling function is even indistinguishable from that for earthquakes suggesting its general validity for fracture processes independent of time, space, and magnitude scales.     

Acoustic emission of ultrashort laser pulses in air and aerosols

The effectiveness of photoacoustic diagnosis in studying ultrashort laser pulse propagation in different media is considered on the basis of results from laboratory experiments on recording acoustic emissions induced by terawatt femtosecond laser pulses affecting aerosols, individual droplets, and air.     

Multi-parameter characterization of the longitudinal plasma profile of a filament: a comparative study

The longitudinal plasma profile of a laboratory scale filament induced by the propagation of an intense femtosecond laser pulse in air was measured simultaneously by three different methods. Each of them is based on a specific property of the filament, including electromagnetic pulse emission, backward emitted nitrogen fluorescence, and acoustic wave generation. Although each of above methods has been separately reported, it is the first time that a simultaneous multi-parameter measurement on a laser filament is presented. Such multiparameter measurement allows a direct comparison between the different methods, and a better understating of the dynamics of a filament from its generation through multi-photon/tunnel ionization (MPI/TI) to its damping through acoustic wave emission. The advantages as well as drawbacks of each method are analyzed in a comparative way. PACS 42.65.Jx; 42.68.Ay; 42.68.Wt     

Difference frequency and its harmonic emitted by microbubbles under dual frequency excitation

Vibro-acoustography is an elasticity imaging method that uses two ultrasound beams of slightly different frequency to excite an object and detects the resulting acoustic emission (AE) at the difference frequency. This method is especially sensitive to bubbles due to their nonlinearity. This study explores the harmonic acoustic emission (HAE) at twice the difference frequency emitted from bubbles. A perturbation method based on the dynamic bubble equation is used to derive the AE and HAE from a single bubble excited by dual frequency waves. Simulation shows that HAE is generated only by microbubbles whose resonant frequencies match the incident ultrasound frequencies. In contrast, AE is more sensitive to resonance at the difference frequency, which is relevant to sub-millimeter bubbles. This finding was confirmed by experiments where HAE was produced from Optison microbubbles, but not from larger air bubbles which are off resonance at the incident ultrasound frequency. In conclusion, harmonic acoustic emission is present for microbubbles. It is very sensitive to the size of the bubble and may be used for selective detection of microbubbles.     

Environmental implications of acoustic aerosol agglomeration

An overview is presented of acoustically induced agglomeration of fine particulates, potential industrial applications of the technology, and its environmental implications in terms of fine particle pollutants. Adverse health effects due to exposure to fine aerosols are discussed as well as recent legislation to reduce the output of such emissions. Based on this, the need for new, more efficient particle filtration technologies is demonstrated. It is shown that acoustic aerosol preconditioning meets all the requirements to reduce the fine particle output of conventional filter systems. The results of laboratory scale experiments are presented to illustrate the underlying mechanisms of the acoustic agglomeration process, while data from pilot scale testing are shown to prove the effectiveness of acoustic agglomeration systems in reducing the fine particle content of aerosols. Other filtration technologies are compared with acoustic agglomeration equipment.     

Contributions to the acoustic excitation of bubbles released from a nozzle

It has recently been demonstrated that air bubbles released from a nozzle are excited into volume mode oscillations by the collapse of the neck of air formed at the moment of bubble detachment. A pulse of sound is caused by these breathing mode oscillations, and the sound of air-entraining flows is made up of many such pulses emitted as bubbles are created. This paper is an elaboration on a JASA-EL paper, which examined the acoustical excitation of bubbles released from a nozzle. Here, further details of the collapse of a neck of air formed at the moment of bubble formation and its implications for the emission of sound by newly formed bubbles are presented. The role of fluid surface tension was studied using high-speed photography and found to be consistent with a simple model for neck collapse. A re-entrant fluid jet forms inside the bubble just after detachment, and its role in acoustic excitation is assessed. It is found that for slowly-grown bubbles the jet does make a noticeable difference to the total volume decrease during neck collapse, but that it is not a dominant effect in the overall acoustic excitation.     

压缩空气微波等离子体发射光谱的实验研究

利用微波等离子体发生装置,以压缩空气为工作气体,在1～5 atm气压下激发了微波等离子体。使用光谱测量系统,对不同气压和不同入射微波功率情况下的压缩空气微波等离子体的发射光谱进行了实验研究。结果显示,在其他条件不变时,随着气压升高,压缩空气微波等离子体的带状连续谱特征逐渐减弱;随着入射微波功率降低,带状连续谱强度逐渐减弱而带状连续谱特征依然显著。实验结果为了解压缩空气微波等离子体的光谱特性和NO活性基团的产生条件提供了实验依据。     

Experimental studies on perturbed acoustic resonant spectroscopy by a small rock sample in a cylindrical cavity

A measurement system for acoustic resonant spectroscopy (ARS) is established, and the effects of resonant cavity geometry, inner perturbation samples and environmental temperature on the ARS are investigated. The ARSs of the small samples with various sizes and acoustic properties are measured. The results show that at the normal pressure, the resonant frequency decreases gradually with the increase of liquid temperature in the cylindrical cavity, while the resonant amplitude increases. At certain pressure and temperature, both the resonant frequency and the amplitude decrease greatly when there exist air bubbles inside the cavity fluid. The ARS is apparently affected by the sample porosity and the sample location in the resonant cavity. At the middle of the cavity, the resonant frequencies reach their maximum values for all of the measurement samples. The resonant frequencies of the porous rock samples are smaller than those of the compacted samples if other acoustic parameters are the same. As the sample is moved from the top to the middle of the cavity along its axis, the resonant amplitude increases gradually for the compacted rocks while decreases for the unconsolidated rocks. Furthermore, the resonant amplitude increases firstly and then decreases if the porosity of the rock sample is relatively small. In addition, through the comparisons between the experimental and theoretical results, it is found that the effects of the acoustic parameters and sizes of the samples and the size of the cylindrical cavity on the laboratory results agree well with the theoretical ones qualitatively. These results may provide basic reference for the experiment study of rock acoustic properties in a low frequency using ARS.     

Song of the dunes as a self-synchronized instrument

Since Marco Polo it has been known that some sand dunes have the peculiar ability to emit a loud sound with a well-defined frequency, sometimes for several minutes. The origin of this sustained sound has remained mysterious, partly because of its rarity in nature. It has been recognized that the sound is not due to the air flow around the dunes but to the motion of an avalanche, and not to an acoustic excitation of the grains but to their relative motion. By comparing singing dunes around the world and two controlled experiments, in the laboratory and the field, we prove that the frequency of the sound is the frequency of the relative motion of the sand grains. Sound is produced because moving grains synchronize their motions. The laboratory experiment shows that the dune is not needed for sound emission. A velocity threshold for sound emission is found in both experiments, and an interpretation is proposed.     

Effect of the state of internal boundaries on granite fracture nature under quasi-static compression

Based on an analysis of the spatial distribution of hypocenters of acoustic emission signal sources and an analysis of the energy distributions of acoustic emission signals, the effect of the liquid phase and a weak electric field on the spatiotemporal nature of granite sample fracture is studied. Experiments on uniaxial compression of granite samples of natural moisture showed that the damage accumulation process is twostage: disperse accumulation of damages is followed by localized accumulation of damages in the formed macrofracture nucleus region. In energy distributions of acoustic emission signals, this transition is accompanied by a change in the distribution shape from exponential to power-law. Granite water saturation qualitatively changes the damage accumulation nature: the process is delocalized until macrofracture with the exponential energy distribution of acoustic emission signals. An exposure to a weak electric field results in a selective change in the damage accumulation nature in the sample volume.     

Influence of viscosity on the diffraction of sound by a circular aperture in a plane screen

The linearized equations of viscous fluid flow are used to analyze the diffraction of a time-harmonic acoustic plane wave by a circular aperture in a rigid plane screen. Arbitrary aperture size and arbitrary angle of incidence are considered. Sets of dual integral equations are derived for the diffracted velocity and pressure fields, and are solved by analytic reduction to sets of linear algebraic equations. In the case of normal incidence, numerical results are presented for the fluid velocity in the aperture and the power absorption due to viscous dissipation. The theoretical results for power absorption are compared to previously obtained results from high amplitude acoustic experiments in air. The conditions under which the dissipation predicted by linear theory becomes significantare quantified in terms of the fluid viscosity and sound speed, the acoustic frequency, and the aperture radius.     

Characteristics of noise attenuators for use in acoustic emission studies

The background noise generated by servo-hydraulic machines commonly used in laboratory tests of materials prevents the extraction of the maximum amount of acoustic emission data during fatigue experiments. Some of the solutions for removing background noise during monotonic tests are not applicable, and modern acoustic emission (ae) monitoring systems cannot detect all ae generated in the presence of continuous background noise. The best technique for reducing the background noise appears to be the insertion of a noise attenuating barrier between the noise source and the test specimen.Multilayer sound transmission theory is outlined to show that noise attenuators must be designed for the particular frequency at which the ae system is to operate. The theoretical basis for obtaining (for a given application) the optimum attenuation from attenuators constructed on an acoustic impedance mismatch basis is provided. Finally, physical properties of materials suitable for use in attenuator construction are tabulated.     

光纤Sagnac传感器对材料频率特性的实验研究

不同的材料由于物理性质不同,在断裂时会产生不尽相同的声发射信号。光纤声发射传感器以其频带宽,抗电磁干扰,灵敏度高,体积小等优点在声发射（AE）信号探测方面有着广泛的应用前景。本文采用光纤Sangac传感器,对不同材料断裂过程的声发射信号进行检测,通过快速傅立叶变换分析其频谱。实验得知同一种材料断裂的声发射信号具有相同的...     

Shock Structures in Charge Variable Dusty Plasmas with Effect of Strongly Coupled Dust Particles

A theoretical investigation has been carried out to study the effect of strong electrostatic interaction on the dust acoustic shock structures in strongly coupled dusty plasma with dust charge fluctuations.The fluid approach is employed,in which the strong electrostatic interaction is modeled by effective electrostatic temperature.A Burger-like equation,the coefficients of which are significantly modified by effects of strong coupling and dust charge Ructuation,is derived.It is shown that the combined effects of dust charge Ructuation,the ion/electron temperature,the ion/electron population,and strong coupling effect modify the basic properties of the dust acoustic waves in such a strongly coupled dusty plasma.The results of this work are compared with those observed by some laboratory experiments.     

自由空间内悬挂气泡破碎时的声学特性

气泡破碎是自然界和工、农业生产中常见的现象。气泡在破碎过程中因气流流动而产生强烈的扰动,导致破碎时产生强烈的气动声学。该文采用实验和理论相结合的方式对自由空间内悬挂气泡破碎时的声学特性进行了研究。研究发现：随着液体表面张力系数的增加,声发射的特征振幅逐渐增大;随着气泡半径的增加,声发射的特征振幅逐渐增大。理论计算结果与实验结果基本吻合。     

联合重叠压缩感知法消除水声通信系统限幅噪声

针对正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)水声通信系统的高峰值平均功率比(Peakto-Average Ratio,PAPR)问题,在发射端采用了压扩变换法和限幅法联合的方法降低PAPR。由于限幅法会产生带内干扰和带外噪声,降低通信系统的误码性能,因此利用限幅噪声的稀疏性,在接收端采用压缩感知(Compressed Sensing,CS)法对限幅噪声进行估计和恢复。限幅噪声的估计受信道估计准确性的影响,为提高限幅噪声估计的准确度,提出了重叠压缩感知算法,在恢复限幅噪声的过程中利用了压缩感知信道估计法估计所得的信道信息和发射数据对限幅噪声进行估计,有效降低了限幅法对系统误码性能的影响。仿真和水池实验验证了该算法的有效性。      

Modeling of the intensification of oil production by an acoustic action on the oil pool from the borehole

An improved model of physical processes that occur under an acoustic action (AA) is presented in the context of both thermal AA mechanism and its nonthermal alternatives. It is assumed that the fluid in an oil pool consists of light and heavy hydrocarbon phases, which are in thermodynamic equilibrium. External actions, such as filtration or AA, can shift the point of equilibrium of the fluid fractions in such a way that the heavy fraction may deposit on the pore walls or dissolve. In this case, the process of long-term mudding is governed by the inhomogeneity of the pressure field around the borehole and, correspondingly, by the change in the equilibrium concentration of the heavy impurity in the course of filtration. In the framework of the proposed model, the effect of an acoustic action can manifest itself both indirectly, as a heating of the surrounding medium because of sound absorption (which also changes the equilibrium concentration of impurity), and directly, as the dependence of the equilibrium concentration and relaxation time on the average density of acoustic energy of ultrasonic vibrations. The proposed model makes it possible to reproduce the characteristic features of the fluid filtration from the borehole before and after the AA, including the long-term intensification of petroleum recovery from the collector with a gradually decreasing recovery rate.     

Photoluminescence studies of Si/SiO2 nanoparticles synthesized with different laser irradiation wavelengths of nanosecond pulse duration

Photoluminescence (PL) properties of Si nanoparticles (Si-np) produced by irradiating the Si wafer with nanosecond laser pulses at 532, 683 and 1064 nm are studied. Si-np are found to be deposited in a doughnut shape around the irradiated spot. The irradiation wavelength is found to be the main cause for the particle size variation. Exposure of the freshly prepared Si-np to air for different periods of time leads to increased PL intensity reaching saturation after few days. The PL spectrum shows two well resolved peaks around 435 nm (2.85 eV) and 441 nm (2.81 eV) within an hour of exposure of the freshly prepared samples to air with broadening of the emission spectrum on further exposure to air. Possible mechanism of particle size variation and PL emission are discussed.