Effect of Mesoscale eddies on underwater sound propagation

The effects of sound speed variables induced by an anticyclonic eddy and a field of three cyclonic eddies on long-range sound propagation are investigated respectively. A deep-sea analytical eddy model [Henrick RF, Siegmann WL, Jacobson MJ. General analysis of ocean eddy effects for sound transmission applications. J Acoust Soc Am 1977;62:860-870] is used to determined sound speed distributions produced by warm-core ring in the southwest of South China Sea. Furthermore, the above analytical eddy model is generalize to include the azimuth angle variation and is used to determined sound speed distributions produced by Gulf Stream rings with different strengths. The theoretical temperature fluctuations induced by the warm eddy and a eddy field including three cyclonic eddies agree qualitatively with the in situ investigation data. The transmission loss of acoustic energy through the cross section of the warm-core ring center and three cyclonic eddy centers is simulated using 2-D parabolic-equation (PE) numerical modeling. It is found that the acoustic field has a significant change with variation of the location of SOFAR axis in the presence of the warm eddy and three cyclonic eddies comparing with the scenario of no eddies. When the source is located in the outside of the warm-core eddy and three cyclonic eddies respectively, and the receiver is located in outside of the eddy, the transmission loss as a function of range is investigated at different receive depth. It is shown that the changes of transmission loss caused by the warm-core eddy and three cyclonic eddies are as much as 20 dB than that of no-eddy situation. In the case of three cyclonic eddies, the largest discrepancy of transmission loss is about 40 dB near the range of 45 km for a 25-Hz source being located at a depth of 1500 m.     

Influence of superconductivity on the sound propagation in disordered metals

The velocity of sound has been investigated in the disordered metal Nb₂₀Zr₈₀ which contains low energy excitations. From measurements in the superconducting as well as in the normal state the interaction between these excitations and conduction electrons has been studied in detail.     

三维浅海环境下孤立子内波对低频声能流的传播影响

针对三维浅海环境下孤立子内波对低频声信号传播特性的影响问题,基于Oxyz坐标系下的三维浅海低频声场有限元计算方法,以声能流为研究对象,仿真分析了内波存在对低频声信号传播特性的影响规律。研究结果表明:受内波影响,在xOz平面,声能流垂直分量的传播偏转角度呈现周期性的起伏规律;随着声源深度的增加,内波对声能流偏转角度的影响深度也随之增加。对xOy平面,当声源位于温跃层以上时,随着接收深度的增加,各深度平面上声能流水平分量的偏转角越大;随着声源深度的增加,内波对各深度平面上声能流的影响逐渐减弱。     

Losses during sound propagation on the shelf

The paper discusses the results of studying how changes in the hydrological conditions affect losses that sound undergoes as it propagates along a stationary track in Vityaz Bay in the Sea of Japan. Measurements were conducted with an Mollusk-07 autonomous vertical acousto-hydrophysical measurement system and an autonomous electromagnetic emitter generating a frequency-modulated signal in the 280–340 Hz band. The modulation frequency was 0.3 Hz. It is shown that tide-, internal-wave-, and upwelling-caused variations in sound losses with a frequency of 285–335 Hz propagating along a stationary track with a length of 1640 m for seven days did not exceed 3 dB.     

Wall effects on sound propagation in tubes

Numerical solutions have been obtained for the exact equations describing the propagation of periodic axisymmetric waves in a rigid cylindrical tube. Results were obtained for air over a range of conditions corresponding to shear wave numbers (s = R √?ω/μ) from 0·2 to 5000 and reduced frequencies (k = ωR/a) from 0·01 to 6. For conciseness and convenient application, the results for the attenuation and phase shift coefficients are given in the form of simple polynomials for the ranges 5 ? s ? 5000 and 0·01 ? k ? 6. This range covers virtually all values of tube diameter and sound frequency likely to be met in practical situations that are consistent with a continuum gas model.     

负梯度水文环境下海山对声传播的影响

利用一次南海海山环境下的声传播实验数据,研究了负梯度水文环境下海底山对声传播的影响。针对实验数据中的传播损失异常,从射线声学角度给出了合理的解释,表明海底山的存在引起传播损失在距离上剧烈波动。在距离接收阵较近的7.6 km处,声源位于海山斜坡上,斜坡的反射使接收传播损失减小约8 dB,体现出斜坡增强特征。当声源位于海山后,海底山的遮蔽作用使23.8 km处的传播损失增加超过20 dB,不同位置处海山遮蔽效应的差异使传播损失随距离起伏。利用抛物模型对实验环境下的声传播进行了定量仿真,仿真传播损失同实验结果符合,验证了实验数据中海底山的反射和遮蔽作用。此外,对实验环境下海山的遮蔽损失进行分析,发现在不同声源位置处,海山遮蔽损失在特定频带上同频率对数具有线性关系。     

Long-range sound propagation in the northeastern Atlantic

Experimental data on the long-range propagation of explosion-generated signals are analyzed. The experiments were performed in the northeastern Atlantic under the conditions of a two-axis underwater sound channel. The sound field in the upper channel was governed by the vertical redistribution of the ray structure and sound energy under the influence of a smooth increase in the depth of the channel’s axis along the propagation path. The explosions were produced in the upper sound channel at a depth of 200 m, which was constant along the path. The time structure of the sound field is analyzed for the upper channel (a reception depth of 200 m) and for deeper layers lying somewhat below the boundary between the upper and lower sound channels (a reception depth of 1200 m). The deviation of the decay law obtained for the sound field level in the upper channel from the cylindrical law is used to estimate the attenuation coefficient. The low-frequency (several hundreds of hertz) attenuation coefficients experimentally determined with allowance for the sound field redistribution agree well with the calculated sound absorption in seawater. The attenuation coefficients determined by the differential method also agree well with the absorption calculated by the formulas proposed earlier. The analysis of the time structure of the sound field near the boundary between the upper and lower channels reveals a permanent insonification of this horizon by weak water-path signals propagating with the velocity typical of the signals traveling in the upper channel.     

Long-range sound propagation in the norwegian sea

The data of repeated experiments on the long-range propagation of explosion-generated and cw signals in the Norwegian Sea in summer conditions (with a fully-developed underwater sound channel) are presented. These data are used to analyze the spatial and time structures of the sound field, as well as to estimate the attenuation coefficient at frequencies within 63–630 Hz and to determine its frequency dependence. The spatial variability of the propagation conditions is analyzed on the basis of the experimental data obtained for the propagation of explosion-generated signals along a 815-km-long path crossing the Norwegian and Lofoten Hollows.     

Long-range sound propagation in the Mediterranean Sea

The data of several experiments on the long-range propagation of explosion-generated and tonal sound signals are analyzed. The experiments are performed by the Acoustics Institute in the Mediterranean Sea with a fully developed sound channel. A substantial difference is observed for the propagation conditions in the western and eastern parts of the sea. This difference concerns the vertical sound speed profiles, the time structures of the sound field in the underwater sound channel, the duration of the explosion-generated signal, and the positions of the convergence zones. The experiment is compared with calculations. The observed difference in the experimental and calculated positions of the first convergence zone is explained by the imperfection of the relation used to recalculate the salinity, water temperature, and hydrostatic pressure to the sound speed. In spite of substantial difference in the propagation conditions on two 600-km paths, the experimental low-frequency attenuation coefficients on these paths (and on some shorter ones) agree well with each other for the frequency band of several kilohertz. The data are also close to those published for another 600-km path. All the paths mentioned run in different parts of the Mediterranean Sea. The frequency dependence of sound attenuation (absorption) can be well described by the relation that accounts for the absorption caused by the boron present in the sea water.     

海洋内波对水下声场的影响

海洋内波是海水内部的波动之一,它与海洋表面的海浪运动有着非常不同的物理特征。海洋内波会引起海水温度剖面发生剧烈的波动,这种波动具有很强的方向性和时变性,对水下声传播影响巨大,并展现出一些很奇特的声波导效应(包括二维波导效应和三维波导效应),给水下目标的探测与定位以及水声通信等带来严重影响。海洋内波对声场的影响研究涉及到海洋动力学和水声学两个学科领域,文章重点介绍海洋内波对声场影响研究领域的相关进展,并对内波环境下典型声波导现象进行了讨论,最后对该领域未来的发展进行了总结和展望。     

Acoustic propagation in the subsurface sound channel

Experimental data obtained in the kilohertz frequency band for the sound propagation in the subsurface channel formed by the wind-caused mixing of subsurface waters are discussed. The data were obtained in different years in the northeastern region of the Atlantic Ocean, where the subsurface waters down to the depths of 40–70 m are mixed by both wind waves and the swell that arrives from distant ocean areas. The hydrological conditions in the subsurface waters of this region are characterized by a good reproducibility. The spatial structure of the sound field and the attenuation of sound propagating in the subsurface channel are analyzed. The origin of the additional attenuation (in comparison with the absorption in sea water) is discussed. The data of our experiments are compared with those obtained by other experimenters and with the calculations performed using the computer code by Avilov. The necessity of improving the computer codes to allow for the scattering of sound beyond the channel under the influence of the swell, whose parameters are unrelated to the wind regime at the experimental site, is emphasized.     

Effects of mesoscale eddies on the spatial coherence of a middle range sound field in deep water

Mesoscale eddies have a remarkable influence on the underwater sound field. Many previous studies have investigated the effects of eddies on transmission loss, the convergence zone, time delay, etc. However, the effects of eddies on spatial coherence are less well studied and remain unclear. In this paper, the effects of eddies on spatial coherence at the subsurface in deep water are investigated. The eddy environments are simulated with Gaussian eddy equations, the complex pressure field is obtained using a range-dependent parabolic equation model and the associated mechanism is analyzed based on ray theory and models. The results show that cold/warm mesoscale eddies affect spatial coherence in a high-intensity zone by changing the locations and width of the convergence zone. In the shadow zone, the horizontal correlation radius and the vertical correlation radius increase with range and decrease with depth, and they are increased by warm eddies and decreased by cold eddies, mainly caused by variation of the multipath structure.     

内陆湖中声传输起伏

本文对于存在负跃层的内陆湖中的内波,及其引起的声传输起伏进行了实验研究,在1.7、0.5km上测得声传输起伏统计特性。声传输起伏功率诺与内波谱有一定的关联性,其统计特性显现所测内波垂直位移谱接近ω^-2,声振幅起伏谱接近ω^-1。本文报导了内陆湖夏季负跃层中存在内波和水下内波的基本面貌特征。给出了湖水介质动态变化的规律和参数,为湖区环境下声传输特性的预报提供了依据性的参数。     

Effect of a sloping bottom on sound propagation

Acoustical Physics - The paper presents the results of field measurements of acoustic fields generated in autumn hydrological conditions of the Sea of Japan shelf by a TON-320Hz autonomous signal...     

Observations on sound propagation in rapidly rotating Bose-Einstein condensates

Repulsive laser potential pulses applied to vortex lattices of rapidly rotating Bose-Einstein condensates create propagating density waves which we have observed experimentally and modeled computationally to high accuracy. We have observed a rich variety of dynamical phenomena ranging from interference effects and shock-wave formation to anisotropic sound propagation.     

Long-range sound propagation in the region of the subarctic front

The data of two experiments performed in the northwestern Pacific are presented. The propagation path crosses the northern boundary of the subarctic frontal zone. The influence of the frontal zone on the time structure and intensity of the sound field is studied. This influence most clearly manifests itself in the range dependence of the level of the normalized sound field at frequencies of 63–800 Hz. In the region of crossing the boundary of the frontal zone, a change of 1.5–2 dB in the sound field level is observed with localization in distance. In this region, a pronounced increase in the frequency-independent component of the exponential attenuation is also observed (by 0.015 dB/km for explosion-generated signals received at a depth of 600 m). At depths of 150–800 m, a zone of weak variations of the propagation loss is present in the vertical structure of the sound field at the 100-km part of the path in the region of crossing the front. In the experiment with explosion-generated signals, phenomena that are unrelated to the frontal zone are observed as well, namely, the appearance of reverberation forerunners (prereverberation) on separate parts of the path and the presence of bottom-reflected signals on one of the path fractions with a local bottom rise.     

深海表面层声速剖面变化对声传播的影响

在我国部分海域，表面声速常常在较短的时间内从弱正梯度变为弱负梯度，这将对表面附近的声传播产生巨大影响。本文通过数值模拟的方法，讨论了表面层声速梯度变化对声传播的影响，同时解释了某次海上实验出现的异常现象：在同一地点同一航向，前后两天海况相同，水文相差不大，第一天的实验可以在50km之外接收到信号，而第二天只能在30km处才能收到信号。