Structure of the sound field in convergence zones of the tropical Pacific

Experimental data are presented on the intensity and angular structures of the sound field in the first and second convergence zones for the tropical region of the Pacific Ocean, southwest of the Hawaian Islands. The experiment is carried out using continuous pseudonoise signals in one-third-octave bands around central frequencies of 1.25 and 3.15 kHz with both source and receiver positioned under the discontinuity layer at a depth of 250 m. The experimental data are compared with calculations based on the ray model. The discrepancies arising between the experimental and calculated data at the entry onto the first convergence zone are compared with the corresponding parameters obtained earlier in the tropical regions of the Atlantic and Indian oceans. The detailed intensity structure of the sound field is correlated with the parameters of internal waves observed during the acoustic measurements.     

Features of the Sound Field Structure in a Two-Channel Oceanic Waveguide

The results of experimental studies of the energy and angular structure of a sound field in the region of the Iberian Basin in the northeastern Atlantic are discussed. The experiments are carried out in a two-channel waveguide whose axes are located at depths of approximately 450 and 2000 m. A continuous pseudonoise signal in the frequency range 2.52–4.0 kHz is emitted. The signals are received by the omnidirectional hydrophones and, simultaneously, by a 10-m-long vertical array, which allows one to realize a narrow beam reception (～2.5°) in the vertical plane. The source and the receiver are located in a 500-m-thick layer within the upper sound channel. The field characteristics are measured in the course of a continuous change of distance from 1 to 65 km. The comparison of the experimental data with calculations shows that the sound field structure formed by the lower channel is much closer to the theoretical results than the structure formed by the upper channel. In the upper sound channel, the shadow zone manifests itself only slightly and the first convergence zone begins approximately 72–11 km nearer to the source than predicted by the calculations. The corresponding angular sound field structure is fairly pronounced in the vertical plane and bears no evidence of the random behavior that is peculiar to the fields scattered by the inhomogeneities.     

Illumination of the shadow zone in a two-channel oceanic waveguide with a fine structure of sound speed inhomogeneities

The experimental data on the sound field structure, which were obtained by emitting a continuous pseudonoise signal (a midfrequency of 3.2 kHz) in a two-channel oceanic waveguide, are compared with the calculations performed by the wave program with allowance for the fine structure of the sound speed inhomogeneities. A considerable increase in the intensity of the sound field with a definite angular spectrum is observed in the upper channel in the first shadow zone, and, in the experiment, the increase begins nearer to the sound source than predicted by the wave and ray calculations for a smooth sound speed profile. These features of the field structure are explained by the illumination of the shadow zone by the regular scattering of signals from highly anisotropic fine-structure inhomogeneities of the sound speed profile, which are clearly pronounced in the region of the given oceanic experiment.     

Energy characteristics of the sound fields in the Barents Sea

Experimental studies of the intensity structure of the sound field in the Barents Sea were carried out. The signal propagation path was ～80 km in length, and the sea depth was ～220–250 m. Pseudonoise signals generated in one-third-octave frequency bands with central frequencies of 1.25 and 3.15 kHz were used. The sound source was positioned at two different points: in the subsurface layer at a depth of 10 m and under the discontinuity layer at a depth of 100 m. The reception depths were 15, 100, and 200 m. The experimental results were compared with the results of ray calculations taking into account the wind waves and the parameters of the ground at the bottom. The latter parameters were obtained by classifying the data available from the literature. As a result of the analysis, a fundamental possibility was demonstrated to predict the intensity characteristics of the field structure in a shallow sea with allowance for the depth dependence of sound velocity and the parameters of the waveguide boundaries.     

Investigation of the sound field structure in the ocean with a deep-water reception

The results of studying the angular and energy structures of a sound field at great depths (880 and 1100 m) with the emission of pseudonoise signals (a frequency range of 1–4 kHz) in the upper layers of the ocean below the velocline (at a depth of 200 m) are presented. The results refer to two ocean regions characterized by the presence of a single-axis deep-water sound channel. The sound field structure at great depths is compared with that recorded at the source depth. The experimental data for the first two convergence zones are compared with the calculations by the ray method. A conclusion is made that, on the whole, the basic regularities of the field structure at great depths are adequately described by the ray theory. However, the spatial positions of the convergence zones observed in the experiment differ from those predicted by the ray calculations. Moreover, a closer examination of the field characteristics shows that the parameters of the angular and energy structures noticeably deviate from the calculated values.     

Correlation characteristics of pseudonoise signals in the ocean at a highly directional reception

Data of experiments with single-path and multipath reception of broadband continuous pseudonoise acoustic signals (0.8–1.3 kHz) in a deep ocean are discussed. The experiments were carried out on tracks from ～ 60 to ～ 420 km in length, which corresponds to the first seven convergence zones. The data on the cross-correlation coefficients and arrival time differences are presented (for the first, second, and seventh zones) for signals received by one highly directional (～2°) vertical array or by two such arrays simultaneously at depths of 200 and 450 m. The correlation characteristics of signals propagating over purely water paths, i.e., without reflections from the waveguide boundaries, and arriving under different angles in the vertical plane are compared. It is found that the origin of the decorrelation of signals received from various directions or at different depths (all other factors being the same) is the incomplete resolution of the received multipath signals in the arrival angle in the vertical plane.     

Long-term stability of the sound field structure in the central part of the eastern Atlantic

Results of studying the angular and energy structures of the sound field formed in the first convergence zone as a result of the transmission of a continuous pseudonoise signal in the frequency range 0.8–1.5 kHz are presented for the case of the corresponding points lying in the upper ocean layer at depths between 150 and 500 m. The sound field characteristics were studied with the use of omnidirectional and narrow-beam (～2°) reception in the vertical plane. The variability of the basic experimental parameters of the field structure is analyzed using the data of three experiments carried out in the eastern part of the Atlantic Ocean, near the Canary Islands, over a period of 11 years. The experimental results are compared with ray calculations. It is shown that, in the region under investigation, the experimental characteristics of the field structure are relatively stable and the extent to which they agree with the calculated parameters is also stable. In addition, some data are presented to illustrate the effect of a small lens of Mediterranean water, which manifests itself in sound velocity variations at the channel axis, on certain field characteristics.     

Scattering measurements from a dissolving bubble

A laboratory-scale study on acoustic scattering from a single bubble undergoing dissolution in undersaturated fresh water is presented. Several experiments are performed with the acoustic source driven with five-cycle tone bursts, center frequency of 120 kHz, to insonify a single bubble located on axis of the combined beam of the set of transducers. The bubble is placed on a fine nylon thread located in the far field of the transducer set, arranged in bistatic configuration, in a tank filled with undersaturated water. Backscattered waveforms from the bubble target are acquired every few seconds for several hours until the bubble has completely dissolved, and detailed dissolution curves are produced from the acoustic data. The rate of bubble dissolution is calculated using the solution developed by Epstein and Plesset [J. Chem. Phys. 18, 1505-1509 (1950)]. The results of the experiments performed are in agreement with the calculations.     

Spectral and energy characteristics of acoustic emission in the laser radiation breakdown of water

Results from experimental investigations of acoustic emission from the optical breakdown zone in liquid formed by laser radiation at a wavelength of 532 nm are presented. The linear dependence of the acoustic pressure on the laser pulse energy is revealed. It is shown that the effect of an additional source of sound at a frequency of 37 kHz enhances the acoustic emission from the optical breakdown zone.     

激光声信号的频谱特性

构建了激光声实验测量系统,利用脉冲激光聚焦击穿水介质产生声信号,由水听器将声信号转换成电信号并送入数字存储示波器。分析了激光声信号的时频域数学模型,实验研究了激光声信号的频域能量分布,以及激光器重复频率和激光声信号频谱特性的关系。结果表明:激光声信号能量主要集中在200 kHz内,其中100~200 kHz内的能量所占比例约50%。激光声信号的幅频响应极大值点可以受到激光器重复频率的控制。     

Orthogonal acoustical factors of a sound field in a bamboo forest.

To investigate the acoustical quality of a sound field in a bamboo forest, acoustical measurements were conducted to obtain orthogonal acoustical factors of the sound field. These results are compared with previous results for a sound field in an ordinary forest [H. Sakai, S. Sato, and Y. Ando, J. Acoust. Soc. Am. 104, 1491-1497 (1998)]. The IACC, which is defined as a maximum value of the normalized interaural cross-correlation function between signals at the ears, was 0.07 (4 kHz) and 0.16 (2 kHz) at positions 20 and 40 m from the source, respectively. These values are much better than those in the previously investigated forest. The measured subsequent reverberation time Tsub was up to 1.5 s in the frequency range above 1 kHz at the position 40 m from the source. For certain music sources with higher frequency components, therefore, sound fields in a bamboo forest have excellent acoustic properties.     

On the predictability of the positions of the convergence zones in the ocean

The disagreement between the experimental and calculated positions of the first convergence zone are known from many publications. The most probable cause for such a disagreement, namely, the incorrect specification of the input data for the calculations, is considered. The lack of simultaneity between the hydrological surveys of the region and the acoustic experiments is emphasized. The experimental data obtained by the author in five ocean regions are presented. These data characterize the diurnal variability of the distance from the source to the nearest boundary of the convergence zone. The relations proposed by different researchers for calculating the sound speed from the temperature, salinity, and hydrostatic pressure are analyzed. It is shown that these relations lead to a substantial difference in the estimated depth dependence of the hydrostatic gradient of the sound speed. The position of the first convergence zone is calculated for the propagation conditions determined by vertical temperature and salinity profiles with the subsequent recalculation of these profiles into sound speed profiles by using eight different formulas known from the literature. It is shown that different formulas lead to different values of the distance to the first zone; this difference is substantially greater than that between the calculations and experiment. The necessity of improving the recalculation relations in view of the experimental data on sound propagation in natural oceanic waveguides, including the data on the actual positions of the convergence zones, is emphasized.     

高强混凝土单轴压缩声发射频率特征试验研究

为研究高强混凝土破裂前声发射信号的频率特征,对C60、C70、C80高强混凝土试件进行单轴压缩下的高、低频双通道声发射试验,得到破裂过程的力学参数和声发射参数,探求高强混凝土不同加载阶段声发射信号频率的分布特征。研究表明,三种高强混凝土在峰值应力前,高、低频通道声发射信号均集中在特定的频段内;临近峰值应力时,高、低频通道的声发射信号频率向低频段移动,同时优势频段内的频率趋于分散,这可作为预测高强混凝土破坏的前兆信息。     

Acoustic properties of humpback whale songs

A vertical array of five hydrophones was used to measure the acoustic field in the vertical plane of singing humpback whales. Once a singer was located, two swimmers with snorkel gear were deployed to determine the orientation of the whale and position the boat so that the array could be deployed in front of the whale at a minimum standoff distance of at least 10 m. The spacing of the hydrophones was 7 m with the deepest hydrophone deployed at a depth of 35 m. An eight-channel TASCAM recorder with a bandwidth of 24 kHz was used to record the hydrophone signals. The location (distance and depth) of the singer was determined by computing the time of arrival differences between the hydrophone signals. The maximum source level varied between individual units in a song, with values between 151 and 173 dB re 1 microPa. One of the purposes of this study was to estimate potential sound exposure of nearby conspecifics. The acoustic field determined by considering the relative intensity of higher frequency harmonics in the signals indicated that the sounds are projected in the horizontal direction despite the singer being canted head downward anywhere from about 25 degrees to 90 degrees. High-frequency harmonics extended beyond 24 kHz, suggesting that humpback whales may have an upper frequency limit of hearing as high as 24 kHz.     

三维浅海下弹性结构声辐射预报的有限元-抛物方程法

利用多物理场耦合有限元法对结构和流体适应性强、抛物方程声场计算高效准确的特点,提出了三维浅海波导下弹性结构声振特性研究的有限元-抛物方程法.该方法采用多物理场耦合有限元理论建立浅海下结构近场声辐射模型,计算局域波导下结构声振信息,并提取深度方向上复声压值作为抛物方程初始值;然后采用隐式差分法求解抛物方程以步进计算结构辐射声场.重点介绍了该方法对浅海下结构声辐射计算的准确性、高效性以及快速收敛性后,对Pekeris波导中有限长弹性圆柱壳的声振特性进行了分析.研究得出,当圆柱壳靠近海面(海底)时,其耦合频率比自由场下的要高(低),当潜深达到一定范围时,与自由场耦合频率基本趋于一致;在低频远场,结构辐射场与同强度点源声场具有一定的等效性,且等效距离随着频率增加而增加;由于辐射声场受结构振动模态、几何尺寸和简正波模式影响,结构辐射场传播的衰减规律按近场声影响区、球面波衰减区、介于球面波和柱面波衰减区、柱面波衰减区四个扩展区依次进行.     

Diversity reception of hydroacoustic signals from the results of experiments in the Black Sea

The results are presented of an experimental study of spatial and frequency correlation of amplitude fluctuations of hydroacoustic signals on two paths of the Black Sea under conditions of a thermal underwater sound channel and tonal-continuous emission at frequencies of 4 and 1.5 kHz. The emitters were located in the coastal wedge on the bottom slope at depths of 35 and 60 m; a receiver system was submerged to a depth of 50–60 m off the side of a ship drifting at various distances in the open sea. Data are presented on the spatial (horizontal and vertical) and frequency intervals of fluctuation correlation on the first path extending 100 km at an emission frequency of 4 kHz under conditions of dominant fast fluctuations, as well as on the second path extending 300 km at an omission frequency of 1.5 kHz under conditions of dominant slow fluctuations. The results of experiments are used to estimate the efficiency of space-diversity and frequency-diversity reception of acoustic signals in application to information transmission over a hydroacoustic channel.     

A non-contact technique for evaluation of elastic structures at large stand-off distances: applications to classification of fluids in steel vessels

A novel technique for non-contact evaluation of structures in air at large stand-off distances (on the order of several meters) has been developed. It utilizes a recently constructed air-coupled, parametric acoustic array to excite the resonance vibrations of elastic, fluid-filled vessels. The parametric array is advantageous for NDE applications in that it is capable of producing a much narrower beamwidth and broader bandwidth than typical devices that operate under linear acoustic principles. In the present experiments, the array operates at a carrier frequency of 217 kHz, and the sound field several meters from the source is described spectrally by the envelope of the drive voltage. An operating bandwidth of more than 25 kHz at a center frequency of 15 kHz is demonstrated. For the present application, the array is used to excite vibrations of fluid-filled, steel containers at stand-off distances of greater than 3 m. The vibratory response of a container is detected with a laser vibrometer in a monostatic configuration with the acoustic source. By analyzing the change in the response of the lowest order, antisymmetric Lamb wave as the interior fluid loading conditions of the container are changed, the fluid contained within the steel vessel is classified.     

Measurement of interference-pattern parameters of the near acoustic field of a deep-sea noise source

We present the results of a study of the space-frequency energy distribution of the near acoustic field of noise radiation from a moving source. Dynamic spectrograms of the received signal of noise radiation in the frequency range of 0–2.0 kHz at distances of up to 1300 m are given. It is shown that the variation in the levels of spectral components of the signal due to interference modulation in the near field can attain 12 dB. The interference pattern is shown to occur over the entire range of considered distances of 100–1300m. Within the framework of ray theory, using hydrological data for C(z) in the region of measurements, we calculate the near field for a lumped noise source. The calculation results are in good agreement with the results of field experiments. It is shown that the sound velocity profile C(z) in the region of measurements has a pronounced influence on the interference pattern of the near field. Allowance for C(z) in the calculations of the near acoustic field within the framework of the existing theory allows one to predict the interference pattern and choose optimal distances for measurements under near-field conditions.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 8, pp. 817–831, August, 1995.     

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.     

深海海底反射会聚区声传播特性

在深海声道条件下,海水折射效应会使得声场出现会聚效应;在不完全声道条件下,深海海底对声场具有重要影响.利用在中国南海海域收集到的一次深海声传播实验数据,研究了深海不完全声道环境下的海底反射对声传播的影响.实验观测到不同于深海会聚区的海底反射会聚现象,在直达声区范围内的海底地形隆起可导致海底反射会聚区提前形成,并使得部分影区的声强明显提高.由于不平坦海底和海面的反射破坏了完全声道环境下的会聚区结构,在60 km范围内存在两个海底反射会聚区,会聚区增益可达10 dB以上,同时在11 km附近的影区和51 km附近形成高声强区域.当接收深度与声源深度相同时,第二会聚区的增益高于第一会聚区.在第一会聚区内,随着接收深度的增加,声线到达结构趋于复杂,多途效应更加明显.使用抛物方程数值分析结合射线理论对深海海底反射会聚区现象产生的物理原因进行了分析解释.研究结果对于声纳在深海复杂环境下的性能分析具有重要的指导意义.