Anisotropic field of background internal waves on a sea shelf and its effect on low-frequency sound propagation

The space-time spectral characteristics of the field of background internal waves (IW) are obtained for two oceanic shelf regions (the Atlantic shelf of the United States and the Kamchatka shelf) and analyzed. Within the framework of a numerical experiment, it is shown that the observed anisotropy of the IW field may considerably affect the low-frequency sound fluctuations in the aforementioned regions and, in particular, may change the interference invariant of the sound field.     

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.     

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。     

The effect of internal waves on the sound propagation in the shelf zone of the sea of Japan in different seasons

Results of the observation of seasonal variations in the vertical distribution of water temperature in the shelf zone of the Sea of Japan are presented, and the effect of this variability on the parameters of internal waves and on sound propagation is studied. The measurements were carried out in different seasons using a vertical acoustical-hydrophysical measuring system. The propagation of sound (tone and noise signals) was studied on a 510-m-long track at a constant depth of 38 m. Using a self-contained resonance (320 Hz) transmitter of the electromagnetic type, which was bottom-moored at a depth of 65 m, a 10.6-km-long stationary acoustic track crossing the shelf was set up. During the in-sea experiments, the spatial characteristics of internal waves were measured along with the distributions of temperature, salinity, sound velocity, and sea level variations.     

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。     

Effect produced on sound propagation by an internal temperature front moving over the shelf

Results of observing the changes that occur in the vertical distribution of water temperature under the effect of an intense atmospheric cyclone and the influence of these changes on sound propagation in the shelf region of the Sea of Japan are presented. The measurement results refer to the autumn conditions. The measuring equipment includes a vertical acoustic-hydrophysical measuring system, a broadband transmitter (both of them being connected with the shore station by cable lines), and a self-contained resonance (320 Hz) transmitter of the electromagnetic type. The sound (tone signals) propagation is studied on a 510-m-long constant-depth (38 m) track (TON-310 Hz) and a 10.6-km-long track (TON-320 Hz), which is set up by placing the self-contained transmitter at the bottom (at a depth of 65 m). Results of field experiments are presented along with those of numerical simulation of the effect produced by an internal temperature front moving toward the coast and formed by the seasonal thermocline on the propagation of 320-Hz sound signals through it. It is shown that refraction and scattering of sound waves propagating through the temperature front moving along the acoustic track may cause intensity variations of acoustic field at the reception point, which occur synchronously at different depths and have amplitudes of up to 14 dB and a period of about 40 min.     

Effect of internal waves on the intensity and interference structure of the sound field in a shelf zone

Experimental data obtained by studying the effect of internal waves with known space-time parameters on the propagation of a 240-Hz tone signal and on the space-time interference structure of the sound field generated by a noiselike sound source are presented.     

Low frequency coupled mode sound propagation over a continental shelf

A two-way integral equation coupled mode method is applied to a continental shelf ocean waveguide proposed for a special session devoted to range-dependent acoustic modeling at the 141st meeting of the Acoustical Society of America. The coupled mode solution includes both sediment trapped and continuum modes. The continuum is approximated by a finite number of leaky modes but neglects the branch cut contribution. Mode coupling matrix elements and the range evolution of the modal amplitudes show the nature of the mode coupling. Transmission loss versus range at 100 Hz predicted by the integral equation approach is compared to the transmission loss predicted by a wide angle parabolic equation method. While there is very good agreement, one observes small differences that can be interpreted as backscattering predicted by the integral equation solution.     

The propagation of sound waves and neutron waves in bounded plane geometry

Integral equations are formulated for neutron wave propagation parallel to the faces of a slab and sound wave propagation in a gas confined between parallel plates. The main physical differences arise from the different conditions satisfied by the particle distribution functions at the boundaries of the systems, i.e. neutrons can pass through a boundary unhindered whilst gas atoms are reflected from it.     

The influence of bulk losses and bulk dispersion on the propagation properties of surface waves in a radially inhomogeneous optical waveguide

The influence of bulk losses and bulk dispersion on the propagation properties of surface waves in a radially inhomogeneous optical fibre is investigated numerically. The fibre is assumed to consist of a homogeneous cladding of either infinite or finite extent and a radially inhomogeneous core of higher permittivity. Bulk losses and dispersion in the core and cladding materials are presented by frequency-dependent complex permittivities. Power series expansions for the electromagnetic field quantities in the core have been used and the attenuation and the phase coefficients for the lower order modes have been calculated numerically. Results are presented for two different permittivity profiles, viz. the parabolic profile and the cubic spline profile, the cladding being of infinite extent in this case. Also, the influence of a finite cladding diameter on the numerical results is discussed.The research reported in this paper has been carried out with partial financial support from the Delfts Hogeschoolfonds.     

Shallow water sound propagation with surface waves

The theory of wavefront modeling in underwater acoustics is extended to allow rapid range dependence of the boundaries such as occurs in shallow water with surface waves. The theory allows for multiple reflections at surface and bottom as well as focusing and defocusing due to reflection from surface waves. The phase and amplitude of the field are calculated directly and used to model pulse propagation in the time domain. Pulse waveforms are obtained directly for all wavefront arrivals including both insonified and shadow regions near caustics. Calculated waveforms agree well with a reference solution and data obtained in a near-shore shallow water experiment with surface waves over a sloping bottom.     

The influence of boundary layer on sound propagation in optical fibers

Physical models of two-layered and three-layered fiber were built. These models were used to calculate acoustic properties of optical fibers. Acoustic properties of fibers with boundary layer and without boundary layer calculated from these model were compared. The propagation of acoustic cylindrical symmetric waves in optical fibers was considered. This problem was treated analytically and numerically in continual approximation. The possibility of simultaneous propagation of two cylindrical symmetric waves in optical fibers with a boundary layer was shown (in a three-layered model). The physical phenomenon of the presence of two waves in the fiber with boundary layer is proposed for study of the boundary layer. Formulas for the calculation of the properties of the boundary layer from the acoustic experiment, verified by numerical calculations, are represented.     

Acoustic effects caused by high-intensity internal waves in a shelf zone

The sound field fluctuations caused by high-intensity, solitonlike, quasi-plane internal waves crossing a fixed acoustic path at different angles are numerically modeled for natural conditions of the shelf zone of the Sea of Japan. The horizontal refraction of sound is considered for the case of an acoustic path parallel to the internal wave front.     

Numerical simulation of nonlinear propagation of sound waves in a finite horn

Nonlinear acoustic propagation generated by a piston in a finite horn is numerically studied.A quasi-one-dimensional nonlinear model with varying cross-section uses high-order low-dispersion numerical schemes to solve the governing equation.Because of the nonlinear wave distortion and reflected sound waves at the mouth,broadband time-domain impedance boundary conditions are employed.The impedance approximation can be optimized to identify the complex-conjugate pole-residue pairs of the impedance functions,which can be calculated by fast and efficient recursive convolution.The numerical results agree very well with experimental data in the situations of weak nonlinear wave propagation in an exponential horn,it is shown that the model can describe the broadband characteristics caused by nonlinear distortion.Moreover,finite-amplitude acoustic propagation in types of horns is simulated,including hyperbolic,conical,exponential and sinusoidal horns.It is found that sound pressure levels at the horn mouth are strongly affected by the horn profiles,the driving velocity and frequency of the piston.The paper also discusses the influence of the horn geometry on nonlinear waveform distortion.     

有限长号筒中非线性声传播的数值模拟研究

研究了大振幅活塞声源经有限长号筒向外辐射声波的非线性声学问题。采用具有频散保持特性的高精度计算格式求解了适用于变截面管道的一维非线性声场模型,并考虑到声波的非线性畸变及管口处的声反射,加入了宽频时域声阻抗边界条件。宽频阻抗模型的共轭复数系数采用优化拟合方法近似求解,并采用递推卷积算法快速求解出时域声阻抗。在弱非线性条件下模拟指数形号筒中的声传播取得了与已有实验相一致的结果,表明模型能够描述声波非线性畸变带来的宽频特性。进而采用本模型数值模拟了大振幅活塞声源在双曲形、锥形、指数形和正弦形号筒中的非线性声传播问题,结果表明号筒出口声压级受活塞振动速度、频率以及号筒形状的影响,并分析讨论了波形畸变与号筒几何形状之间的关系。     

有限长号筒中非线性声传播的数值模拟研究

研究了大振幅活塞声源经有限长号筒向外辐射声波的非线性声学问题。采用具有频散保持特性的高精度计算格式求解了适用于变截面管道的一维非线性声场模型,并考虑到声波的非线性畸变及管口处的声反射,加入了宽频时域声阻抗边界条件。宽频阻抗模型的共轭复数系数采用优化拟合方法近似求解,并采用递推卷积算法快速求解出时域声阻抗。在弱非线性条件下模拟指数形号筒中的声传播取得了与已有实验相一致的结果,表明模型能够描述声波非线性畸变带来的宽频特性。进而采用本模型数值模拟了大振幅活塞声源在双曲形、锥形、指数形和正弦形号筒中的非线性声传播问题,结果表明号筒出口声压级受活塞振动速度、频率以及号筒形状的影响,并分析讨论了波形畸变与号筒几何形状之间的关系。     

Observations of acoustic surface waves in outdoor sound propagation

Acoustic surface waves have been detected propagating outdoors under natural conditions. Two critical experimental conditions were employed to ensure the conclusive detection of these waves. First, acoustic pulses rather than a continuous wave source allowed an examination of the waveform shape and avoided the masking of wave arrivals. Second, a snow cover provided favorable ground impedance conditions for surface waves to exist. The acoustic pulses were generated by blank pistol shots fired 1 m above the snow. The resultant waveforms were measured using a vertical array of six microphones located 60 m away from the source at heights between 0.1 and 4.75 m. A strong, low frequency "tail" following the initial arrival was recorded near the snow surface. This tail, and its exponential decay with height (z) above the surface (approximately e(-alpha z)), are diagnostic features of surface waves. The measured attenuation coefficient alpha was 0.28 m(-1). The identification of the surface wave is confirmed by comparing the measured waveforms with waveforms predicted by the theoretical evaluation of the explicit surface wave pole term using residue theory.