Low-frequency Sound Propagation Research Articles

Estimation of Water-Like Bottom Length along an Acoustic Track in Shallow Water

Analytically and within the framework of numerical simulations, remote sensing of the water-like bottom in shallow water based on low-frequency sound propagation loss is considered. Water-like bottom sediments are understood as sediments in which the sound speed is close to the sound speed in water, but having a significantly higher density. A model statistical analysis of depth-averaged transmission loss is carried out for acoustic tracks of a fixed range in one of the areas of the Kara Sea with a known structure of the upper layer of the bottom, which includes water-like areas. A good correlation between low-frequency transmission loss and the length of water-like bottom is demonstrated. Based on this result, a method for remote integral estimation of the water-like bottom size at an acoustic track between a single source and a vertical array of hydrophones is proposed.

Low-frequency non-reciprocal sound propagation features in thermoacoustic waveguide.

Thermoacoustic waveguides are systems of hollow tubes and thermally graded porous segments that can operate as active materials where acoustic waves receive energy from an external heat source. This work demonstrates that by adjusting the pore geometry several unique low-frequency propagation features arise from the complex-valued band structure of periodic thermoacoustic waveguides that reflect into the acoustic pressure field within finite-length systems. Numerical methods have been employed to model waveguides with porous segments constituted by cylindrical inclusions (parallel pins). In periodic structures, a critical frequency emerges where the sign of the refractive index in one direction of propagation changes, thus zero- and negative-unidirectional refractive index, unidirectional energy transport, and amplification/attenuation crossover effects may take place. On the other hand, the study of the acoustic pressure field shows that, for wave packets with either direction of propagation, finite-length waveguides may behave as active acoustic metamaterials with zero- or negative-refractive index. The acoustic pressure field in the waveguide, generated by an upstream source, may exhibit increasing amplitude and phase recovery farther away from the source, mimicking the field created by a downstream source, propagating upstream in a non-active medium.

Solutions to muddy (geoacoustic inversion) problems

Waveguide propagation is a ubiquitous topic in ocean acoustics. This presentation focuses on low-frequency (f < 500 Hz) sound propagation in coastal oceans (water depth shallower than 500 m). These environments are highly dynamical and complex, and act as dispersive waveguides, bounded by the sea-surface and the seafloor. The underwater acoustic field can thus be described by a set of modes that propagate with frequency-dependent speeds. Propagation of transient acoustic signals to a distant receiver (range > 1 km) accrues multi-modal dispersion information about the propagation medium. To extract relevant information about the oceanic environment from the acoustic recording, physics-based processing methods must be used. In this presentation, I will briefly review modal propagation and time-frequency analysis. I will then show how these approaches can be combined into a non-linear signal processing method dedicated to extracting modal information from a single receiver: this information is the foundation of a transdimensional inversion method used to characterize the oceanic environment. This method will be used to perform geoacoustic inversion on the New England Mud Patch. Using several datasets collected under different oceanographic conditions, I will notably demonstrate a successful estimation of the seafloor properties, consistent with geophysical core measurements and other inversion studies, even when the water column is dynamical and mostly unknown.

A spectrally discretized wide-angle parabolic equation model for simulating acoustic propagation in laterally inhomogeneous oceans.

Sound waves can be used to carry out underwater activities. Rapidly and accurately simulating sound propagation is the basis for underwater detection. The wide-angle parabolic model has a good computational speed and accuracy and is currently the main numerical model for mid- and low-frequency sound propagation. The classical wide-angle parabolic equation model is discretized by the finite difference method and a low-order difference scheme is generally adopted. In this paper, a wide-angle parabolic equation model based on a spectral method is proposed. The depth operators of each layer are discretized via the Chebyshev spectral method and then assembled into a global matrix for the forward step. Lateral inhomogeneity is addressed by updating the global depth matrix while stepping forward. In the proposed spectral algorithm, both soft and hard seabeds can be accurately simulated by imposing boundary conditions, and the perfectly matched layer technique is used to truncate the unbounded acoustic half-space. Several representative numerical experiments prove the accuracy and efficiency of the proposed algorithm. However, the spectral method requires that the thickness of the layers does not change during the forward step. Thus, the current spectral algorithm cannot simulate waveguides with terrain undulation, which is its main limitation.

Acoustic characterization of the seabed with a single-element time-reversal mirror

Motivated by the 2012 Florida Straits Noise Interferometry Experiment, this paper investigates a time-reversal-based approach to acoustic remote sensing of the ocean. The signal processing mimics operation of a physical time-reversal mirror. The input data for the simulated time-reversal mirror can be obtained using either a compact, broadband sound source or cross-correlation of the diffuse noise recorded by spatially separated receivers. Low-frequency sound propagation is considered over ranges that are large compared to the water depth. The approach exploits the notion that the “best” focusing of the backpropagated time-reversed signal occurs at the “right point,” when the backpropagation takes place in the same propagation medium as the one, where the data have been acquired. Various metrics of the focusing quality are considered. A combination of spatial and temporal characteristics of the focus is proposed that leads to a robust and unique solution of the geoacoustic inverse problems considered for a single-element passive time-reversal mirror. Inputs with rather low signal-to-noise ratio prove acceptable, which is particularly important in the passive remote sensing context.

Spectral analysis of acoustic plane waves in ductworks with uniform and varying cross-section waveguides

The problem of low-frequency sound propagation in ducts involving varying cross-sections is analyzed. Spectral finite element formulations for the dynamic analysis of varying cross-section ducts are presented as tapered spectral elements. These elements can be used in connected waveguides through dynamic stiffness relations. Linear and polynomial duct types are considered. The elements’ shape functions are established as well as the dynamic stiffness matrices describing the spectral relation between the acoustic pressure and the volume velocity. The presented spectral approach describes the acoustic wave motion at any position along arbitrary multiply connected duct waveguides within the one-dimensional plane-wave assumption. The derived expressions are verified experimentally, and by comparison with 3-D finite element solutions. The modeling of varying cross-section duct waveguides can play an important role in the design of acoustic excitation devices and horn-type loudspeakers. The advantage of the presented formulations lies in the low computational cost when compared to the finite element approach.

Effects of non-ideal seabed topography on low frequency sound propagation in shallow sea

In order to study the influence of seabed topography on sound propagation under non-ideal conditions, a simulation model of low frequency sound propagation of whole waveguide is established by using finite element method (FEM). The action mechanism of the topography with seamounts on the components of the seismic wave field is studied in time domain. The simulation results show that the propagation of various seismic wave components can be seen intuitively from the time domain. Scholte waves can be excited easily in a suitable range from the seabed, and can propagate along the seabed for a long distance with small attenuation and strong stability. The presence of a seamount allows Scholte waves to scatter, and Scholte waves can travel along the surface of the seamount to the back of the seamount. The path and energy intensity of sound wave change with the change of terrain.

Deep-sea explosion sound signal propagation loss measurement and model verification

This paper researched the low-frequency sound field propagation characteristics in the continental slope sea area, the sound field propagation loss is calculated by using the radiated sound field data of the deep-fixed explosive sound sources with 50 m and 200 m depth in the continental slope sea area, and the experimental results are verified by combining with the Kraken normal wave model. The model verification results are in good agreement with the experimental measurement data.

Investigation on low-frequency sound propagation characteristics of fluid-filled steel pipeline

With the development of vibration reduction and noise reduction technology, the propeller noise, hydrodynamic noise and equipment base noise in submarines have been effectively suppressed. The sea pipeline system contains mechanical noise and fluid pulsation noise, which is one of the main noise sources of submarines. The fluid-filled steel pipeline is the basic component of the sea pipeline system. It is of great significance to study the low-frequency sound propagation characteristics of the fluid-filled steel pipeline system to effectively suppress the low-frequency noise of the pipeline system to the sea and achieve quiet submarines. At present, there are few studies on the sound propagation of fluid-filled steel pipeline, and most of them refer to gas-filled pipes. It is believed that there are low-frequency plane waves in the pipeline, which leads to the design of muffling devices only for liquids, the role of the pipe wall in the propagation of low-frequency sound is ignored. In this paper, according to the relationship between normal waves and medium wave velocity, a sound propagation model of fluid-filled steel pipeline is proposed, which is verified by experimental research. Through this model, the propagation path of low-frequency sound in the fluid-filled steel pipeline is clarified, which has an important guiding role in the vibration and noise reduction of the fluid-filled pipeline system.

Modal structure of the sound field in a shallow-water waveguide with a gassy sediment layer: Experiment and theory

Motivated by a series of experiments in the Lake Kinneret (Israel), the paper investigates low-frequency sound propagation in the three-layer model of the shallow water waveguide, where a thin layer of gas-saturated sediment is situated between a homogeneous fluid sub-bottom and a continuously stratified water column. Typical values of the thickness and sound speed in the gassy layer are 1 m and 250 m/s. The layer is thin compared to typical water depth of about 40 m. Normal mode structure of the acoustic field is analyzed. It is shown that with increasing frequency each normal mode transforms into a mode trapped in the gassy layer. These modes have unusually small phase and group speeds that are significantly less than the sound speed in water. Theoretically predicted normal mode dispersion curves are compared to the dispersion curves retrieved from observations of shipping noise. Phase speeds of normal modes are measured by cross-correlatingthe noise recorded on two sparse vertical arrays. A technique is proposed for solving the inverse problem of determining the parameters of a gas-saturated layer by matching the measured and modeled frequency dependencies of the normal mode phase speed.

Low-Frequency Sound Propagation in an Underwater Waveguide with a Giant Gassy Pockmark

Bottom formations known as pockmarks basically arise due to extensive gas emission. Active pockmarks are characterized by exceptionally high gas saturation and substantially reduced sound speed. The latter circumstance leads to strong attenuation of sound waves contacting with a pockmark. In the present paper, we study low-frequency sound propagation in a 10-km long waveguide crossing a giant pockmark. A new method of acoustic waveguide scanning based on measurement of the wavefield propagator is represented. This method allows one to explore attenuation anomalies associated with the presence of the gas-saturated bottom region. In particular, one can find out which beams fall into a pockmark area and therefore experience strong losses. Identifying such beams, as well as beams which avoid pockmark-assisted losses, one can estimate probable locations of the pockmark segment in the waveguide, provided information about the background medium is sufficient.

The coordinated arctic acoustic thermometry experiment—CAATEX

The coordinated arctic acoustic thermometry experiment (CAATEX) was a joint U.S.-Norwegian trans-Arctic acoustic propagation experiment with a design comparable to the 1994 TransArctic Propagation (TAP) experiment. The goal was to measure the changes in low-frequency sound propagation due to changes in ocean heat content and salinity, and ice conditions. Two 35 Hz acoustic transceiver moorings, one in the Nansen Basin and one in the Beaufort Sea, were deployed along with four other receiving moorings. All six moorings were equipped with vertical hydrophone arrays, recording transmissions every 36 h from fall 2019 to fall 2020. Each mooring also recorded temperature and salinity time series along the vertical extent of the hydrophone arrays, ice thickness using an upward-looking sonar, and ocean bottom pressure. The acoustic travel-times allow comparison of present-day heat content to the 1994 measurement, but there are several other points of comparison that are sensitive to environmental changes. These include transmission loss, acoustic scattering, and the acoustic arrival structure. These measurements of ocean and ice processes, and the acoustic propagation and ambient sound will improve our ability to monitor, communicate, and navigate in the Arctic Ocean.

Modelling of sound propagation across the Arctic Ocean using oceanographic fields and oceanographic data

During the coordinated arctic acoustic thermometry experiment (CAATEX), two acoustic sources transmitted 35 Hz binary m-sequences in a yearlong experiment. The signals were transmitted across the Arctic Ocean and detected at distances up to 2700 km. The aim was to make measurements similar to those carried out in 1994 and 1998, to investigate the changes in sound propagation due to changes in the ocean temperature, ocean stratification, and thinning of the sea ice. Changes in propagation conditions over the last decades were investigated using time-series of data from ocean reanalysis, as well as available oceanographic data along the transect. Range-dependent fields were constructed and used as environmental input to acoustic models to produce time series of arrival times and vertical arrival structure. This approach allows for an examination of the sensitivity of low-frequency sound propagation to both vertical and large-scale horizontal changes in the ice-ocean environment. The predicted time series of acoustic travel times and arrival structure will be presented and compared with the observations.

Three-dimensional coupled-mode model and characteristics of low-frequency sound propagation in ocean waveguide with seamount topography

Large-scale topography, such as a seamount, substantially impacts low-frequency sound propagation in an ocean waveguide, limiting the application of low-frequency acoustic detecting techniques. A three-dimensional (3D) coupled-mode model is developed to calculate the acoustic field in an ocean waveguide with seamount topography and analyze the 3D effect. In this model, a correction is introduced in the bottom boundary, theoretically making the acoustic field satisfy the energy conservation. Furthermore, a large azimuth angle calculation range is obtained by using the operator theory and higher-order Padé approximation. Additionally, the model has advantages related to the coupling mode and parabolic equation theory. The couplings corresponding to the effects of range-dependent environment are fully considered, and the numerical implementation is kept feasible. After verifying the accuracy and reliability of the model, low-frequency sound propagation characteristics in the seamount environment are analyzed. The results indicate lateral variability in bathymetry can lead to out-of-plane effects such as the horizontal refraction phenomenon, while the coupling effect tends to restore the abnormal sound field and produces acoustic field diffraction behind the seamount. This model effectively considers the effects of the horizontal refraction and coupling, which are proportional to the scale of the seamount.

Passive time-reversal mirror for acoustic characterization of the seabed

This paper investigates application of a signal processing technique, which is motivated by time reversal, to geoacoustic inversions in a shallow-water waveguide. The signal processing mimics operation of a physical time-reversal mirror. The input data for the simulated time-reversal mirror can be obtained using either a compact, broadband sound source or cross-correlation of the diffuse noise recorded by spatially separated receivers. Low-frequency sound propagation is considered over ranges that are large compared to the water depth. The approach exploits the notion that the “best” focusing of the backpropagated time-reversed signal occurs at the “right point,” when the backpropagation takes place in the same propagation medium as the one, where the data have been acquired. Various metrics of the focusing quality are considered. A combination of spatial and temporal characteristics of the focus is proposed that leads to a unique solution of the geoacoustic inverse problem for a single-element passive time-reversal mirror. [Work supported by NSF.]

Geo-acoustic inversion using polynomial chaos expansion

In order to improve the computational efficiency of algorithms while exploring the method to overcome the ambiguity problems in underwater geo-acoustic inversion, we use the data of transmission losses at the broadband sound frequencies and multiple propagating distances with the matrix of polynomial chaos expansion coefficients of transmission losses to invert the speed (<i>c</i>), attenuation (<i>α</i>) of compression sound wave and the density ratio of seabed to seawater (<i>ρ</i>) in their prior searching intervals. When approximating the transmission loss with the polynomial chaos expansion, the expansion coefficients are the functions of parameters including sound frequency, source and hydrophone’s position while the polynomial bases are functions of the above geo-acoustic parameters which are uniformly distributed in their respective intervals. The expansion coefficients are calculated by embedding the orthogonal polynomial bases into the acoustic wide-angle parabolic equation model. After that, the coefficients are deduced using the Galerkin projection and least angel regression. Under the situations of low sound frequency, short or medium sound propagation distance and short or medium length of intervals of geo-acoustic parameters, the polynomial chaos expansion can approximate the transmission losses accurately with the relatively error less than 1%. In the simulation case, with the high signal to noise ratio and the low errors of relative distances between source and receivers, the geo-acoustic parameters can be inverted accurately when the appropriate truncated powers are chosen. And the time cost is reduced by at least an order of magnitude compared with that of traversal grids searching procedure.

Influence of Mesoscale Vortex on Underwater Low-Frequency Sound Propagation

To discuss the effect of an ocean mesoscale vortex on underwater low-frequency sound propagation, a theoretical model of a sound field under the action of an ocean mesoscale vortex was established based on acoustic wave theory and ray theory, using the finite element method. The influences of vortex properties and vortex intensity on the characteristics of sound propagation are discussed. The simulation results indicate that, in a cold vortex, the acoustic convergence zone moves toward the sound source, the span of the convergence zone decreases, and the convergence gain increases. Alternatively, for a warm vortex, the convergence zone moves away from the sound source, the span of the convergence zone increases, the sound rays diffuse, and the convergence gain decreases; the greater the vortex intensity, the greater is the effect on the motion and gain of the convergence region.

Characteristics of Very Low Frequency Sound Propagation in Full Waveguides of Shallow Water.

This work is concerned with the characteristics of very low frequency sound propagation (VLF, ≤100 Hz) in the shallow marine environment. Under these conditions, the classical hypothesis of considering the sea bottom as a fluid environment is no longer appropriate, and the sound propagation characteristics at the sea bottom should be also considered. Hence, based on the finite element method (FEM), and setting the sea bottom as an elastic medium, a proposed model which unifies the sea water and sea bottom is established, and the propagation characteristics in full waveguides of shallow water can be synchronously discussed. Using this model, the effects of the sea bottom topography and the various geoacoustic parameters on VLF sound propagation and its corresponding mechanisms are investigated through numerical examples and acoustic theory. The simulation results demonstrate the adaptability of the proposed model to complex shallow water waveguides and the accuracy of the calculated acoustic field. For the sea bottom topography, the greater the inclination angle of an up-sloping sea bottom, the stronger the leak of acoustic energy to the sea bottom, and the more rapid the attenuation of the acoustic energy in sea water. The effect of a down-sloping sea bottom on acoustic energy is the opposite. Moreover, the greater the pressure wave (P-wave) speed in the sea bottom, the more acoustic energy remains in the water rather than leaking into the bottom; the influence laws of the density and the shear wave (S-wave) speed in the sea bottom are opposite.

Sound Field Modeling with Bottom Reflective Parameters (P,Q) Based on Parabolic Equation

Acoustical properties of the sea bottom can be described using geoacoustic (GA) models. Most existing propagation models use GA parameters as the bottom properties. It is difficult to obtain GA parameters for a layered bottom because of inter parameter coupling. These problems can be solved by inverting the model-independent reflective parameters P and Q. For a multilayered bottom, a sound field computation model, RamPQ, is developed using the mapping of GA and (P, Q) spaces. The mean square error of the transmission loss in numerical simulations and experimental data for low-frequency sound propagation are employed to validate RamPQ and demonstrate the performance of the model.

SOFTWARE DEVELOPMENT FOR ESTIMATION OF LOW FREQUENCY SOUND PROPAGATION IN GAS GUIDES OF POWER PLANTS TAKING TO ACCOUNT ACTIVE SOUND SOURCES

This paper is devoted to the problems of modelling and calculation of propagation of low frequency sound in gas guides of power plants taking to account active sound sources. The structure of software for prediction and calculation of low-frequency sound propagation in gas guides have described. Software uses four-pole method and takes to account radiation from additional (active) sound course. By using software it is possible to estimate sound source parameters to provide efficient sound attenuation. Examples of software application to calculation of intake and exhaust noise of internal combustion engine are described. The results of calculations show the possibilities of four-pole method software using to design acoustically the parameters of gas guides and mufflers for the different fields of applications.