Journal of Theoretical and Computational Acoustics

This paper discusses the value added by using a single vector sensor over a conventional pressure-only hydrophone for geoacoustic inversions. Inversion methods based on genetic algorithms are used to estimate the seabed properties. Synthetic signals of impulsive arrivals first are modeled using KRAKEN and RAM propagation models, each being modified to predict components of the vector field. While KRAKEN is utilized to directly compute dispersion curves, RAM provides full-field results that require the application of time warping to separate the modal arrivals. Combinations of dispersion curves utilizing all vector sensor channels are compared to curves estimated with the pressure-only channel. Within the time warping analysis, both binary masking and band-pass filter masking methods are applied to compare stability of results. The environment modeled for the synthetic analysis and inversion method utilize sound speed profiles measured during the Monterey Bay 2019 at-sea experiment and assume a sediment layer of constant thickness overlying a deeper sub-bottom type. White noise is added to the synthetic data at different signal-to-noise ratios to evaluate the impact of signal excess on the results. A hybrid optimization approach is used to improve the results of the genetic algorithm method. The analysis with synthetic data is consistent with the analysis of broadband, impulsive data collected from the experiment, indicating that the additional information from the vertical velocity channel further improves the geoacoustic parameter estimates.

The sound field of a harmonically radiating monopole moving along arbitrary trajectories in three-dimensional space is studied in the frequency range and represented as a convolution integral in Cartesian coordinates. The observation time of the motion of the source can be finite or infinite. This convolution integral is referred to as the “Cartesian Convolution Integral ” and is applied to point sources moving on special orbits described by circular and conical helices. For such helical orbits, an alternative approach in cylindrical or spherical coordinates leads to closed form expressions in the form of infinite series for the frequency spectra. The comparison of the results of the Cartesian Convolution Integral with those of the series expansion shows a very good agreement. Finally, the Cartesian Convolution Integral is used to calculate the spectral sound field of a source moving along an elliptical helix.

Acoustic metamaterials have emerged as alternative solutions to achieve useful physical effects that differ from the ones obtained with traditional materials. In terms of sound absorption, previous works have addressed their potential as compact surfaces with high performance. Nevertheless, studies on their angle-dependent behavior are scarce. In this work, an analytic model and a numerical model to estimate the performance of periodic surfaces with unit cells composed of 2D Helmholtz resonators are presented. By making use of these modeling tools, the absorption of surfaces with one and three different resonators is studied as a function of both incidence angle and frequency. Changes in the incidence angle can cause variation of the maximum absorption coefficient, the frequencies at which the maximum performance is observed, and the frequency range of significant absorption. Furthermore, the rate at which the performance changes as a function of the incidence angle is larger as the angle increases. Given the angle dependency of these absorbers, a strategy to optimize the dimensions of the surface elements to maximize the absorption performance for predefined ranges of incidence angles and frequencies is presented.

A semi-analytical approach is presented to solve three-dimensional dynamic Green’s function for an infinitely extended acoustic field with multiple spheres subjected to the Robin boundary conditions. The multipole expansions of the acoustic field induced by a time-harmonic point source are expanded with spherical wave functions. As an alternative to the complex addition theorem, the multipole expansion is computed in a straightforward way. By taking the finite number of terms, an algebraic system is constructed and is used to obtain Green’s function. This result of one sphere agrees with the available analytical solution. For the case of more than one sphere, the proposed results are verified by the numerical method such as the boundary element method (BEM). It indicates that the present solution is more accurate than that of the BEM and shows a fast convergence. Finally, the parameter study is performed to explore the influences of the exciting frequency of the point source, the surface admittance, the number and the separation of spheres on the dynamic Green’s functions. The proposed results can be applied to solve the acoustic scattering problems and to increase the application of boundary integral equation method in the way of numerical Green’s functions.

Sensitivity analysis is a powerful tool for analyzing multi-parameter models. For example, the Fisher information matrix (FIM) and the Cramér–Rao bound (CRB) involve derivatives of a forward model with respect to parameters. However, these derivatives are difficult to estimate in ocean acoustic models. This work presents a frequency-agnostic methodology for accurately estimating numerical derivatives using physics-based parameter preconditioning and Richardson extrapolation. The methodology is validated on a case study of transmission loss in the 50–400Hz band from a range-independent normal mode model for parameters of the sediment. Results demonstrate the utility of this methodology for obtaining Cramér–Rao bound (CRB) related to both model sensitivities and parameter uncertainties, which reveal parameter correlation in the model. This methodology is a general tool that can inform model selection and experimental design for inverse problems in different applications.

In this paper, an elastic seafloor with the sinusoidal interface is modeled to study the effect of the fluctuant interface on Scholte wave excitation. The reflection and transmission coefficients of sound waves are derived from the scattered wave theory to investigate the acoustic field characteristics. In numerical simulations, longitudinally invariant finite element models are constructed to demonstrate the excitation of Scholte waves at the seafloor. The effect of sinusoidal interface parameters on Scholte wave excitation is analyzed in this research. Results show that stronger Scholte waves still can be excited when the sound source is away from the seafloor with a sinusoidal interface. In this process of Scholte wave excitation, the period $\mathrm{\Lambda }$ and height $h$ of sinusoidal interfaces are related to the excitation frequency and intensity, respectively.