An elastic medium with inclusions that are small compared to the sound wavelength and differ in density is considered. If the inclusions are resonators that respond equally to the influence of waves coming from different directions, then the effective density of the medium in a certain frequency band becomes negative. If the direction of the dipole moment of the resonators is fixed, then the medium with inclusions has an anisotropic effective density. The Helmholtz equation for such a medium was obtained, and the field of a point source was studied.

This paper presents a method for rapidly reverse designing the absorption performance of acoustic coatings, utilizing the principles of a concatenated deep neural network. It enables the swift acquisition of effective input parameters. By cascading a reverse neural network with pre-trained forward neural networks, a concatenated neural network is obtained. This network maps the absorption spectrum response to structural and material parameters, thereby resolving the nonuniqueness issue in traditional reverse design. The paper describes the detailed process of reverse designing the absorption performance of acoustic coatings and validates the correctness of the reverse design using finite element methods. A comparative analysis investigates the impact of different loss functions on result accuracy. The findings demonstrate that the proposed modified loss function algorithm significantly enhances precision compared to traditional direct reverse design. This advancement allows for the customization of acoustic coatings with specific acoustic properties, providing technical groundwork for vibration and noise reduction in underwater vehicles.

The pulse width of multibubble sonoluminescence flashes in an aqueous NaCl solution was measured by a correlation method for the spectral range of 300–800 nm. The flash pulse width had a maximum value of 21 ns in the spectral region adjacent immediately to the Na D-line peak (589 nm) and decreased to 2 ns with distance from the line peak. The measured dependence of the flash pulse width on the wavelength agreed with the dynamic Na line shape model proposed by us earlier, where the spectral line width and shift were governed by a fast change in the emitting medium density during bubble collapse. Using the correlation method, the sequence of metal and continuum flashes was determined to measure the relative delay between them. The results showed that Na emission takes a longer time as compared to continuum emission and occurred almost symmetrically in time around a continuum flash with a vanishingly small delay of 0.21 ns after the continuum flash. Using the same method for a CeCl3 solution, a cerium line flash (350 nm) was revealed to occur after a continuum flash with a delay of 31 ns close to the Ce emission lifetime of 33 ns to be indicative of essential distinction between the mechanisms of Na and Ce emission under multibubble sonoluminescence.

We consider the problem of estimating the position of a local inhomogeneity on a stationary acoustic path between a single sound source and vertical receiving array in a shallow water waveguide in the presence of background disturbances. A local bottom rise and a soliton-like internal wave are chosen as model inhomogeneities. It is proposed to determine the distance from the source to the inhomogeneity by cepstral analysis of the amplitude of the first waveguide mode isolated on the array, with preliminary deformation of the frequency axis. Numerical modeling is used to study the stability of this approach in the presence of several local inhomogeneities or additional disturbances: bottom slope, background internal waves, wind waves, and bottom irregularities. Estimates of the signal-to-noise ratio required to implement the approach are pro-vided.

A numerical model for investigating the aerodynamic sound generated by the interaction between flow and bluff body is developed, and then applied to the computation of near-field noise induced by the turbulent airflow passing through a generic side mirror. The flow field is simulated by employing the viscous vortex method. Then the sound sources within the computational domain are extracted from the simulated results with a vortex sound equation. The sound waves, sum of radiated sound and scattered sound, are determined using a time-domain boundary element method combined with the convolution quadrature method for improving the stability of the time marching algorithm. Further, the fast multipole method is adopted to enhance the computational efficiency. The computed pressure coefficients and surface pressure fluctuations match the measurements and simulations very well, and the obtained spectra of near-field aerodynamic sound are also close to the measured results. The comparisons of computed results of two additional cases with the previous studies demonstrate convincingly that the proposed model can effectively predict the flow-induced near-field noise.

An experimental verification of the possibility of estimating the coordinates of a mobile marine robot using spatially offset small-sized vector–scalar arrays, a broadband source placed on board the robot, and a remote polyharmonic permanently installed source—a beacon, which is used to eliminate bearing shifts caused by rotating receiving arrays under the action of underwater currents. It is shown that the use of technical means installed in the waveguide makes it possible to solve the triangulation problem and determine the horizontal coordinates of the robot, while taking into account the ray structure provides a depth estimate.

The study considers the possibility of creating a broadband sound-absorbing coating for hydroacoustic measuring tanks and chambers with inertial or soundproof walls, consisting of a layer of viscous liquid with gas bubbles. The coatings are calculated using the well-known theory of sound propagation in a liquid medium with bubbles, as well as the Kramers–Kronig integral dispersion equations. It is shown that the volumetric size distribution function of bubbles should be constant over the entire range of their sizes. A viscous liquid is designed to increase bubble damping to a value on the order of unity that is optimal for coatings by adding viscous losses in the surrounding liquid to small thermal losses. Low-frequency compensating resonators are used in coatings for soundproof walls. Several examples of calculating the acoustic characteristics of coatings are given.

A finite element method is presented for calculating hydrodynamic noise excited by turbulent fluid fluctuations in the presence of an elastic body. The conventional approach to solving this problem by direct solution of the Lighthill equation requires a large amount of calculations. It is demonstrated that the situation is considerably simplified when noise components are calculated at relatively low frequencies, which correspond to wavelengths that exceed the dimensions of the turbulent zone. In this case, the noise field can be expressed in terms of turbulent fluctuations in pressure on the surface of an elastic body, which is found in the incompressible fluid approximation. The article is based on a report presented at the IX Russian Conference “Computational Experiment in Aeroacoustics and Aerodynamics,” Svetlogorsk, September 26–October 1, 2022.

This work introduces a method of one-dimensional deconvolution with Tikhonov regularization for enhancing three-dimensional optoacoustic images in vivo. The method employs adaptive self-calibration to eliminate frequency-dependent distortions associated with ultrasound propagation and detection. By adapting to the inhomogeneous frequency characteristics of the examined medium, the method eliminates the need for additional calibration experiments. The processing time for three-dimensional optoacoustic data of size 200 × 200 × 100 voxels is less than 5 ms, facilitating the real-time enhancement of angiographic images and improving the effective spatial resolution by more than 50%.

The numerical method is proposed for predicting the far-field noise using Ffowcs Williams–Hawkings (FW–H) equation with high-order finite-difference method for the time derivative. The results of this method for second-, fourth-, and sixth-order finite difference approximations are compared with that of analytic applications, such as monopole and dipole. It is observed that the use of the high-order time derivatives is an efficient approach to improve the prediction accuracy of the radiated acoustic pressure, particularly when the temporal resolution is not sufficiently high owing to the limited time step size. Our findings in this study provide evidence that for higher-order approximations, the RMS error for the first and second derivatives is smaller. In addition, the RMS error for the sixth-order approximation decreases considerably compared to that for the second-order approximation, with an increase in the number of points per period. This study and its results are expected to serve as a guide for noise prediction, indicating the temporal accuracies of the acoustic analogy according to the high-order approximation of time derivatives.