Acoustical Physics最新文献

Chuprov’s interference invariant (II) well describes the properties of a sound field in shallow water. However, the question of how applicable Chuprov’s II concept is to deep water, where the patterns of sound field decay with distance are more complex has been insufficiently studied. Therefore, the authors studied the II properties in the near and far fields of acoustic illumination, as well as in the shadow zone. A new definition of the invariant was proposed and studied, and its characteristics were compared with Chuprov’s II as a function of distance, reception and emission depths, and summer or winter propagation conditions. The new invariant is called the phase-energy invariant (PEI), since orthogonal components of the phase gradient are used to describe the spatial sound energy distribution. The stability of the new invariant, its independence on different influencing factors, and its natural change with distance from zero to one are shown. It has been established that in winter conditions, at almost all distances, the PEI is equal to unity, and the II does not have stable values and varies jumpwise over a very wide range. In summer conditions, in the shadow zone, with increasing distance, the PEI increases, just like the II, from close to zero to one. In the near and far fields of acoustic illumination, the PEI is approximately equal to unity, and the II in these zones, both in summer and winter, is characterized by unlimited oscillations, caused by division by a value close to zero. It is shown that the definition of PEI is valid both in single-mode waveguides and in free unbounded space with a dispersive medium.

Abstract

The inverse problem of acoustic sounding of a three-dimensional nonstationary medium is considered, based on the Cauchy problem for the wave equation with a sound speed coefficient depending on the spatial coordinates and time. The data in the inverse problem are measurements of time-dependent acoustic pressure in some spatial domain. Using these data, it is necessary to determine the positions of local acoustic inhomogeneities (spatial sound speed distributions), which change over time. A special idealized sounding model is used, in which, in particular, it is assumed that the spatial sound speed distribution changes little in the interval between source time pulses. With such a model, the inverse problem is reduced to solving three-dimensional Fredholm linear integral equations for each sounding time interval. Using these solutions, the spatial sound speed distributions are calculated in each sounding time interval. When a special (plane-layer) geometric scheme for the location of the observation and sounding domains is included in the sounding scheme, the inverse problem can be reduced to solving systems of one-dimensional linear Fredholm integral equations, which are solved by well-known methods for regularizing ill-posed problems. This makes it possible to solve the three-dimensional inverse problem of determining the nonstationary sound speed distribution in the sounded medium on a personal computer of average performance for fairly detailed spatial grids in a few minutes. The efficiency of the corresponding algorithm for solving a three-dimensional nonstationary inverse sounding problem in the case of moving local acoustic inhomogeneities is illustrated by solving a number of model problems.

The information content of the parameters of a spectral voice source model in an automatic voice identity recognition problem is studied. For the voice parameters, the identity recognition error was 20.8%; using these parameters together with the pitch period reduced the error to 13.8%. Lastly, the combined use of the spectral model parameters with the pitch period and mel-frequency cepstral coefficients provided the highest accuracy (the recognition error was 1.2%).

The low-frequency (74 kHz) shear elasticity of the homologous series of normal hydrocarbons (alkanes) is studied using acoustic resonance. The shear modulus and mechanical loss tangent are measured, and the relaxation frequency and effective viscosity are calculated. The dependences of these parameters on homologue viscosity are established. It is shown that the mechanical loss tangent of all studied liquids is less than 1, demonstrating that the relaxation frequency is below the experimental frequency.

The acoustic characteristics of barriers with a T-shaped profile are studied by finite element simulation. It has been found that the sound reduction efficiency of this barrier is related not only to diffraction, but also to sound interference at the leading and trailing edges of the barrier. It is shown that the sound interference at the trailing edge of the barrier, in contrast to the sound interference at the leading edge, affects the sound field only at short distances from the rear surface of the barrier. The influence of the sound frequency and geometrical dimensions of the barrier on these processes is analyzed.

Abstract

The acoustic properties of suspensions based on pure glycerol and diamond powder with a particle size of 1–2 μm and different concentrations were studied using a resonator with a longitudinal electric field. A disk resonator made of langasite with round electrodes on both sides of the plate with a frequency of 4.1 MHz, operating on a longitudinal acoustic wave, was completely immersed in a liquid container with the studied suspension. Based on the measured frequency dependences of the real and imaginary parts of the electric impedance of the resonator using an equivalent electromechanical circuit, the longitudinal elastic modulus and longitudinal viscosity coefficient of the samples were determined. Comparison of the experimental dependences of the longitudinal elastic modulus, viscosity coefficient, and longitudinal acoustic wave velocity on the volume concentration of diamond particles in the suspension with the calculated dependences demonstrated good agreement.

A method is proposed for solving the inverse problem of reconstructing acoustic wave sources from field measurements on some surface using wavefront reversal in the particle dynamics method. In this method, the studied medium is represented as a set of interacting particles (material points or solid bodies), for which classical equations of motion are written. The paper considers the representation of a medium as a set of particles in a body-centered cubic crystal lattice. The case of a linear dependence of the force of attraction of particles on distance is considered. The advantage of this approach is the ability to take into account wave propagation in arbitrarily inhomogeneous media using a single numerical model. The possibility of visualizing two spherical acoustic wave sources in water behind an obstacle has been demonstrated numerically and experimentally, despite the presence of transverse waves in the considered model of a solid body; their influence is negligible in this case. The method was tested experimentally on a soundproof screen with an aperture simulating a sound-emitting object of complex shape. A wave from a point source of short pulses passes through the aperture. Using a receiving acoustic sensor mounted on a two-dimensional scanner, the spatiotemporal distribution of sound vibrations on the water surface was measured. By processing the data using wavefront reversal in the particle model, the image of the aperture in the soundproof screen was reconstructed.

Abstract

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%.

Abstract

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.

Abstract

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.