Vol 65, No 1 (2019)

Acoustic metamaterials have become a novel and effective way to control sound waves and design acoustic devices. In this study, we design a 3D acoustic metamaterial lens (AML) to achieve point-to-point acoustic communication in air: any acoustic source (a speaker) in air enclosed by such an AML can produce an acoustic image where the acoustic wave is focused (the field intensity is at a maximum, and the listener can receive the information), while the acoustic field at other spatial positions is low enough that listeners can hear almost nothing. Unlike a conventional elliptical reflective mirror, the acoustic source can be moved around inside our proposed AML. Numerical simulations are given to verify the performance of the proposed AML.

Studying the interaction of sound with a coated cylindrical shell immersed in water is essential for improving existing underwater target detection and classification algorithms. According to the impedance theory of sound scattering, in vacuo structural admittance describes the relationship between the sonar-induced forces and the resulting vibration on the surface, which can be used to solve the problem of the acoustic scattering and radiation. In this work, we investigate numerically and theoretically the structural admittance of a coated cylindrical shell. Analytical expressions of the structural admittance are derived for different external forces: a plane acoustic wave, a normal point force, and a random noise field. The structural admittance is also numerically evaluated. The results show that the structural admittance is independent of exterior medium and fluid loading. According to the impedance theory of sound scattering, the scattered field of a coated cylindrical shell is calculated by combining the structural-, acoustic-, and internal-admittance matrices. Because of the non-local property of structural surface admittance, we build an algebraic model of a coated object by nonlinear curve fitting and study a local approximation of the structural admittance. We also find that simplifying the large matrices is useful for research on structural vibrations. Thus, this work presents a systematic study of the acoustic scattering characteristics of structural admittance of an infinite, coated cylindrical shell.

Processes that accompany propagation of time-limited pulsed signals in a relaxing medium are investigated for the case of a nonlinear medium with power-law (quadratic or cubic) nonlinearity or nonanalytic nonlinearity (modular or quadratically cubic one). Instead of ordinary integro-differential equations with exponential or fractional-power kernels, a simplified model of a medium with finite “memory time” is used. Such a medium “remembers” its prehistory within a limited time interval, and the corresponding kernel of the integral term is nonzero only within a finite interval. For this model, the problem is reduced to solving a difference-differential equation, which considerably reduces the amount of calculations, as compared to the initial integral equation. The processes that accompany evolution of pulses, namely, the formation of compression and rarefaction shock fronts and the appearance of triangular and trapezoidal nonlinear structures, are described. Effect of relaxation time on these processes is revealed.

We study signal-accumulation and noise-suppression methods and algorithms that improve the accuracy and stability of reconstructing the geoacoustic parameters in layer-by-layer reconstruction of bottom layers and use parametric models of the formation of signals reflected from an elastic layered half-space under coherent pulsed probing of the sea shelf bottom. We present the results of analyzing the efficiency of the algorithms by stochastic numerical simulation.

The paper describes the results of an experiment on studying long-range surface reverberation in the deep-water part of the Black Sea. The experiment employed tone pulses with a filling frequency of 2 kHz and a duration sufficient for narrowband spectral analysis. The source and receiver of acoustic signals were spatially offset in the water area; therefore, directional reception and time strobing made it possible to study different situations: forward scattering, bistatic scattering, and monostatic (back) scattering. The paper analyzes the Doppler scattering spectra for different values of the bistatic scattering angle.

The paper presents the results of an experiment that uses the MLS technique to measure the impulse response when a signal passes through an air jet. The data are compared with the numerical simulation results obtained by solving the equation for sound propagation in a steady flow using a finite-difference method. It is shown that the longitudinal component of the flow causes drift and focusing of the signal.

The paper presents the results of comparative experimental studies on the effect of a significant difference in the radiation patterns of individual acoustic modes from an open duct in steady-state and in-flight modes in the AC-2 anechoic chamber. When there is an incoming flow, the directivity changes and the amplitude of the maximum of individual modes may vary severalfold. This effect was originally discovered by numerical simulation, and the need to experimentally confirm this difference in the characteristics of individual modes set the task of developing a new technique for projection of the results of acoustic bench tests of aircraft engines to in-flight conditions within an airplane.

A photoacoustic spectrometer based on a three-colored light-emitting diode (with peak emission wavelengths of 465, 525, and 640 nm) designed to determine the intensity of photosynthesis in different deep layers of plant leaves is described. The physical properties of the photoacoustic signal were studied at different wavelengths and light modulation frequencies. It was shown that the proposed spectrometer can be employed for quantitative evaluation of heat dissipation and photochemical assimilation of the absorbed light energy in this medium.

The review analyzes the research and development associated with dolphin and fish acoustics. The main focus is on the structure of echolocation and communication sounds of dolphins under conditions of intense noise impact. Sounds were recorded and emitted directly on the head of a dolphin at the hypothetical input points for acoustic information. The study also involved sounds within the nasal passage in order to more accurately localize emission sources. Fish sounds were recorded in the tank in which they were kept. The analysis showed that in order to raise their noise immunity, dolphins can intentionally change the structure of emitted sound signals when impacted by extremely intense noise. Hypotheses on the fish sound emission mechanism are proffered.

The improved generalized sidelobe canceller (GSC) based on eigenvalue decomposition beamforming technique for ultrasound imaging is proposed. Firstly, the signal subspace is obtained by performing eigenvalue decomposition on the covariance matrix of received data. Secondly, the weighting vector of GSC is divided into adaptive and non-adaptive two parts. Then the non-adaptive part is projected into the signal subspace to obtain a new steer vector. Subsequently, based on the orthogonal complementary space of the new steer vector, the blocking matrix is constructed. Finally, the weighting vector is updated by projecting the final weighting vector into the signal subspace. In order to verify the proposed algorithm, the simulations of the point targets and the cyst phantom were conducted in Field II. The experimental results indicate that the proposed method has better resolution and contrast ratio than the conventional algorithms. In addition, the algorithm is robust to noises. Furthermore, combining with coherence factor, the contrast ratio of the proposed algorithm can be further improved in comparison with a conventional GSC with coherence factor.

Longitudinal ultrasonic pulse propagation over the cross section of an elliptic cylinder is modeled for the case of transmission–reception of waves by a contactless electromagnetic acoustic transducer along the cylinder envelope. Modeling is based on ray approximation and finite-element method. Formation of series of multiple reflections within the cross section of an elliptic cylinder is studied both theoretically and experimentally. New informative parameters of determining the cylinder ellipticity by the mirror-shadow electromagnetic acoustic method of multiple reflections are justified: the modulation period of a series of multiple reflection pulses, the delay time of reflected pulses, and the probabilistic-statistical characteristics of the data array of multiple reflection series.