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.
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.
The hydrodynamic processes occurring in water under the action of pulsed laser radiation with a power of 20 W, wavelength of 1.94 m, and a pulse duration of 100 ns were investigated. Such radiation leads the excitation of jet flows in water, as well as microbubbles and broadband acoustic vibrations. It was established that the main energy of these vibrations lies within the range of 10-15 kHz; they are excited according to the thermal cavitation mechanism and the regime corresponds to superintense nucleate boiling. It was shown that laser-induced hydrodynamic processes exert a pronounced biological effect on Daphnia magna crustaceans, leading to their increased fertility for acoustic doses of 35 and 350 J/m(2). The experimental data and theoretical estimates show that stimulation of the reproductive function in crustaceans is due to the action of laser-induced low-intensity broadband acoustic vibrations and is not associated with temperature effects. The death of crustaceans and appearance of individuals with maldevelopments were observed for the maximum exposure (300 s). It is shown that the negative effects are related only to the effect of high-temperature microjets on Daphnias.
The acoustic scattering from an infinite cylindrical shell with double internal rigid plates at normal incidence is studied by theoretical and experimental approaches. The two rigid plates are attached to the shell along lines parallel to the shell axis symmetrically. The dominant feature of frequency-angle spectra is the interference fringes caused by the reflection waves of attachments in the illuminated region and specular reflection. For a cylindrical shell with double internal plates, there may be several attachments in the illuminated region simultaneously. There will be bright spots when two interference fringes caused by two attachments in the illuminated region intersect, while only interference fringes can be observed for one attachment in the illuminated region. Theoretical and experimental results show that the number of attachments in illuminated region can significantly affect the scattering characteristics, and it is a prominent feature to identify the location of internal plates.
The hydrodynamic processes occurring in water under the action of pulsed laser radiation with a power of 20 W, wavelength of 1.94 μm, and a pulse duration of 100 ns were investigated. Such radiation leads the excitation of jet flows in water, as well as microbubbles and broadband acoustic vibrations. It was established that the main energy of these vibrations lies within the range of 10–15 kHz; they are excited according to the thermal cavitation mechanism and the regime corresponds to superintense nucleate boiling. It was shown that laser-induced hydrodynamic processes exert a pronounced biological effect on Daphnia magna crustaceans, leading to their increased fertility for acoustic doses of 35 and 350 J/m2. The experimental data and theoretical estimates show that stimulation of the reproductive function in crustaceans is due to the action of laser-induced low-intensity broadband acoustic vibrations and is not associated with temperature effects. The death of crustaceans and appearance of individuals with maldevelopments were observed for the maximum exposure (300 s). It is shown that the negative effects are related only to the effect of high-temperature microjets on Daphnias.
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.
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.
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.
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.
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.
Psychophysical experiments on listeners with normal hearing were conducted to discriminate the rippled spectra of an acoustic signal against maskers with different positions of the spectral band relative to the signal band. As the signal level changed from 50 to 80 dB SPL, the on-frequency masker level changed by 29 dB, whereas the low-frequency masker level (the position of the center of the spectral band was from –1.25 to –1 octave relative to the signal) changed by 8.7–9.8 dB. These results are interpreted as 0.3 dB/dB compression of responses to the signal and no compression of the effect of low-frequency maskers. If the spectral bands of the signal and masker partially overlap, discrimination of the spectral structure occurs predominantly in the part of the spectrum that does not overlap the masker spectrum and is subjected to low-frequency masking that is not compressed.
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.
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.
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.
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 possibility of using a single time realization of a vortex ring to study its aeroacoustic properties is considered. For each realization, averaging is performed over six microphones located around the path of the ring at the same distance from its axis. Spectra obtained as a result of such averaging are presented. It is shown that the specific features of noise from an individual vortex ring agree well with earlier results when averaging over an ensemble of realizations. It is proposed for the first time to localize the sources of noise produced by a vortex ring during its motion using a multimicrophone array with a beamforming algorithm adapted for the considered problem. The position of the noise sources on the localization map is in good agreement with the trajectory measurements of the vortex ring. A beamforming array was used to obtain the averaged spectra of single realizations corresponding to localization of sources for different delays from the moment of vortex launch.
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.
Correlation analysis of low-frequency reverberation signals has been carried out by decomposing the reverberation into reference functions, formed as a set of signals reflected from a point scatterer moving along a circular trajectory. The choice of the reference signal is related to the model of sound scattering by inhomogeneities (bubbles) occurring in the near-surface layer and moving together with the orbital currents of wind waves. The study demonstrated that the correlation functions differ significantly (by 5–6 dB) for the region of position and negative Doppler frequencies of reverberation signals. A stable increase has been revealed for the maximum of the correlation function at certain radii of scatterer motion, which are related to the amplitudes of wind waves.
The phenomenon of “wave resonance” which occurs at excitation of traveling waves in dissipative media possessing modular, quadratic and quadratically-cubic nonlinearities is studied. The mathematical model of this phenomenon is the inhomogeneous (or “forced”) equation of Burgers type. Such nonlinearities are of interest because the corresponding equations admit exact linearization and describe real physical objects. The presence of “accompanying sources” (traveling with the wave) on the right-hand side of the inhomogeneous equations ensures the inflow of energy into the wave, which thereafter spreads throughout the wave profile, flows to emerging shock fronts, and then dissipates due to linear and nonlinear losses. As an introduction, the phenomenon of wave resonance in ideal and dissipative media is described and physical examples are given. Exact expressions for nonlinear steady-state wave profiles are derived. Non-stationary processes of wave generation, spatial “beating” of amplitudes with different relationship between the speed of motion of the sources and the natural wave velocity in the medium are studied. Resonance curves are constructed that contain a nonlinear shift of the absolute maxima to the “supersonic” region. The features of the resonance in each of the three types of nonlinearity are discussed.
We study the interaction of noise and regular signals with a front whose steepness increases or decreases owing to nonlinear distortion of the profile of an intense pumping wave. Projective transformation is used, which is a result of one of the Burgers equation symmetries. Signal interaction with the pumping wave at its leading edge results in an increase in signal amplitude, a decrease in its time scale, an increase in the signal evolution rate, and earlier merging of discontinuities. At the trailing edge, an increase in signal amplitude, an increase in the time scale, and deceleration of the evolution rate occur. Formulas are obtained that describe the transformation of the spectrum and the correlation function of noise. Laws of the change in noise energy for both small and large Reynolds numbers are found. We study the interaction of weak noise with a nonstationary shock front in a medium with a finite viscosity. It is shown that, owing to competition between amplification at the shock front and high-frequency attenuation, the dependence on the noise intensity on distance has a nonmonotonic character, and at large distances, the intensity tends to zero, while the correlation time tends to a finite value.