Acoustical Physics最新文献

A computational and theoretical study of the properties of the well-known Chuprov waveguide invariant (CI) was carried out in a plane-parallel Pekeris waveguide. In contrast to earlier works, in which predominantly omnidirectional (monopole) sources were used as a source and sound pressure fields (scalar fields) were studied, in this work not only scalar, but also vector fields formed in the waveguide by directional-combined multipole sources with directivity in both horizontal and vertical planes are investigated. A differential equation has been obtained that makes it possible to fairly accurately calculate the CI values under different conditions of signal propagation and different depths of the sources and receivers. This makes it possible, in a simpler way than “total computer simulation,” to predict the invariance (stability) of the CI when both the hydrophysical conditions in the waveguide and the geometry of the experiment are varied. It is shown that the directivity of sources in the horizontal plane has virtually no effect on the properties of the CI, and the directivity in the vertical plane leads to a shift in the fan structure of the signal amplitude fields, but has little effect on the CI values. The properties of the fan structure change similarly when using vertical projections of the vibrational velocity vector: despite the fact that another analytical relation different from scalar fields is used to calculate the CI, the CI value is close to (1) at all frequencies and distances, except for those at which new modes or dislocations appear. At these frequencies and in these zones, alternating emissions with different signs and magnitudes occur. It is concluded that the stability of the CI allows the application of signal processing algorithms developed for scalar fields and nondirectional sources to vector–scalar fields generated, including with the use of directional sources.

Abstract

A SAW resonator based on langasite (LGS) substrate with (0°, 138.5°, 116.6°) cut was designed and fabricated. Simulation and experimental results demonstrate that the resonator has two resonance modes, one Rayleigh wave and the other leaky surface acoustic wave (LSAW). The temperature dependent resonance frequencies of both Rayleigh wave and LSAW had been studied. The results show that the turnover temperature of LSAW mode is below absolute zero, namely, the resonance frequency of LSAW changes monotonically in rather wide temperature range, such as from cryogenic to ultrahigh temperatures. Real-time temperature measurement shows that the temperature sensor based on the LSAW mode can effectively monitor the environment temperature, as the thermocouple does. Our work suggests that the LSAW temperature sensor has great application potential in aerospace field for wide temperature range sensing.

The ability of the dolphin auditory system to recognize and classify noise signals according to certain invariant characteristics under the influence of noise interference and in conditions of spatial uncertainty of the simultaneous presentation of positive and negative signals was investigated. Bottlenose dolphins trained to differentiate such signals had to solve this problem in conditions simulating real sea conditions, when the perception of a useful noise signal occurs against a background of similar signals and against a noise interference background. First, noise signals were sequentially presented to the animal against a background of white masking noise. Subsequently, the dolphin had to identify a signal of a positive class from several simultaneously sounding sound sources. The animal’s performance was assessed at several specified noise interference levels. In this case, the actual noise interference was both white noise and simultaneously sounding negative signals. It has been shown that the efficiency and noise immunity of the dolphin’s auditory system depends on the degree of alternativeness of the spatial uncertainty of the simultaneous presentation of signals.

Applying numerical modelling approach the accuracy in determining the distance between underwater sound sources and receivers is assessed at a range of several kilometers from each other in the Kara Sea in autumn. It is suggested that the main source of errors in determining the distance is the lack of accurate data on the vertical sound speed profile along the acoustic signal propagation path. Data from September and November were analyzed, in the interval between which significant changes in the profile take place, when the vertical sound speed gradient changes from negative to positive. Characteristic values of sound speed variations were obtained by statistical processing of hydrological data taken from the World Ocean Database. The results are important for analyzing the capabilities of underwater acoustic navigation.

Abstract

Cracks are one of the most common damages to thin plates. Therefore, rapid and accurate detection of cracks is of great significance in the nondestructive testing of thin plates. Most existing nondestructive testing technologies focus on crack localization rather than the quantification of the crack length and orientation. Therefore, this paper proposes a damage index, considering the wave attenuation during propagation, to quantify the crack length and orientation. Lamb waves are transmitted and received by a circular sensor array. Based on the abnormal reflection of the A0 mode of Lamb waves, the reflections at cracks are extracted by the Hilbert transform. The influence of the crack length and orientation are studied parametrically, and the relations between the proposed damage index and crack length and angle are given based on regression analysis.

Abstract

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.

The method of selecting the configuration of dissipative mufflers with the required acoustic efficiency is considered. A specific feature of the approach is the use of an integral indicator of acoustic efficiency and dimensionless geometric parameters. The studies were carried out using finite element calculations. In the finite element model of dissipative mufflers, the acoustic characteristics of a fibrous sound-absorbing material obtained from experimental studies were used.

Abstract

In this study, phonon crystal structures embedded in acoustic black holes are discussed. The low-frequency band gap is widened by exploiting the low-frequency, broadband and multimode properties of the acoustic black hole. The energy band properties of the acoustic crystal structure embedded in an acoustic black hole are calculated by means of a finite element method. The mechanism of band gap generation is investigated. The vibration transfer characteristics of finite period structures are analyzed. The influence of the structural parameters of the acoustic black hole is analyzed. The results show that the acoustic crystal structure embedded in an acoustic black hole has multiple band gaps in the 500 Hz band and the band gap coverage is increased to 45.18%. The starting bandgap is 16.10% lower than before embedding in the acoustic black hole and the width of the first bandgap expands to 173.03% of that before embedding in the acoustic black hole. The onset and termination frequencies of the first band gap are mainly determined by the vibrational modes of the scatterer and the acoustic black hole structure. The vibrational transfer of the finite period structure is analyzed and shows good damping characteristics in the bandgap interval. Finally, vibration experiments verify the vibration damping effect of the proposed coupled acoustic black hole phononic crystal, and the relevant findings of this paper can be used in the vibration damping design of plate structures, enriching the experience of research related to acoustic black holes.

The article describes the methodology and presents the results of a model quantitative assessment of the efficiency of solving the problem of detecting and tracking a low-noise underwater object using three algorithms for spatial signal processing at the output of a multielement array—the nonadaptive Bartlett algorithm, the Capon algorithm, and the Capon algorithm, combined with a projection procedure for limiting the signal power of strong local sources.

Abstract

High-intensity focused ultrasound (HIFU) is widely used in the treatment of benign and malignant tumors due to its advantages of noninvasiveness and high therapeutic efficiency. However, how to improve the efficiency of heat deposition in a short period of time is a key problem during HIFU thermal ablation. The acoustic cavitation excited by pulse HIFU has been proven to achieve HIFU efficiency enhancement. However, the real-time monitoring of acoustic cavitation is still an issue. In this study, a real-time detection method of acoustic cavitation is established based on self-sensing ultrasound, and the synergistic effect of acoustic cavitation excited by pulse HIFU is researched. The influences of the output power, pulse duration, irradiation depth on cavitation duration are respectively discussed by using the established cavitation detection method compared passive cavitation detection (PCD). The relationship between cavitation intensity and synergistic effect is discussed. The results have shown that the cavitation detection can real-time measure cavitation duration compared with PCD. In addition, during the cavitation detection of pulse HIFU, the synergistic effect of acoustic cavitation is obvious in HIFU ablation.