The article discusses the results of an experiment conducted in the Japan sea in March 2016 on the acoustic track length of 194 km at the winter hydrological conditions. It was the most complicated case of propagation of pulsed pseudo-random signals from the shelf into the deep sea in the presence of acoustic track the vortex formation. Analysis of experimentally obtained impulse response showed that apparent at all points in the maximum first arrival of acoustic energy is in good agreement with the calculation. This suggests that the horizon of reception of the first pulses come, the past in the near-surface sound channel at the shortest distance and at small, close to zero angles.
During autumn period of 2015-2016 intensive bionoises were recorded in shallow waters of the eastern coast of the Black Sea (Suhum bay) using broadband hydrophone (upper frequency limit - a few hundred kilohertz). When placing a hydrophone in 30-40cm above the bottom surface at a depth of 5 to 10 m the recorded signal contained high frequency noise generated by snapping shrimps. Along with these background signal we observed relatively rare (usually several times per minute), but very intense (up to a few hundred Pa) clicks. They apparently were radiated by single shrimp localized in the vicinity of the hydrophone. Such conditions are allowed to carry out a time-frequency analysis of the signals in natural conditions of the habitat. It has been shown that they consist of the following sections: a short precursor generated by the water jet emitted during the closing of the claws, a powerful ultrasound pulse arising due to the collapse of a cavitations bubble, and multiple reflections from the surrounding elements of the bottom surface (gravel, empty shells of mussels). The rise time of the main pulse may be 7-10 microseconds with total duration of the signal from 0.1 to 0.4ms, varying specificity due to reflections. In many cases along with the main pulse the delayed echo from local artificial underwater objects and from the surface also can be observed. The possible practical applications of the data are discussed.
M. B. Salin, G. A. Baidakov, O. A. Potapov, B. M. Salin, A. V. Stulenkov, D. D. Razumov
Memoirs of the Faculty of Physics 2017. N 5.
Narrow-band spectra of sound backscattering were investigated in a series of experiments. Two types of scattering spectra are distinguished. In the first case, side-lobes are observed at a certain distance from the carrier frequency. This effect is caused by Bragg scattering on surface waves. In the second case, a smooth broadening of the spectrum around the carrier frequency is observed. In this case, the reverberation spectrum is interpreted using the phenomenological model of sound scattering at subsurface inhomogeneities. These inhomogeneities are moving along circular tracks under the action of wind waves currents, which velocities are rather smaller than the phase velocities of surface waves.
Speech waveforms are known to be remarkably difficult to compress. Lossless entropy compression methods yield about 30% reduction of the initial waveform size. Delta-compression (and also somewhat similar to it methods based on the ideas suggested by A. Haar and I. Daubechies) allow to improve this result by another 30%. Lossy compression methods yielding much higher compression rates take advantage of the psychoacoustic peculiarities of sound perception by removing certain components of the speech signal that have the smallest effect over its perceived quality, e.g. portions of the spectrum that are “in the shadow” of prominent spectral peaks. It must be observed that all the abovementioned ideas are based on the existence of some a priori knowledge about such kind of data as the speech signal (for instance, that the ordinates of two adjacent points of the speech waveform normally differ by a small value, or that the spectrum of the speech signal usually contains several prominent peaks that determine the perceived quality of the sound). Following this logic, it can be suggested that the a priori knowledge that the speech signal is a container enclosing linguistic data can just as well be used to significantly improve the efficiency of both lossy and lossless compression technologies by including in the data source model the regularities appertaining to the linguistic component of the speech signal. This paper deals with the possible results of this suggestion being tested.
Statistical problem of a low-frequency sound propagation in shallow-sea waveguide with two-dimensional random inhomogeneities is considered. Transmission loss law for average intensity in the case of fluctuating thermocline and absorbing liquid bottom is investigated. Within the framework of local-mode approach it is shown that mode coupling with the distance is of local character if there are no strong fluctuations of modal wavenumbers in stochastic waveguide. So, in the local-mode approach the mode coupling to be the certain supplement to adiabatic approximation of intensity, and it does not change seriously the average transmission loss. Comparison between results of local-mode and diffusion approximation is also carried out.
V. A. Grigorev$^1$, V. G. Petnikov$^2$, A. V. Shatravin$^3$
Memoirs of the Faculty of Physics 2017. N 5.
Within the framework of numerical modeling the feasibility of mode description of the sound field is analyzed for shallow water with an Arctic type bottom: a liquid gas-filled sediment layer overlaying an elastic half-space (permafrost). It is determined that the optimum description of the field in the water layer at short distances from the sound source (order of 1÷10 depths of the waveguide) is achieved by using of the discrete spectrum of normal modes and quasi-modes calculated with use of the Pekeris cut. Computations of the transmission loss in the waveguide with the sediment layer thickness comparable to or higher than the acoustic wavelength in the sediments, shown that the seabed is acting as a uniform liquid half-space. If the thickness of the sedimentary layer is less than a quarter of the wavelength, then the layer can be neglected. The transmission loss abruptly increases with the sound speed in the sediments approaching the sound speed in the water. A method is proposed for estimating the sound speed in the sediment layer. The method based on the analysis of attenuation of the sound fields of higher modes.
In this report, temporal and spatial focusing properties of time-reversal mirrors are studied for leakage detection. The acoustic remote sensing of subsea gas leakage traditionally uses sonars as active acoustic sensors and hydrophones picking up the sounds generated by a leak as passive sensors. When gas leaks occur underwater, bubbles are produced and emit sound at frequencies intimately related to their sizes. Two factors: the local character of the acoustic emission signal caused by the leakage and a resonant nature of the bubble radiation at their birth make particularly effective use, for active location, time-reversed emission signals in order to obtain a powerful scattering signal. The propagation of emission and time-reversed signals is described in terms of Green’s function of the Pekeris waveguide. This study expands the range of applications that use the mechanism of time-reversal focusing to specific conditions – the vicinity of drilling platforms located on the sea shelf.
A method of acoustic wavefield modeling in a randomly-inhomogeneous underwater sound channel is considered. The method is based on the construction of a propagator in the basis of normal modes of the waveguide. Mode coupling is modeled using random matrices with uncorrelated elements whose variances are determined by inhomogeneity spectrum. Spectral statistics of such propagator is compared with the corresponding data obtained via direct solution of the parabolic equation. It is shown that the propagator constructed via random matrices describes fairly the process of wavefield decoherence with increasing range.
Experimental data was obtained using the Doppler log installed on the boat tacks – North water area of Ladoga lake at depths greater than 100 m. The Gain at the gate of the synchronous receive 4 channels was constant, the ratio s/n >> 1. The obtained coefficients of variation, temporal and spatial correlation. Estimates are compared to traffic conditions with a small velocity and stop the boat. Examines the relationship of ratings with the top layer of the bottom, having in some seas.
In the study for the first time on the material of the Russian language, acoustic (spectral and temporal) characteristics of stressed vowels from word cut from spontaneous speech of children aged 14-16 years, recognized by auditors with different probability, are described on the basis of the Russian language. Perceptual analysis of children's speech was conducted, aimed at recognizing the meaning of adolescent's words by adults. For words recognized with different probabilities (0.75-1.0 and 0-0.25), the values of the duration of the stressed vowels, the duration of their stationary sections, the pitch frequency value (F0) are described, the values of the formant frequencies (F1, F2). Formant triangles of coordinated vowels for the Russian language (a, y, u) are constructed and their areas are defined.