Effect of random hydrodynamic inhomogeneities on low frequency sound propagation loss in shallow water

Low frequency (100–500 Hz) sound propagation loss on the US Atlantic continental shelf and in the Barents Sea in the presence of stochastic surface waves, and for the US Atlantic shelf also in the presence of internal waves, is studied for the range of up to 150 km by means of numerical simulations. Qualitative difference between sound propagation loss behavior on the US Atlantic shelf and in the Barents Sea is demonstrated for summertime conditions even without random inhomogeneities. It is shown that whereas internal waves have a weak effect on propagation loss, surface waves result in its considerable increase in both areas under wintertime conditions with a wind speed of more than 9 m/s.     

Losses during sound propagation on the shelf

The paper discusses the results of studying how changes in the hydrological conditions affect losses that sound undergoes as it propagates along a stationary track in Vityaz Bay in the Sea of Japan. Measurements were conducted with an Mollusk-07 autonomous vertical acousto-hydrophysical measurement system and an autonomous electromagnetic emitter generating a frequency-modulated signal in the 280–340 Hz band. The modulation frequency was 0.3 Hz. It is shown that tide-, internal-wave-, and upwelling-caused variations in sound losses with a frequency of 285–335 Hz propagating along a stationary track with a length of 1640 m for seven days did not exceed 3 dB.     

Four-magnon decay of magnetostatic surface waves in yttrium iron garnet films

The four-magnon instability of magnetostatic surface waves (MSSWs) in yttrium iron garnet epitaxial films is investigated experimentally. It is shown that four-magnon instability for MSSWs with wave numbers 30–600 cm⁻¹ is a decay instability and develops for values of the wave magnetization close to the threshold level for second-order parametric instability of a homogeneous transverse pump wave. When the supercriticality of the MSSW power is 15–20 dB, the generated parametric spin waves themselves become unstable with respect to the four-magnon interaction, so that kinetic instability develops in the film. It is shown that the pump signal transmitted through the signal and the length of the “nonlinear” part of the film, where a MSSW is capable of exciting parametric spin waves, increase as the pump power is increased. Fiz. Tverd. Tela (St. Petersburg) 38, 330–338 (February 1997)     

Peculiarities in the formation of the sound field structure of a point source in the Black Sea underwater sound channel

This paper studies unique (characteristic only of the Black Sea) peculiarities of the underwater sound channel (USC). Changes in the sound velocity of at depths of 50–250 m, forming the lower boundary of the Black Sea USC, differ fundamentally from the corresponding areas of the profile c(z) in other regions of the world ocean. With lowering from 40–50 m (by 5–10 m lower than the level of the channel axis) to 200–250 m, the sound velocity gradient decreases monotonically from 0.08–0.22 to 0.02 1/s (and does not increase like in the majority of regions of the world ocean). The end portion of an explosion signal received in the Black Sea USC at a distance of 200 km or more from the source represents a quasi-harmonic signal with a gradually changing frequency. Moreover, the end portion of the signal has an explicitly block structure, which agrees well with the block structure of the spectrum of an explosion signal element. In the truncated τ(R) diagram, there is no sharp bend characteristic of the majority of regions of the world ocean. At comparatively small depths of the Black Sea, a sufficiently rapid increase in the complete duration of a multiray signal with distance is observed. A comparative analysis is conducted of experimental materials obtained with a difference of seven years on virtually the same long-range propagation path of explosion signals. The main reasons for the interannual variability in the conditions of sound channel propagation in the Black Sea are explained.     

南海北部海域内波环境下声传播损失起伏统计特性

浅海内波会引起声传播能量随时间的起伏变化,进而影响水声设备的工作性能.本文利用2015年南海北部一次浅海声场起伏实验数据,对比分析了浅海线性内波和孤立子内波条件下的声传播损失统计特性.在孤立子内波条件下,声传播损失起伏明显加剧,可达11 dB,且分布明显展宽,相对于线性内波的环境,声传播损失起伏可增加5 dB.从简正波...     

Experimental study of the horizontal correlation of an acoustic field in an inhomogeneous shallow-water waveguide

We present the results of experiments on measuring the characteristics of a wideband low frequency (10–200 Hz) acoustic field in an inhomogeneous shallow-water waveguide in the Barents Sea area. Correlation functions of the signals received by hydrophones of autonomous bottom stations with independent reception, which are horizontally spaced 18 and 112 km apart, are measured. A pneumoacoustic radiator with an overpressure of 10.5 MPa and a volume of 7.5 l was used as a sound source. During the measurements, the signal source was shifted from the line of receivers to distances 10–225 km. The dependences of the correlation coefficient of the received signals on the radiation frequency and the interferometer-baseline length and orientation are studied. It is shown that pronounced local minima in the interval 10–40 Hz due to the difference of the inhomogeneities in the propagation channels and, therefore, the frequency characteristics of the medium are present in the frequency dependences of the correlation coefficient against the background of a monotonic decrease with increasing frequency. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 3, pp. 208–215, March 2009.     

The sound speed in southern deepwater zone of the Caspian Sea,off Anzali Port

Sound velocity determination in seawater is a key component of modern hydrographic surveying; however, little data exists on sound velocity characteristics of the southern Caspian Sea. Hence, a study was undertaken in 2008 to examine the seasonal variability of sound speed in deep-waters of the South Caspian Sea near the Iranian coast. The seasonal cycle of seawater temperature and thermal stratification in the Caspian Sea water created a wide range of spatial and temporal changes of sound speed with relevant differences between shallow water (over the continental shelf) and deep-water area. The collected data showed that seasonal variations of the sound speed were most important in the upper 100 m water depth, while below this level that is in deepwater the changes were small. The maximum values of sound speed were observed at the surface in midsummer around 1517–1519 m s⁻¹ over the continental shelf while the speed of sound was about 1453 m s⁻¹ between 450–470 m depths with no major seasonal variations. Variations in vertical structure of the sound speed were in agreement with temperature changes, while effects of the salinity on the sound speed were little.     

Frequency shifts of the sound field interference pattern on oceanic shelf in summer conditions

Considerable shifts Δf/ϕ ≈ 10⁻¹ of the low-frequency sound field interference pattern in the frequency domain, associated with barotropic tide and internal tidal waves, were observed in the Shallow Water’06 experiment on the New Jersey shelf in the summer of 2006. The acoustic frequency shifts appear to be strongly dependent on the modes of the sound field. By examining different modal structure, it is possible to analyze the overall interference pattern and find which part is more sensitive either to the surface tide or the internal waves. This feature can be exploited to acoustically monitor tidal waves of different kinds.     

Detection of magneto-gravity waves in the ionosphere by analysis of the maximum observable frequencies on oblique-sounding paths

The existence of magneto-gravity waves stipulated by the substorm activity can lead to the occurrence of traveling ionospheric disturbances, whose velocity exceeds the sound speed. For detection of magneto-gravity waves, we used experimental data on propagation of decameter radio waves on the midlatitude and subauroral oblique sounding paths Inskip–Rostov-on-Don, Cyprus–Rostovon-Don, Irkutsk–Rostov-on-Don, and Norilsk–Rostov-on-Don in December 2006 and March 2007 under conditions of weak geomagnetic disturbance. Time delays between the AE indices of polar electrojets and the maximum observable frequencies for the considered paths were established by calculation of linear correlations. These delays correspond to the times required for transport of gravity disturbances from the auroral region to the reflection points of radio waves on respective paths. Among the obtained time shifts, we mention the 5–10-min ones which correspond to increased velocity of the disturbances compared with the usual velocities of acoustic-gravity waves for the paths under study. Such cases can be related to the transport of magneto-gravity waves. Study of the spectral composition of the AE-index disturbances and recorded maximum observable frequencies shows consistency of their spectral features in the cases of increased correlation for small time delays. It is also found that the spectral features of the AE-index disturbances coincide with the spectral features of the disturbances of the horizontal component of the geomagnetic field on ground-based magnetic stations. According to calculated dispersion curves, the frequencies of magneto-gravity waves were estimated, and they turned out to be equal to ω ≈ (1–2) ・ 10⁻⁴ Hz.     

Multimode optical fiber demultiplexer-star coupler using a single gradient-index-rod lens and a multi-facet grating

An optical fiber multi-function device consisting of a single gradient-index-rod lens and a multi-facet blazed reflection grating is proposed to simultaneously realize functions of wavelength demultiplexing and optical signal distribution in a multimode optical fiber transmission system. We analyzed the demultiplexing characteristics and the tolerance of optical components using the ray trace method. This device can realize not only low loss optical signal distribution but also offers improved demultiplexing characteristics in comparison with the previously proposed demultiplexer-multiposition switch. The following characteristics are expected from the design using commercially available optical components: a working band of 0.64–0.88 μm, channel separation of 34–36 nm, 3 dB bandwidth of 27–28 nm, channel cross-talk of less than - 40 dB and minimum excess insertion loss of 0.9–2.1 dB.     

Sound fluctuations caused by internal waves in a shallow sea

Estimates are presented for the fluctuations of the parameters of low-frequency sound fields in shallow-water regions of the Barents Sea, in the presence of seasonal internal gravity waves. The objective of the experiments is to reveal the main mechanisms that govern the sound fluctuations and their statistical parameters on paths of moderate lengths (50–60 to 100–120 km). Another objective is to determine the features of the sound interaction with internal waves for the sound speed profile of the summer—autumn type for which the water stratification is most pronounced. As the probing signals, continuous tonal ones produced by bottommoored sources at the frequencies about 100 and 300 Hz are used along with the 1/3-octave noise signals with the central frequency 1000 Hz, which are generated by a source deployed from a vessel. For the signal reception, both fixed bottom-moored hydrophones and a vertical chain of hydrophones are used, the chain also being deployed from the vessel. The water temperature, the salinity, and the thermocline displacements are monitored with standard hydrographic sensors. The following main results are presented: the estimate of the degree of correlation between the sound fluctuations and the parameters of the water layer, the comparison of the fluctuations in the signal amplitude envelope with the data obtained in other regions, and the estimate of the statistical parameters of the signal amplitude fluctuations, including their dependence on the path length. One more result consists in the proof of the wave nature of the interaction of sound and internal waves, which manifests itself in a strong dependence of the sound interaction with internal waves of discrete frequencies on the frequency of the probing signal and on the angle at which these wave beams intersect. An attempt is made to explain the observed phenomena by the synchronism in the interacting sound and gravity waves. The data obtained can be used to analyze and compare the fluctuations of the sound fields in the ocean, especially in shallow-water regions.     

Simultaneous complete elastic and electromagnetic band gaps in periodic structures

We study elastic and electromagnetic properties in periodic structures and present “deaf and blind” structures, i.e. materials having simultaneous complete phononic and photonic band gaps, that is, transverse electric (TE) and transverse magnetic (TM) electromagnetic waves, pure shear elastic waves, and mixed shear and dilatation elastic waves, cannot propagate within these structures. These composite materials can control the flow of light and sound at the same time. The existence of complete gaps for electromagnetic and elastic waves can lead to the simultaneous localization of light and sound, a novel phenomena that can have strong influence on photon–phonon interactions. We study the dependence of the simultaneous and complete gaps on material parameters to provide design guidelines on how to create these photonic–phononic crystals. PACS 78.66.Sq; 78.20.Bh; 63.20.-e     

南海东沙海域线性内波引起的声场统计特性

海洋内波对声传播及水声探测具有重要影响.利用南海东沙附近海域一次低频声传播起伏实验同步获取的声学与水文观测数据,从水文连续观测数据中提取了内波特征参数,验证了修正线性内波频谱公式,用蒙特卡洛方法统计分析了存在线性内波条件下的声场起伏特性,并用射线理论解释了声源与跃层相对位置对声场起伏的影响机理.结果 表明:随着频率、平...     

Sounding of an artificially perturbed ionosphere by means of an ionosonde/position finder with chirp modulation of the signal

We describe the operation of an ionosonde/position finder with chirp modulation of the signal. The first results of measuring the characteristics of short-wave radio signals scattered by artificial small-scale inhomogeneities, which were obtained by means of an ionosonde/position finder on the IZMIRAN—“SURA”—Rostov-on-Don path are presented. It was found that under certain ionospheric conditions, the angular and frequency selection of the scattered signals take place, in which case the signals are observed simultaneously in several frequency intervals (mainly, in three, namely, 6–9.5 MHz, 10–12 MHz, and 15–18 MHz) with different angles of incidence of radio waves in the vertical plane. In this case, the incidence angles were 20◦–35◦, 18◦–32◦, and 10◦–20◦ from the horizon for the first, second, and third frequency interval, respectively. Ionograms of oblique sounding were modeled allowing for the scattering of radio waves by artificial small-scale inhomogeneities. It is shown that at frequencies from 10 to 12 MHz, aspect conditions are fulfilled for the signals ducting along the high-angle beam (Pedersen mode). At frequencies 15–18 MHz (higher than the maximum observable frequency of the forward signal on the path IZMIRAN—Rostov-on-Don), aspect scattering conditions are fulfilled for the signals incident on a scattering area in the ascending part of the trajectory. At low frequencies 6–9.5 MHz (below the maximum observed frequency of the forward signal on the IZMIRAN—Rostov-on-Don path), the observable additional signals are caused by the scattering of radio waves by artificial inhomogeneities with subsequent relfection of the scattered signal from the Earth on the “SURA”—Rostov-on-Don path. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 4, pp. 267–278, April 2009.     

Sura-wind radar: Studies of nonlinear effects using VHF-wave occultion of the ionosphere

We present experimental results on reception of VHF signals of the transmitting facility “Sura” of the Radiophysical Research Institute by the NASA spacecraft WIND. The experiments were performed during daytime during the summer seasons 1997–1998. The dependence of power and spectral characteristics of the VHF radio waves on the power of the sounding radiation is analyzed. We find that, in a wide range of effective powers of the VHF radiation from about 40 kW to 160 MW, the phenomena observed in the radiation received onboard WIND (such as focusing, scintillations, or frequency deviations of signals) does not usually depend on the operation mode of the “Sura” facility. At the same time, broadening of the radiation pattern of the facility and decrease of the mean level of the received signal by about 6 dB toward the direction of maximum of the radiation pattern were observed at the peak radiation power of the facility. The experimental results are compared to present concepts on effects of self-focusing instability and nonlinear defocusing of VHF radio waves in the ionospheric plasma. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 799–809, August 1999.     

Similarity relations for physical modeling of the acoustic fields in a shallow sea

Using the mode formalism, we consider the similarity relations for scale modeling of the acoustic fields in a shallow sea. The corresponding conditions for physical modeling of sound propagation and diffraction in a waveguide with regular parameters are obtained. The similarity relations for spatio-temporal mode correlation functions are also found in the case of acoustic fluctuations caused by wind waves and random internal waves. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 4, pp. 318–327, April 2009.     

Variations in the radio-wave absorption in the ionosphere due to internal gravity waves

We estimate the influence of internal gravity waves on the radio-wave absorption in the Earth’s atmosphere. It is shown that the internal gravity waves can lead to significant spatio-temporal variations in the absorption. We conclude that riometric measurements can be used for the diagnostics of internal gravity waves in the Earth’s atmosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 3, pp. 194–198, March 2006.     

Backscattering at the <Emphasis Type="Italic">E</Emphasis> and <Emphasis Type="Italic">F</Emphasis> layers for the vertical incidence of radio waves on the ionosphere

We present the results of the vertical ionosphere sounding at a frequency of 9.02 MHz using the “Sura” facility. Intense backscatter signals from meteor trails were observed at altitudes 100–130 km. Increased background of the scattered signal, which was about − 100 dB with respect to the mirror-reflected signal, was observed at altitudes of about 190–200 and 270–280 km. According to the Doppler-shift measurements of the scattered-signal frequency, the wind velocity was more than 30 m/s at altitudes 100–130 and 270–280 km and was significantly smaller in the altitude range 190–200 km. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 1, pp. 23–27, January 2009.     

Extracting microwave energy from a cavity by mode conversion at a coupling window

A study is made of the production of high power nanosecond rf pulses by extracting microwave energy from an oversized cavity by means of conversion, at a coupling window, of the high-Q working mode to an auxiliary mode which is strongly coupled to an external load. It is shown that microwave rf pulse compressors with copper storage cavities and energy extraction by mode conversion at a coupling window can provide gains of 5–13 dB with output signal durations of 20–150 ns and peak powers of 5–10 MW in the 3-cm band and 50–100 MW in the 10-cm band. Rf pulses lasting 30 ns with peak powers of 0.5 MW have been obtained experimentally at a frequency of 9.4 GHz with a gain of 9 dB. Zh. Tekh. Fiz. 68, 92–96 (July 1998)     

Optoacoustic diagnostics of absorbing media

We have calculated the intensities of ultrasonic waves excited by counter-propagating light waves of different frequencies and scattered by spherical defects in absorbing isotropic media. We have analyzed the intensity of the scattered sound as a function of the scattering angle and the position of the defect relative to the entry face of the acoustic waveguide. We have shown that for hollow spherical defects, the intensity of the scattered sound tends toward zero for scattering angles close to 90o, and for significantly smaller angles for dense inclusions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 686–689, September–October, 2006.