Calculation of acoustic scattering of a nonrigid surface using physical acoustic method

In this paper the physical acoustic method or the Kirchhoff approxima-tion is extended to treat the scattering of a nonrigid surface in order to estimatethe target strength of targets with absorbing coatings.By using the locally planewave approximation,the relationship between the sound pressure and its normalderivative on the surface can be represented by the plane wave reflectioncoefficient and the acoustic impedance of the surface.The resulting modifiedKirchhoff approximation involves the plane wave reflection coefficient.For aimpedance sphere,a comparison between the physical acoustic method and theexact solution shows that the physical acoustic method still is a good approxima-tion at higher κα values.     

The influence of optical properties of shallow shelf seas on temperature structure and acoustic propagation

The penetration of solar radiation below the sea surface is not always taken into account in numerical ocean models, even though its influence on the temperature (and therefore sound speed), structure of the water column can be significant, especially in shallow shelf seas where water turbidity tends to be high. Variations in sound speed structure have a consequent effect on the propagation of sound underwater which, in turn, influences the performance of sonar systems. A double exponential parameterization for the penetration of solar radiation with depth was implemented in an N × 1D turbulence closure model of the UK shelf seas. The model was run along a section through the Celtic Sea Front, with sets of optical extinction coefficients representing different water clarities, for one month to simulate the generation of the front, and the results were compared with a control model run in which all solar radiation was absorbed in the surface layer. Temperature structure and sound propagation were only affected by changes to the optical parameterization on the stratified side of the front, where consequent variations in acoustic propagation loss of up to 10 dB relative to the control were simulated using an acoustic model. Changes in propagation loss were greatest for acoustic sources placed in the stratified water above the thermocline. Similar changes to the optical parameterization made on the well-mixed side of the front had no discernible effect.     

Sonic stop-bands for cubic arrays of rigid inclusions in air

Extensive band structures have been computed for cubic arrays of rigid spheres and cubes in air. Complete stop bands are obtained for the face-centered-cubic (fcc) structure; however, there is no gap for the body-centered-cubic (bcc) and simple-cubic (sc) structures. These gaps start opening up for a filling fraction of (27%) for spherical (cubic) inclusions and tend to increase with the filling fraction, exhibiting a maximum at the close-packing. We also propose a tandem structure that allows the achievement of an ultrawideband filter for environmental or industrial noise in the desired frequency range. This work is motivated by the recent experimental measurement of sound attenuation on the sculpture, by Eusebio Sempere, exhibited at the Juan March Foundation in Madrid (Nature 378, 241 (1995)) and complements the corresponding theoretical work (Appl. Phys. Lett. 70, 3218 (1997)). Received: 24 October 1997 / Revised: 1 December 1997 and 26 January 1998 / Accepted: 6 March 1998     

On the generation of internal solitary marine waves

Summary A nonlinear model of generation of internal ?solitary? marine waves is discussed: when a large surface wave—for instance a superficial wave or a bore of tidal origin—passes over a submarine mountain or crosses a strait, packets of internal waves may often be detected. We study this phenomenon taking into account the effect of the air-sea surface; we show that the phenomenon can be schematized, in an approximate but realistic way, by using the solutions of an inhomogeneous KdV equation. The forcing term depends on the air-sea surface elevation and on the bottom topography. We then apply our model to the marine currents, of tidal origin, flowing through the straits of Gibraltar.

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Riassunto Si studia un modello non lineare della generazione di onde interne marine. Quando una grande onda superficiale passa sopra una montagna sottomarina o attraversa uno stretto si osservano pacchetti di onde interne. Si studia questo fenomeno tenendo conto della interfaccia aria-mare, si mostra che il fenomeno può essere rappresentato in maniera approssimata dalle soluzioni di un'equazione di KdV non omogenea. Il termine forzante dipende dalla superficie aria-mare e dalla topografia del fondo. Infine il modello proposto è applicato alle correnti di marea dello stretto di Gibilterra.

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Резюме Обсуждается нелинейная модель образования внутренних ?одиночных? морских волн: когда большая поверхностная волна проходит над подводной горой или приливная волна пересекает пролив, часто могут детектироваться пакеты внутренних волн. Мы исследуем зто явление, учитывая влияние границы раздела воздух-море. Мы показываем, что это явление может быть схематизировано приближенным, но реалистическим способом, используя решения неоднородного уравнения Кортевега-де Вриса. Силовой член зависит от возвышения границы раздела воздух-море и от топографии дна. Затем мы применяем нашу модель для морских течений (приливного происхождения), проходязих через Гибралтарский пролив.

     

Enrichment and transport of surfactants in marine aerosol formed during particular weather conditions

Summary Surfactants appear responsible for the presence and the enrichment of microfloatable components in marine aerosol. According to the first set of both surface thermodynamic and chemical data, a qualitative model of enrichment and mass transfer is proposed for marine aerosol, near the coast in particular weather conditions. The present data also suggest a quality test for coastal sea-water, integrated over large areas, in those weather conditions in which a direct surface water sampling is very difficult. Paper presented at the 1° Congresso del Gruppo Nazionale per la Fisica dell'atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

Acoustic Analysis of the Interaction of Choral Arrangements, Musical Selection, and Microphone Location

Acoustic differences were evaluated among three choral arrangements and two choral textures recorded at three microphone locations. A choir was recorded when singing two musical selections of different choral texture, one homophonic and one polyphonic. Both musical selections were sung in three choral arrangements: block sectional, sectional-in-columns, and mixed. Microphones were placed at the level of the choristers, the conductor, and the audience. The recordings at each location were analyzed using long-term average spectrum (LTAS). The LTAS from the mixed arrangement exhibited more signal amplitude than the other arrangements in the range of 1000-3500Hz. When considering the musical selections, the chorus produced more signal amplitude in the region of 1800-2200Hz for the homophonic selection. In addition, the LTAS produced by the choir for the homophonic selection varied across the microphone locations. As for the microphone location, the LTAS of the signal detected directly in front of the chorus had a greater slope than the other two locations. Thus, the acoustic signal near the choristers differed from the signals near the conductor and in the audience. Conductors may be using acoustic information from the region of the second and third formants when they decide how to arrange a choir for a particular musical selection.     

Nonlinear acoustics of bounded solid

Reflection and refraction are investigated when the incidental angles of transverse waves are larger than the critical one. An interesting result is obtained that at this angle all the second-order displacements on the stress-free surface have a very strong maximum module and a phase leap; thereupon an appropriate second-order harmonic surface wave could be produced. Project supported by the National Natural Science Foundation of China (Grant No. 19334062).     

Discernment of non-Newtonian behavior in liquids by acoustic means

An experiment was conducted with the use of a „sonic viscometer” to demonstrate that a change in the constitutive nature of a fluid will alter the overtones that occur from the nonlinearity of the propagation of a sine wave through the fluid. Pure sine waves ranging octave-wise from 500 to 8000Hz were introduced into distilled water, and FFT analyses of the signals received were compared with those resulting from transmission of the same set of sine waves through a 1% solution of polyethylene oxide in distilled water. Differences in the FFT spectra which occur as the result of introducing the polymeric solute indicate that the device could prove viable in indicating small changes in viscous properties. Received: 10 February 1998 Accepted: 1 June 1998     

The semi-diurnal tides in the Sicily Strait

Summary The distribution of the semi-diurnal tides in the Sicily Strait is studied. The aim of this paper is not a best fitting of the observed tides by means of a mathematical model; such a purpose would be based on much more stations, better placed and having long series of data in order to obtain a clear definition of the phases. The present work is a test probing the reliability of what was devised by Sterneck (by means of semi-empiric consideration): the hydrodynamic numerical model confirm the Sterneck suggestions. Moreover the paper is a demonstration of the possibility to use models commonly employed in closed or semi-closed basins also, under some conditions, in the case of open basins.

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Riassunto Viene studiata con un modello idrodinamico la distribuzione delle maree semidiurne nel Canale di Sicilia. Lo scopo del lavoro però non è quello di approssimare le osservazioni col modello matematico (che dovrebbe essere un confronto basato su molte piú stazioni di quelle disponibili) bensí quello di verificare quanto ricavato in maniera piú approssimativa in un vecchio lavoro dello Sterneck. Il modello impiegato conferma la presenza del nodo delle maree semidiurne presso Pantelleria e dà anche conto delle elevate ampiezze dell'ondaM2 a Gabes, dove si verificano le maree piú forti del Mediterraneo. Il lavoro è anche una applicazione di un modello già usato in bacini marini semichiusi a bacini aperti, come il Canale di Sicilia, tenendo presenti alcuni accorgimenti di impiego che qua vengono proposti.

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Резюме Исследуется распределение полусуточнных приливов в Сицилийском проливе. Цель зтой статьи не является получение наилучмей подгонки набяюденных приливов с помощью математических моделей. Настоящая работа представляет проверку надежности результатов приближения, использованного в старой работе Стернека (используя полузмприческое расмотрение): гидродинамическая численная модель модтверждает предположения Стернака. Более того, в работе показывается возможность использования моделей, обычно применяемых для замкнутых или полузамкнутых бассейнов, а также при определенных условиях для случая открытых бассейнов.

     

Circulation and heat exchanges of the Northern Adriatic in the winter season 1992/93

Summary The sea currents, temperature and conductivity were measured continuously at two stations and two depths of the eastern part of the Northern Adriatic during the winter season 1992/93. Meteorological elements were measured simultaneously at the two nearest meteorological stations on the Croatian coast (Rovinj, Pula). The sea level was also measured in that period at Rovinj. One severe bora storm was observed in the period of measurements. Qualitative good agreement between observed currents and velocities of the earlier developed models for the response of the Northern Adriatic to the bora and sirocco wind forcing has been shown. The sea currents on the Rovinj-Po transect are of the opposite direction to the bora wind because of tis spatial heterogeneity. The sea level observed at Rovinj station rose during the bora blows and it was much higher than what expected in the developed models. The response time of the currents to the bora forcing is computed by using correlation function between currents and wind. The heat exchanges on the sea-atmosphere surface are calculated. The correlation between same wind components and between total heat exchanges for two different stations was made. The sea temperature evolution calculated from meteorological data agrees well with what observed, especially for the Pula station. At the end of the measurement period the new dense water (σ_t=29.3 kg m⁻) was observed. We dedicate this article to the memory of colleague Antonio Michelato. D. Morožin is not employed at the Institute R. Boŝković.