Home HomeAdrian D. Jones, Defence Science and Technology Organisation (Australia)
The performance of sonar systems, particularly in shallow ocean areas typical of a continental shelf, is complicated and limited by the variability of the acoustic signals which are received. Here, signals vary over time, with spatial location and with frequency, often in ways that are poorly understood. The present state of knowledge of this acoustic variability is reviewed, and some recent developments which have de-mystified some aspects of the phenomena are described. As is shown, average descriptions of the variation of signals with frequency, position and time may be determined, based on the properties of the acoustic signal field. It is shown how these descriptions suggest that the performance of sonar systems may be optimised. It is also shown how these descriptions lead to the specification of parameters for system performance and effectiveness modelling.
sv970392.pdf (Scanned)
TOP
Xiqing Zhu, China Ship Scientific Research Center (China)
Wusheng Wu, China Ship Scientific Research Center (China)
Line-Spectrum noise of counter-rotation propellers has constructed the main part of the radiated noise of underwater vehicles. The line-spectrum noise of the counter-rotation propellers is due to the interaction between fore or aft propeller and wake of the vehicle, and the interaction between fore and aft propeller, Based on a combination of the lifting surface theory and acoustic thecniques, the prediction method of line-spectrum noise is presented in this paper, Theoretical calculation method, characteristics and numerical prediction of the line-spectrum noise are detailed in this paper. The effect of different wake and different distance between fore and aft propeller on the propeller noise is also studied by numerical method, The agreement of predicted results compared with existing experimental data is quite satisfactory. Therefore, the methods are to have important values for performance prediction of the vehicles, inspection of the noise sources and acoustic design of counter-rotation propellers ,etc,
sv970255.pdf (Scanned) TOP
Henry Lew, Defence Science and Technology Organisation (Australia)
R.F. Barrett, Defence Science and Technology Organisation (Australia)
In active sensing systems, such as radar and sonar, the transmitted waveform has an important influence on target detection (especially in reverberation limited environments), parameter estimation and the resolution capability of the system. A method of generating waveforms which maximizes the output signal-to-noise (ambient plus interference) ratio of a replica correlator receiver is given. These waveforms are optimal only for a particular realization of the noise. A large set of such waveforms can be generated by Monte Carlo simulation. From this set, the robust optimum waveform, i.e., the one least sensitive to different noise realizations, can be determined.
sv970475.pdf (Scanned) TOP
Dmitri Indeitsev, Institute for Problems in Mechanical Engineering (Russia)
V. Alekseev, Institute for Problems in Mechanical Engineering (Russia)
Yu Mochalova, Institute for Problems in Mechanical Engineering (Russia)
The phenomenon of trapped modes (standing waves) near the underwa-ter constructions has been studied. The influence of construction dynamic on the process of trapped modes formation has been studied. It has been also shown that existence and a number of resonance frequencies essentially depend on elastic construction and channel parameters. The influence of liq-uid nonhomogenity on the initiation of trapped modes has been investigated separately. It is shown that only for specific gradient of liquid density the phenomenon of trapped modes can exist.
sv970447.pdf (Scanned) TOPAshley Ian Larsson, Defence Science and Technology Organisation (Australia)
The prediction of underwater acoustics is important to show the capabilities of a theoretical sonar system and improving sonar systems. Unfortunately under water acoustics is complicated and awkward to model - especially the sea floor acoustics. Often many assumptions are made in order to model the acoustics of the sea floor. Most under water models are covered under four categories, which are Normal mode theory, Ray theory, Parabolic equation and Fast field transform methods, but all require input of ocean floor acoustic properties. Those properties themselves are often poorly known and need to be estimated from geological data. There are several models which link seafloor geological parameters to acoustic properties - for example Hamilton(l) and Biot(2). For the Australian region the AUSEABED database Jenkins (3) provides data on the Iithology and other geophysical properties of the seafloor, from which acoustic parameters may be estimated Jenkins(4), Using this resource and a Normal mode model, transmission loss was calculated and compared with in
sv970353.pdf (Scanned) TOP
Graham D Furnell, Defence Science and Technology Organisation (Australia)
Graham J. Day, Defence Science and Technology Organisation (Australia)
The acoustical features of the shallow water environment can have a harsh effect on the performance of sonar systems. By no means the least important of these features are low grazing angle values of bottom reflection loss, which can largely determine transmission loss encountered. This paper presents the initial results of measurements of low grazing angle bottom loss conducted in the shallow waters of the Timor Seato the north of Australia. The measurements were performed over the frequency band 0.5-4 kHz via a technique which exploits the interference field created by the interaction between direct path and bottom reflected sound.
sv970338.pdf (Scanned) TOP
David G. Blair, Defence Science and Technology Organisation (Australia)
Jim Thompson, Defence Science and Technology Organisation (Australia)
Stuart Anstee, Defence Science and Technology Organisation (Australia)
It is intended to produce an underwater acoustic imaging system with three-dimensional images of resolution around 1 mm at 1 m range. For this purpose, sparse random arrays have considerable cost and feasibility advantages over filled arrays. There is no degradation of beamwidth, while the average distant sidelobe level (ADSL) can be reduced to a level expected to be satisfactory for imaging surfaces such as sea mines, although unsatisfactory for imaging continua such as human tissue. We have simulated the imaging of point targets by random arrays of elements. The simulation uses exact path lengths in the near field. Good range resolution is obtained through either a short toneburst or a cross-correlated chirped signal. It is confirmed that, as the array is made more sparse, the ADSL rises. For broadband signals, the ADSL is found to be markedly lower than the monofrequency value. The use of partially random arrays, constructed out of identical subarrays for ease of manufacture but oriented randomly, leads to an appreciable degradation in ADSL. For example, a system of 100 square subarrays with four subarray orientations resulted in an ADSL penalty of 5 dB.
sv970333.pdf (Scanned) TOPMarshall V. Hall, Defence Science and Technology Organisation (Australia)
Acoustic transmission losses (TL) measured at 16 Hz along a track in shallow water have been inverted to obtain seabed acoustic properties. The data are represented by an analytic function of range with two free parameters. The seabed is modelled as an equivalent uniform half-space. A uniform space has 5 unknowns: sound and shear speeds, sound and shear absorption coefficients, and density. The number of unknowns is reduced to three by using regression equations to relate density and shear absorption to sound-speed. By holding shear-speed fixed for subsets of the process, the number of unknowns is reduced to two (sound-speed and absorption). The parameters of the TL analytic function are computed over intervals of these unknowns by fitting outputs of the "Oases" mathematical model, and solutions of the resulting pair of simultaneous equations are sought. Finally, these solutions are expressed as functions of shear-speed, and criteria are presented for selecting the optimum results. The equivalent half-space is not necessarily related to the actual properties of the seabed and is liable to vary with frequency. It should however, produce correct results when an appropriate TL model is run at the original frequency, for arbitrary source and receiver depths and water-column conditions.
sv970328.pdf (Scanned) TOP
Sergey M. Grudskii, Rostov State University (Russia)
S.S. Mikhalkovich, Rostov State University (Russia)
The paper is devoted to the problem of sound propagation in stratified waveguide which is a non--homogeneous liquid layer overlaying multi-layered liquid-elastic bottom with the thin ise--floe of finite width laying on a surface. Influence of the ice--floe to characteristics of an acoustic field is investigated. For the problem solution the ice layer is replaced by equivalent in some sense condition on a surface of "defreesed ice". Problem with this condition can be easily reduced to the equation of convolution on a finite interval or, in Fourier images, to so-called modified Wiener--Hopf equation. To solve this equation we use a method of matrix Riemann boundary problem. It allows to construct asymptotic formulas for elements of reflection and transmission matrices by a small parameter, characterizing thickness of ice. These formulas are uniform on ice-floe width. Finally some numerical calculations are given to obtain quantitative estimates for so-called "critical" ice-floe lengths and to illustrate influence of absorption in ice on attenuation of acoustic field.
sv970262.pdf (Scanned) TOP
David N. Matthews, Defence Science and Technology Organisation (Australia)
Marshall V. Hall, Defence Science and Technology Organisation (Australia)
B.L. Jessup, Defence Science and Technology Organisation (Australia)
To accurately predict sound propagation in shallow water (12-60m) over a wide frequency band (5Hz-25 Khz) and at ranges up to 2000 m using a single acoustic propagation model is a difficult task. Probably the only model that claims to come near to fulfilling this requirement is OASES[1] although the maximum range achieved drops to only ~100 m at the higher frequencies. There are however a number of other models available, each of which has a "domain of applicability". This paper compares two of these models, a ray model (ISO-RAY)[2] and a normal mode model (STOKES) [3] , and assesses their useability over the frequency range stated above. In addition it also investigates the operational limitations of OAST (the transmission loss module of OASES), since this model is generally considered a benchmark in underwater acoustics. By combining the ray and normal mode model it was possible to nearly fulfil the above requirement .
sv970141.pdf (Scanned) TOP
Edwin R. Galbreath, Defence Science and Technology Organisation (Australia)
Ross L. Dawe, Defence Science and Technology Organisation (Australia)
This paper describes a method to extend the effective detection range of a pair of directional sonar arrays such as sonobuoys by cross correlating beamformed time series data. This technique effectively treats the sonobuoys as a field instead of independent sensors and gives a theoretical improvement in system detection threshold of 5log10(n)dB, where n is the number of sonobuoys. The time series output from the sonobuoy beams are compared and the resulting modulus of the normalised cross correlation coefficient is plotted on a map display overlaid with the buoy positions. Results using both simulated and experimental data indicate the technique can be successful in assisting the observer to detect and localise weak acoustic targets.
sv970056.pdf (Scanned) TOP
Patrick Yeung, The University of Hong Kong (Hong Kong)
Allen T. Chwang, The University of Hong Kong (Hong Kong)
Using a bubbly liquid is an economical and effective technique in controlling underwater sound signals. The sound speed in a bubbly liquid can be much different from that in a pure liquid and it has a very strong dispersive effect. When a sound signal passes through a bubbly layer, some of the energy is reflected and some is lost due to dissipation. Thus, the signal is attenuated. This property can be used to control underwater noise emitted from a submarine or an underwater construction site.In this paper, in order to apply this technique to real engineering problems, a harmonic point source enclosed by a cylindrical bubbly layer is considered. Due to the randomness and nonlinear behaviour of the mixture, rigorous models are hard to be developed. However, by assuming that bubbles are small in spherical shape and there is no interaction between the bubbles, classical theory for dilute bubbly liquid can be employed. Applying Taylors expansion and numerical integration, transmitted pressure fields through a thin cylindrical bubbly layer are obtained in an infinite domain, near a free surface, near a flat seafloor and in water with finite depth. It is found that if the bubbly liquid obeys the theory, the transmissim is very low when the incident frequency is higher than the natural frequency of the bubbles. This is called the Dead Zone of the bubbly liquid. There will be no propagating mode for an acoustic wave when it passes through the "Dead Zone.In addition, experimental results of a cylindrical bubbly layer with air volume fraction about 10% are obtained and compared with the theoretical results, lt is found that the experimental results agree with the theory if the incident frequency is near the bubble natural frequency. However, when the incident frequency is further increased there is a significant difference between experiment and theory. The transmission coefficient obtained in the experiment is much higher than that predicted by the dilute bubbly liquid theory. This seems that the Dead Zone does not appear in the experiments. The classical theory cannot be applied to this finite air volume fraction bubbly liquid. Discussion is given for the explanation of the phenomena and the design of bubbly layer on underwater sound control.
sv970021.pdf (Scanned) TOPDouglas H Cato, Defence Science and Technology Organisation (Australia)
The wide range of oceanic conditions around Australia result in wide variation in the characteristics and the levels of ambient sea noise, showing significant differences to observations of noise in the northern hemisphere. Contributions from independently varying sources result in variations in noise level of more than 30 dB with variations of up to 20 dB occurring commonly. Spectral shapes vary widely depending on the dominant sources at the time. Because of low levels of traffic noise in many areas, sea surface generated noise is often dominant at low frequencies (below about 200 Hz) in contrast to most northern hemisphere observations. Biological noise is usually dominant in tropical waters, except for conditions of high winds or heavy rain. Biological choruses that result when countless fish or invertebrates are calling are widespread in tropical and temperate waters, some showing regular diurnal variation. These choruses are often rise more than 20 dB above the background noise, Intense transient signals from whales also make significant contributions to the ambient noise.
sv970015.pdf (Scanned) TOPAlexey S. Besov, Lavrentyev Institute of Hydrodynamics (Russia)
The influence of magnetic field on the cavitation processes in pulse rarefaction waves in water is investigated experimentaly. It is shown, that water is transformed into anew state. This state is characterized by a temporary increasing of the threshold of cavitation inception and the significant influence of magnetic field on the dynamics of a bubbly cluster. The life time of new state of water is found to depend on the intensity of loading. The mechanism of the influence of a magnetic field on the cavitation ability of water is proposed. It is based on the change of pfysical properties of cavitation nuclei under pulse loading at magnetic field presence. The possible influence of hysteresis properties of water [2] is excluded by preliminary loading the water samples with the chosen amplitude shock wave. A rarefaction wave is produced by reflecting a plane short (a few microseconds) shock wave from the free surface of water [1]. For the detection of the threshold of cavitation inception and dynamics of bubbly cluster the variable capacitance transducer method [1] is used.
sv970283.pdf (Scanned) TOP