Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1994-11-07
1996-05-14
Tudor, Harold J.
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
310337, 310800, 367162, 367165, 367166, H04R 1700
Patent
active
055174671
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an acoustic antenna for receiving low-frequency undersea waves.
Such an antenna is intended to detect and locate sources of undersea acoustic noise; in order to obtain good performance both in detection and in location, it is necessary to work over a low-frequency spectrum (by "low frequencies" will be understood frequencies lower than 2 kHz, typically lower than 1 kHz), and to make use of an antenna the gain of which is considerable so as to obtain a satisfactory signal
oise ratio (in numerous applications, a gain of 20 dB is necessary).
These two requirements (low frequencies and high gain) necessarily dictate antennae of considerable dimensions.
To that end, a first possibility consists in towing behind the naval vessel (ship or submarine) a streamer of hydrophones, thus forming a linear antenna of very great length.
Such a type of antenna may be much longer than the submarine and thus have very high performance at low frequency; it exhibits numerous drawbacks, however, in implementation (winch system, etc. and increase in the drag of the submarine) and above all a complete absence of directivity in the vertical plane by reason of the linear configuration of the streamer.
Another possibility consists in placing, over a large part of the length of the submarine, an antenna formed by an assembly of point sensors (hydrophones of small dimensions linked together in an appropriate way). It is thus possible to have available a two-dimensional array, which makes it possible to have directivity in the vertical plane and thus to locate the direction of the acoustic source in this plane.
This hydrophone-array antenna nevertheless exhibits a certain number of drawbacks: constituting the antenna acoustically with respect to the vibrations and resonances of the hull and of the attached structures of the submarine (especially vibrations and resonance originating from the machinery of the submarine) and from the hydrodynamic flow noise of the water on the sensors which, in the absence of appropriate decoupling, would produce a perturbing acoustic pressure masking the incident signal, generally of very low amplitude; through the hull for each sensor; badly withstand the hydrodynamic forces to which they are subjected, in addition to the fact that they often cause troublesome disturbance to the flow of streams of water along the hull of the submarine.
In order to remedy these various drawbacks, the intention proposes an undersea acoustic antenna no longer produced from a set of point sensors, but from two surface sensors, typically of several tenths of a square meter of pick-up surface area each.
The use of essentially surface sensors would make it possible, by direct integration effect, to mask the major part of the parasitic or flow noises mentioned above, which would always be more or less picked up, previously, with the antennae formed from an assembly of point sensors.
It will also be seen that the antenna of the invention, despite its very large dimensions, only very slightly disturbs the hydrodynamic behaviour of the submarine, and further offers excellent resistance to hydrodynamic stresses and to impacts.
To this end, according to the invention, this acoustic antenna for receiving low-frequency undersea waves, includes at least one surface sensor formed by a stack of conducting layers forming electrodes and of dielectric layers of piezoelectric material interposed between these conducting layers, this sensor being enclosed in a sheathing of flexible material, the assembly thus constituted forming an attached flat panel mounted against the wall of the hull of a naval vessel, especially of a submarine, this panel exhibiting a degree of freedom in bending so as to allow it to follow the shape of this hull.
According to a certain number of advantageous characteristics: respective electrodes of which are electrically linked in parallel, the set of elementary sensors being placed in a common leaktight sheathing. strip machined in such a way as to divide it into separate elementary pl
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Fichaux Robert
Fromont Bernard
"Thomson-CSF"
Tudor Harold J.
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