WO1991019996A1

WO1991019996A1 - Low-frequency signal acoustic detection method

Info

Publication number: WO1991019996A1
Application number: PCT/FR1991/000443
Authority: WO
Inventors: Michel Lagier
Original assignee: Thomson-Csf
Priority date: 1990-06-15
Filing date: 1991-06-04
Publication date: 1991-12-26
Also published as: FR2663489A1; FR2663489B1; AU7964391A

Abstract

Acoustic field intensity meters (102, 103) placed on either side of a source of interference (100) are used instead of hydrophones and their output signals are added, whereby the external noise signal can be obtained while interference from the source to which the field intensity meters are connected is substantially suppressed. In particular, the method provides for improved detection of naval vessels.

Description

METHOD FOR ACOUSTIC DETECTION OF LOW FREQUENCY SIGNALS

The present invention relates to acoustic detection systems which make it possible to detect signals at low or even very low frequencies which propagate within the sea. It is known that current submarines are designed to be extremely noisy in order to precisely to limit the possibilities of detection by listening to radiated noise by these submarines. Taking into account the precautions which are taken, one notes that the only frequencies emitted at a level allowing a possible detection are located at the bottom of the spectrum, below 100 Hz, even of 10 Hz. These noises correspond essentially to the fundamental of machines and resonances of the structure, as well as some higher harmonics. To detect and locate the noise source at these frequencies, it is not possible to make an acoustic antenna with dimensions smaller than those of the submarine, which leads to the use of towed linear antennas, called flutes.

In addition, the use of conventional sensors, that is to say hydrophones, proves impossible at these frequencies since the noise radiated by the tractor building of the flute is of the same nature as the noise to be detected and therefore masks this latest.

To overcome these problems, the invention proposes a method for acoustic detection of low frequency signals in the presence of a source of parasitic noise, characterized in that at least two intensimeters are used placed on either side of said source and that we sum the signals delivered by these intensimeters, which substantially eliminates parasitic noise in the signal resulting from this summation. Other features and advantages of the invention will appear clearly in the following description given by way of nonlimiting example with reference to the appended figures which represent: - Figure 1, a schematic view of a submarine and a noise source;

- Figures 2 and 3, sectional views of an intensimeter;

- Figure 4, a top view of a submarine provided with intensimeters;

- Figure 5, a sectional view of an intensimeter fixed to a shell;

- Figure G, a side view of a shell provided with a set of intensimeters; and FIG. 7, a diagram of a system for processing the signals received by the intensimeters.

There is shown in Figure 1 a very schematic section of the hull 100 of a submarine taken in a plane perpendicular to the longitudinal axis of this submarine. An acoustic source 101, located for ease of explanation in the section plane, is located at a certain distance from the submarine with an elevation angle relative to the horizontal. Two intensifiers 102 and 103 were placed on either side of the submarine hull on a horizontal diameter which are acoustically decoupled from this hull. Remember that intensimeters are devices that measure acoustic intensity, not pressure like hydrophones.

At the frequencies considered, the submarine shell vibrates in phase and therefore this shell contracts and expands according to the arrows shown in the figure, at the frequency of the acoustic waves it emits corresponding, as we said, to the vibrations due machines than structural resonances. Under these conditions, and since the intensimeters are decoupled from this shell, they measure acoustic intensities corresponding to waves which move away from the shell in two diametrically opposite directions for the two intensifiers. Thus the sum of the acoustic intensities measured by these intensimeters and coming from the hull of the submarine is zero.

On the other hand, for the distant noise source 101, this intensity is not zero, since it can be considered that from a certain distance the paths between this source 101 and the intensimeters 102 and 103 are substantially parallel and the distances substantially equal. The acoustic intensities at the locations of the intensimeters are therefore substantially equal in the same direction and in the same direction. If we therefore add the measurements from the two intensimeters, the signals from the submarine subtract and cancel each other while the signals from the distant source add and strengthen.

The system according to the invention therefore makes it possible to eliminate the noises specific to submarines in order to keep only the noises coming from external sources to be identified;

As an intensimeter, it is possible to use, for example, a geophone type sensor such as that described in French patent n ° 87 0G990 filed on 19/5/87 in the name of the applicant and which is shown diagrammatically in Figures 2 and 3. The figure 2 represents a half-section in a vertical plane passing through the axis oz and FIG. 3 a section in a horizontal plane xoy represented by the dotted line A- A 'in FIG. 2. This sensor comprises a housing formed by a cylindrical tube 200 closed by two circular flanges 201 and 202. An inertial sphere 203 is enclosed inside this housing and is held substantially in the center of it by six bimetallic strips 204 to 209 made of piezoelectric polymer which are supported by their central part on the spherical inertial mass. Each bimetallic strip is prestressed in bending by pressing at its ends on the inside of the housing by means of stops such as 211 and 210. The two bimetallic strips 204 and 205 corresponding to the oz axis are connected to a differential amplifier 212 which delivers an output voltage Vz substantially proportional to the movements of the inertial mass inside the housing and which therefore measures the speed of movement of the housing under the effect of the acoustic waves which move the intensimeter along the axis oz. Two other differential amplifiers connected to the other bimetallic strips enable the output voltages Vx and Vy to be obtained respectively and correspond to the speeds along the axes ox and oy.

We know that the acoustic intensity is the scalar product of the acoustic pressure by the acoustic speed measured at the same point. To obtain the sound pressure, for example, a ceramic tube is used which forms part 200 of the intensimeter housing. This ceramic tube is connected to an amplifier 213 which delivers the value of the sound pressure at the level of the intensimeter, without reference to any direction. Known electronic circuits make it possible to obtain the three acoustic intensities in the three directions xyz from the signals P, Vx, Vy and Vz.

To obtain a sufficient signal, and also to be able to form directional channels in reception, such intensimeters are placed on both sides of the submarine all along its sides, as shown in FIG. 4 where particularize the two intensimeters 102 and 103 already shown in FIG. 1.

As described above, it is necessary to decouple the intensimeter from the submarine hull. It is, however, of course necessary for the intensimeter to be maintained relative to the submarine and moreover the additional drag caused by these devices must be minimized. For this, for example, as shown in FIG. 5, it is possible to overmold these intensimeters in a dome 300 made of elastomeric material which is shaped to be as hydrodynamic as possible. This dome is itself glued to the hull 100 to fix the intensimeter 102 on the submarine. Inside the dome was also placed the electronics 301 for processing the signals from the intensimeter. The output connections 302 of this electronics leave the dome to be grouped on a rail which leads to a sealed crossing of the hull making it possible to group these connections on the processing assemblies located inside the submarine.

The elastomeric molding material used to make the dome 300 is for example polyurethane. Such a material is known for making sonar domes of all kinds. Its very low shear coefficient makes it possible to obtain the desired decoupling between the intensimeter and the hull.

As a variant represented in FIG. 6, when the submarine is provided with passive flank antennas, known under the name of "flank array", and formed from piezoelectric panels 401 embedded in an elastometer coating 400 itself glued to the sides of the submarine, it is advantageous to drown the intensimeters 102 in this same coating which will form a boss at this location. This boss will of course be hydrodynamic in shape.

The intensimeters placed on the port side and the intensimeters placed on the starboard side therefore constitute two groups which can be compared to the two intensimeters 102 and 103 in FIG. 1. The sources of low-frequency noise located at a distance are then detected by summing the intensities of these two groups then adding the two signals thus obtained. To form channels W directives both site as shown in Figure 1, that deposit Θ as shown in Figure 4, this is carried out ^• summing known manner using the projections of the intensities on the steering axis θ, that is to say by multiplying these intensities by the sine 'or the cosine of the angle considered according to the component x, y or z used.

By calling I. " the real part of the intensity corresponding to the sensor i and to the direction D, this processing consists in carrying out the sum:

NOT

(1) Σ α. I = _W i = l ^ll DD In this formula N is the total number of the intensimeters and the d. represent coefficients which optimize in a known way the signal to noise ratio.

To obtain these coefficients, we start from the following two relations: _r -, = 0

(2)

(3) N

Σ o. = G i = l

The relation (2) is obtained in the absence of external signal and the relations (3) with the external signal only, G being a gain established starting from the profits of the electronic chains of processing.

As there are so many signals. that there are ways we get, if M is the number of ways, a linear system of M + 1 equations. This system can be reversed by choosing N = M + 1. Another calculation method consists in minimizing the relations (2) starting from the initial value provided by the relation (3).

FIG. 7 shows a block diagram of the system which makes it possible to obtain from the intensimeters 701 to

70N the channel signals W ^ to _DM .

The signals representing the pressure P and the values of the velocities V x, V y and V z are applied to a circuit 710 which makes it possible to digitize and multiplex them over time. AT starting from the signals thus digitized and multiplexed, this same circuit performs a multiplication and an integration which performs the operation:

In this formula the parameter k represents x, y or z.

The calculation of the intensities I— is carried out in a circuit 711 where the direction of the path W ,, 1 varying from 1 to M is introduced sequentially. This calculation corresponds to a simple geometric calculation known elsewhere in the state of art.

By taking for example directions only defined in the horizontal plane, that is to say only in bearing, the direction of the channel W. is identical to θ, and the operation corresponds to:

(5) ^l i _θl = 1 II cos (φ - _θ] )

In this formula is given by the expression:

< ⁶ > tg φ = LX ix and the modulus of vector I by the expression:

(7) I - GI ix ² . I ι.y ² - I i.zV- '

The reference axis in these calculations is the x axis.

Having thus obtained the values -., We then carry out in a calculation circuit 712 from the values of the coefficients. determined by calibration as seen above, the calculation of the channel signals _D1 to W _™ . by applying formula (1). It will be noted more particularly that the process thus described allows effective filtering of flow noises, also known under the name of pseudo-sounds, since this only concerns the imaginary part of the acoustic intensity whereas, as we saw, the calculations relate only to the real part of this acoustic intensity.

Claims

1. Method for acoustic detection of low frequency signals in the presence of a source of parasitic noise (100), characterized in that at least two intensimeters (102, 103) are used placed on either side of said source and that we sum the signals delivered by these intensimeters, which substantially eliminates parasitic noise in the signal resulting from this summation.

2. Method according to claim 1, characterized in that two sets of intensimeters (102, 103) are used aligned on either side of the source of parasitic noise

(100) and that the signals delivered by these intensimeters are processed so as to form directional channels.

3. Method according to any one of claims 1 and 2, characterized in that the intensimeters make it possible to determine the acoustic intensity in the three directions

(Oxyz) of a reference trihedron and that the processing of the signals provided by these intensimeters makes it possible to determine channels in bearing (θ) and in site (φ).

4. Method according to any one of claims 1 to 3, characterized in that the intensimeters (102,

103) to the source of parasitic noise (100) while acoustically decoupling them from this source.

5. Method according to claim 4, characterized in that this fixing without elastic coupling is made using an overmolding of polymer material with low shear constant.

6. Method according to any one of claims 1 to 5, characterized in that a geophone type intensimeter (200-211) is used making it possible to obtain the acoustic speed in the 3 directions (Oxyz) and a sensor for pressure (200) to obtain the acoustic pressure (P) and that obtains the desired acoustic intensity by carrying out the scalar product of the acoustic pressure by the acoustic speed.