GB1560681A

GB1560681A - Sound source detecting apparatus

Info

Publication number: GB1560681A
Application number: GB4701162A
Authority: GB
Original assignee: CSF Compagnie Generale de Telegraphie sans Fil SA
Current assignee: Thales SA
Priority date: 1961-12-28
Filing date: 1962-12-12
Publication date: 1980-02-06
Also published as: FR1605544A; NL160393C; NL160393B; DE1441485C1

Description

(54) SOUND SOURCE DETECTING APPARATUS (71) We, CSF - COMPAGNIE GÉNÉRALE DE TÉLÉGRAPHIE SANS FIL, a French Body Corporate, of 79, Boulevard Haussmann, Paris, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to apparatus for detecting the sources of sound, for example, submarine sources of sound.

According to the invention there is provided apparatus for use in providing an indication of the bearing and frequency of a source of sound, the apparatus comprising: an acoustic head having two omnidirectional hydrophone dipoles superposed in crossed relationship; and electronic means for detecting respectively phase differences between signal voltage outputs from the two poles of each dipole, and for applying voltages proportional respectively to such phase differences to the deflection plates of an oscilloscope tube.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 illustrates the propagation of a plane wave towards an acoustic dipole; Figure 2 illustrates, very diagrammatically, how the bearing and the frequency may be displayed on the screen of an oscilloscope tube; Figures 3a and 3b represent, respectively in elevation and in plan, an acoustic head suitable for use in an apparatus embodying the invention; Figure 3c shows a detail of Figure 3a; Figure 4 is a block diagram of an apparatus embodying the invention and including the acoustic head of Figures 3; Figure 5 illustrates how ambiguity as to direction is removed by means of the apparatus; and Figure 6 shows the screen of an oscilloscope tube forming part of the apparatus.

The operating principle of the apparatus will be first set forth.

Figure 1 shows two identical hydrophones 1 and 2, arranged as an acoustic dipole. They are omnidirectional and their axes are normal to the plane of the figure.

A plane wave, propagating along the direction A and forming an angle 6 with a line perpendicular to the base 1.2 of the acoustic dipole, reaches hydrophones 1 and 2, respectively at instants tl and t2, 6 being the bearing angle of the sound source in the example considered.

At instant tl, the front of the plane wave in the plane of the figure 1 lies along line 1.21, forming angle 0 with base 1.2.

The time elapsed between the instant t1 at which the wave has reached hydrophone 1 and the instant t2 at which it has reached hydrophone t2 is : t2 - t1, which is equivalent to a phase difference of 2:: dc . f sin 0; where d is the length of base 1.2, f the frequency of the wave transmitted and c the velocity of propagation.

In Figure 2 are shown the dipole 1.2, the base of which may be, for example, normal to the axis of the boat carrying the apparatus, and the dipole 3-4 at right angles with dipole 1.2.

Figure 2 is a simplified diagram of the bearing determining apparatus of Figure 4, which also measures the frequency of the sound received. In this apparatus, a phase detector 1.2 provides an output voltage which is proportional to the phase difference between the waves, received by hydrophones 1 and 2, and a similar deivce 3 4 does the same for hydrophones 3 and 4.

The two output voltages thus obtained are applied to the pairs of plates 28.1, 28.2 and 28.3, 28.4 of an oscilloscope tube 28.

In the absence of the voltage applied to the plates the spot is at the center of the screen.

In the presence of the phase difference measuring voltages, the spot is deflected along a vector cp,,, shown as being parallel to 1.2, proportionally to the phase difference measured at dipole 1.2 and along a vector 3 4, shown to be parallel to 3.4, proportionally to the phase difference measured at dipole 3.4.

These two vectors are respectively proportional to 91.2 i.e. to t2 - t and consequently to f Sin 0, and to 3.4 i.e. to t4 - t3 and consequently to f Cos 6.

Accordingly, the location on the screen of the spot corresponding to the detected sound source has a module proportional to frequency f of the acoustic wave while its argument is the bearing H of this source.

Figures 3a, 3b, 3c show the acoustic head of an apparatus embodying the invention, and Figure 4 shows a block diagram form the electronic circuit thereof.

The acoustic head comprises two dipoles at right angles to each other, as shown in elevation in Figure 3a, in the plan in Figure 3b and, on an enlarged scale in elevation, in Figure 3c.

In each dipole, one of the poles comprises two hydrophones, 1 and 1' or 3 and 3', which have the same vertical axis and are connected in parallel.

Figure 3c shows the spacing between the hydrophones 1 and 1' and the spacing of the vertical axis of pole 1 - 1' with respect to the vertical axis of hydrophone 2.

These spacings are, for example, 200 mm and 37.5 mm. The acoustic head is located in a cylindrically shaped dome, having a diameter of 250 mm, and topped by a spherical portion, the total height of the dome being thus 500 mm.

As will be shown later in the description, these spacings enable the decorrelation of clutter noises between the two poles of each dipole. If the spacing between the two hydrophones of the same pole 1-1' is too large, this pole has its own directivity, and is no longer omnidirectional. A spacing of 200 mm gives a directivity of + 10 in the vertical plane at 3 d Bs below the maximum, at a frequency equal to 10,ode cs, and of + 22 at a frequency equal to 5,000 cs.

The signals derived from the sounds received by the pole comprising hydrophones 1, 1', are amplified in amplifier 11 and those from hydrophone 2 in amplifier 21. The amplified signals are modulated in modulators 12 and 22, which may be, for example, single side band modulators. The carrier is provided by an oscillator 5, whose frequency is, for example, 30 kc/s. A phase-shifter device 6 provides a phase-shift ofwbetween the carrier waves, respectively applied to modulators 12 and 22.

The output signals of modulators 12 and 22 are amplified and clipped in channels 13 and 23 respectively and phase detected in a phase detector 7. The output signal of unit 13 (or 23) has, consequently, a substantially rectangular wave-form, comprising two alternating levels which correspond respectively to the two alternations of the input sinusoidal wave.

The phase difference between two sine waves, can be derived after clipping and amplifying, from the time interval during which the corresponding levels have the same value and this is the purpose of the phase detector 7.

The output voltage of this phase detector is a d.c. voltage which is a linear function of said phase difference. Taking into account thesphase shift introduced by the phase shifter 6, this voltage is equal to zero for a phase difference between the sine waves equal to Amplifier 25 amplifies the signal before it is applied to the oscilloscope tube 28.

The circuit connected to dipole 3-4 is entirely similar. It comprises amplifiers 41 and 31, modulators 42 and 32, clippers 43 and 33, a phase detector 8 and an amplifier 27.

Switches 19 and 20 normally connect attenuators 24 and 26 before amplifiers 25 and 27 respectively. If desired, for enlarging the scale of the vector , for example at relatively low frequencies, attenuators 24 and 26 may be disconnected by switches 19 and 20.

The operation of the apparatus shown in Figure 4 will now be described. The sound wave received has a frequency f comprised, for example, between 2 000 and 10 D00 c/s and its direction of propagation is, for example, substantially horizontal.

The dipole comprising hydrophones 1, 1', and 2 will be considered first. It will be noted that in the case of a plane wave derived from a distant source, the sum of the signals received by hydrophones 1 and 1' is equal to that which would be received by a single hydrophone, located midway between hydrophones 1 and 1'.

The spacing between hyrophones 1 and 1' is sufficiently high for the sea clutter noise signals received by hydrophones 1 and 1' not to be correlated with those received by hydrophone 2.

The same is done for the other dipole.

It may be shown, that, for a sufficiently loud sound, with the above indicated phase-shift bym the output signals of phase detectors 7 and 8 are proportional respectively 2 to: 4 f. disc. sin 6 and 4 f. d/c. cos 6 where: d: the horizontal spacing of the two poles of each dipole; c: velocity of the incident wave; H: bearing angle of the sound source.

It may be shown that there will be no ambiguity as to the position of the source if d ch being the shortest wavelength of the received signals. However, distance x is too small for preventing any correlation of the noise signals received by the two poles of a normal dipole.

This is why one of the poles of each dipole is here made up of two similar hydrophones which are spaced from each other.

In the apparatus shown there is in fact no ambiguity.

This is so, because, the combination of the output signals of the two correlators 7 and 8 makes it possible to identify the quadrant which corresponds to the location of the source of sounds; since one of these signals depends on sin 6 and the other on cos 6 as shown in Figure 5.

Accordingly the following chart may be drawn: Output of Output of Quadrant where the Correlator 7 Correlator 8 source is located + + 1 + - 2 3 + + 4 The position of the sound source may be determined both in bearing from the ship and in true bearing. If this latter positioning is desired, a bearing corrector system corrects the output of phase detectors 7 and 8 by the bearing of the ship. This system is very diagrammatically shown at 14 to 18 and comprises a computor 38. It is well known in the art and therefore need not be described in this application.

Switches 9 and 10 are provided for connecting the output of phase detectors 7 and 8 to the oscilloscope tube 28 either directly or through the bearing corrector system.

Up to this point, it was assumed that the information obtained as to the direction of the incident wave was not influenced by the reflection of the incoming signals on the hull of the boat or any interfering action of the radioelectric assembly.

To minimize this perturbing action the acoustic head is located as far as possible from the hull which may reflect the incoming sonic signals. It will be placed, for example, ahead of the hull or above it.

As to the field distortion due to the dome and to the hydrophones themselves, the resulting error is reduced due to the symmetry of the acoustic head.

It is preferred to keep the modulated carrier feed to the clippers at a constant level of the order of 10 volts, for example by means of a compressor device with filtering of the harmonics.

On the whole, the total error in bearing will generally not exceed + 2" and may even be lowered to about + 1".

Figure 6 shows the screen of the oscilloscope tube upon which three signals A, B and C are received.

The directivity pattern of the apparatus of Figures 3 and 4 is more rigorously circular than that of sound source detecting hydrophone arrangements used hitherto.

WHAT WE CLAIM IS: 1. Apparatus for use in providing an indication of the bearing and frequency of a source of sound, the apparatus comprising: an acoustic head having two omnidirectional hydrophone dipoles superposed in crossed relationship; and electronic means for detecting respectively phase differences between signal voltage outputs from the two poles of each dipole, and for applying voltages proportional respectively to such phase differences to the deflection plates of an oscilloscope tube.

2. Apparatus according to claim 1, wherein one pole of at least one of said hydrophone dipoles comprises two hydrophones having a common axis and spaced apart one from the other along said axis so as to make possible a decorrelation of clutter noises between the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

It may be shown, that, for a sufficiently loud sound, with the above indicated phase-shift bym the output signals of phase detectors 7 and 8 are proportional respectively 2 to:

4 f. disc. sin 6 and 4 f. d/c. cos 6 where: d: the horizontal spacing of the two poles of each dipole; c: velocity of the incident wave; H: bearing angle of the sound source.

It may be shown that there will be no ambiguity as to the position of the source if d ch being the shortest wavelength of the received signals. However, distance x is too small for preventing any correlation of the noise signals received by the two poles of a normal dipole.

This is why one of the poles of each dipole is here made up of two similar hydrophones which are spaced from each other.

In the apparatus shown there is in fact no ambiguity.

This is so, because, the combination of the output signals of the two correlators 7 and 8 makes it possible to identify the quadrant which corresponds to the location of the source of sounds; since one of these signals depends on sin 6 and the other on cos 6 as shown in Figure 5.

Accordingly the following chart may be drawn: Output of Output of Quadrant where the Correlator 7 Correlator 8 source is located + + 1 + - 2

3 + + 4 The position of the sound source may be determined both in bearing from the ship and in true bearing. If this latter positioning is desired, a bearing corrector system corrects the output of phase detectors 7 and 8 by the bearing of the ship. This system is very diagrammatically shown at 14 to 18 and comprises a computor 38. It is well known in the art and therefore need not be described in this application.

Switches 9 and 10 are provided for connecting the output of phase detectors 7 and 8 to the oscilloscope tube 28 either directly or through the bearing corrector system.

Up to this point, it was assumed that the information obtained as to the direction of the incident wave was not influenced by the reflection of the incoming signals on the hull of the boat or any interfering action of the radioelectric assembly.

To minimize this perturbing action the acoustic head is located as far as possible from the hull which may reflect the incoming sonic signals. It will be placed, for example, ahead of the hull or above it.

As to the field distortion due to the dome and to the hydrophones themselves, the resulting error is reduced due to the symmetry of the acoustic head.

It is preferred to keep the modulated carrier feed to the clippers at a constant level of the order of 10 volts, for example by means of a compressor device with filtering of the harmonics.

On the whole, the total error in bearing will generally not exceed + 2" and may even be lowered to about + 1".

Figure 6 shows the screen of the oscilloscope tube upon which three signals A, B and C are received.

The directivity pattern of the apparatus of Figures 3 and 4 is more rigorously circular than that of sound source detecting hydrophone arrangements used hitherto.

WHAT WE CLAIM IS: 1. Apparatus for use in providing an indication of the bearing and frequency of a source of sound, the apparatus comprising: an acoustic head having two omnidirectional hydrophone dipoles superposed in crossed relationship; and electronic means for detecting respectively phase differences between signal voltage outputs from the two poles of each dipole, and for applying voltages proportional respectively to such phase differences to the deflection plates of an oscilloscope tube.
2. Apparatus according to claim 1, wherein one pole of at least one of said hydrophone dipoles comprises two hydrophones having a common axis and spaced apart one from the other along said axis so as to make possible a decorrelation of clutter noises between the

two poles of that dipole.
3. Apparatus as claimed in claim 2, wherein one pole of each said hydrophone dipoles comprises two hydrophones having a common axis and spaced apart one from the other along said axis so as to make possible a decorrelation of clutter noises between the two poles of each dipole.
4. Apparatus according to claim 1, 2 or 3, wherein said electronic means comprise in combination a local oscillator having a first and a second output. a phase-shifter which, when the apparatus is in use, will provide a phase-shift equal to 2 and which is coupled to said secound output and has a third output; a first and a second pair of modulators respectively coupled to said dipoles, the modulators of each pair being also coupled to said first and third outputs respectively, for providing modulated signals; and means, connected to receive the said modulated signals, for producing from the modulated signals voltages proportional respectively to such phase differences.
5. Apparatus according to claim 4, further comprising true bearing correction means for modifying said voltages proportional respectively to such phase differences.
6. Apparatus for use in providing an indication of the bearing and frequency of a source of sound, substantially as hereinbefore described with reference to the accompanying drawings.