GB2227317A

GB2227317A - Infrasonic hydrophone signal correction arrangement

Info

Publication number: GB2227317A
Application number: GB8927435A
Authority: GB
Inventors: Robert Dechnik; Dean Gillies
Original assignee: Plessey Australia Pty Ltd
Current assignee: Plessey Australia Pty Ltd
Priority date: 1988-12-05
Filing date: 1989-12-05
Publication date: 1990-07-25
Also published as: JPH02269988A; FR2640106A1; GB8927435D0; DE3940198A1

Description

This invention relates to an infrasonic hydrophone correction control and

in particular it relates to a method and means for correcting errors which occur due to pressure fluctuations caused by vertical acceleration of the hydrophone which cause the performance of the hydrophone to be severely degraded at 5 infrasonic frequencies.

INFRASONIC HYDROPHONE CORRECTION CONTROL It will be realised that the acoustic signals to be measured are pressure signals and that vertical acceleration also produces a pressure signal, and when for instance a hydrophone device is suspended from a sonobuoy which floats on the surface of the ocean, the measuring hydrophone is subject to acceleration due to wave motion.

It is an object of this invention to correct the error caused by such acceleration. Correction is achieved according to this invention by using an accelerometer such as a pie;to ceramic element which is shielded from acoustic pressure, and a signal from this device is then applied through electronic means as a correction to the infrasonic signal.

The method of hydrophonic correction control according to this invention comprises suspending an acoustic sensor in the ocean by floatation means at a selected depth, receiving acoustic signals in the acoustic sensor, recording vertical movement of the acoustic sensor due to wave motion by means of an acceleration sensor, and applying the signal of the acceleration sensor to the signal of the acoustic sensor to remove the effect of the rise and fall of the acoustic sensor due to the wave motion.

Thus the method comprises incorporating acceleration related pressure recording means with the hydrophone, and canceling the signal of the acceleration pressure recording means from the output signal of the hydrophone.

The infrasonic hydrophone correction control device thus comprises an acoustic sensor adapted to be supported at a selected depth in the ocean from floatation means, a vertical acceleration sensor coupled to rise and fall with the acoustic sensor when the acoustic sensor rises and falls due to wave motion, means to shield the acceleration sensor from acoustic signals, and means to apply the 2 signal from the acceleration sensor to the signal from the acoustic sensor to cancel acceleration effects from the acoustic sensor output.

The device conveniently comprises an acoustically transparent encapsulant having a housing carrying an acceleration sensor which is shielded from acoustic pressure signals to read only vertical acceleration.

Thus the invention comprises a hydrophone unit wherein a hydrophone forms an acoustic pressure sensor and the output signal of which would contain an error signal caused by vertical acceleration related hydrostatic pressure at the hydrophone, characterised by means to sense acceleration pressure at the hydrophone, and by means to cancel the acceleration related pressure from the output signal of the hydrophone.

The actual method and apparatus can of course be widely varied within the spirit of the invention but to enable the nature of the invention to be fully appreciated an embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a schematic view illustrating the invention in general.

FIG. 2 is a block diagram showing the apparatus of the invention.

FIG. 3 is a schematic sectional view showing a hydrophone embodying the invention, and FIG. 4 is a circuit diagram showing a typical application of the invention.

3 Referring first to FIG. 1, to give the background of the invention, the following formula demonstrates the principles involved.

P= pgh = p2- Ph - Hydrostatic Pressure signal p - density of water S2 g gravitational constant h - instantaneous depth cc - acceleration of hydrophone s - Laplace complex variable The 1/S2 factor implies that this effect is extremely large at very low (infrasonic) frequencies, and rolls of at 40d131decade of frequency.

Referring now to FIG. 2 which shows schematically the arrangement of the invention, the acoustic sensor 1 produces a general pressure signal at 2 which is the voltage signal including the hydrostatic component, while the acceleration sensor 3 produces at 4 the hydrostatic component. The hydrostatic component is subtracted from the general voltage signal at 5 and this is passed to a pre20 amplifier 6 to give the corrected signal.

The hydrostatic canceling hydrophone 7 comprises an acoustically transparent encapsulant 8 for the acoustic sensor 1, but shielded within the housing 10 is the acceleration sensor 3 so that this sensor 3 measures only acceleration and 25 not acoustic pressure.

The correction'unit, including the pre-amplifier 6, can be housed in the extension 12 of the device and the hydrostatic cancelling hydrophone 7 is arranged to be suspended from a sonobuoy 11 or the like to be located at a 30 required depth in the ocean.

The circuit diagram of FIG. 4 shows a circuit which can be used to process the signal from the acoustic sensor 1 and the acceleration sensor 3 to produce the corrected signal.

From the foregoing it Will be seen that the solution to the problem of degraded 4 infrasonic signals lies in active signal cancellation and shows how an accelerometer may be used to measure the vertical acceleration of the hydrophone.

This signal from the acceleration sensor 3 is filtered and amplified to give the correct level, and is then subtracted from the actual pressure signal measured by the hydrophone. That is, if the measured pressure signal is Pr, where Pr = Ph+ PS PS - Required signal Ph -Hydrostatic contamination Then Ph is derived from the acceleration, and subtracted from Pr, to yield the required signal PS. This is illustrated diagrammatically in FIG. 2.

As explained the acceleration sensor 3, in the form of a centre piezoceramic element, operates as an accelerometer and is shielded from acoustic pressure. The acoustic sensor 1 preferably comprise two outer piezo-ceramic bender elements which act together as the acoustic sensor 1, whose outputs are combined in order to minimise inertial acceleration sensitivity, and maximise pressure sensitivity.

The electronic circuitry required to perform the amplification and filtering may be packaged in the same housing as illustrated, a suitable circuit configuration employing - 20d131decade pre-emphasis to the resultant signal as illustrated in 2 5 FIG. 4.

Claims

1. The method of infrasonic hydrophonic correction control which comprises incorporating an acceleration sensor with an acoustic sensor to form a hydrostatic cancelling hydrophone and cancelling the hydrostatic pressure signal of the acoustic sensor from the output of the hydrophone.

2. The method of infrasonic hydrophone correction control which comprises suspending an acoustic sensor by flotation means in the ocean at a selected depth, receiving pressure signals in the acoustic sensor, measuring vertical movement of the acoustic sensor due to wave motion by means of an acceleration sensor, and applying the signal of the acceleration sensor to the signal of the acoustic sensor. to cancel the effect of the rise and fall of the acoustic sensor due to wave motion.

3. The method of claim 2 wherein the acoustic sensor is housed in an acoustically transparent encapsulant, and the acceleration sensor is carried in a housing. - in the said encapsulant to be actuated by acceleration forces but shielded from acoustic signals.

4. The method of claim 3 wherein the signals from the acoustic sensor and the acceleration sensor are passed to electronic means to subtract the acceleration related pressure signal from the signal of the acoustic sensor.

5. An improved hydrophone correction control comprising a hydrophone and flotation means, characterised by an acoustic pressure sensor having as its output a general pressure signal which would contain an error signal caused by acceleration related pressures at the hydrophone, an acceleration sensor to sense the hydrostatic component of the hydrophone, and means to 30 cancelthe hydrostatic component related pressure from the output signal of the hydrophone.

1.

6 6. An improved hydrophone correction control embodied in a hydrostatic cancelling hydrophone according to claim 5 suspended from a sonobuoy

7. An infrasonic hydrophone correction control comprising an acoustic sensor adapted to be supported in the ocean at a selected depth, flotation means to support the acoustic sensor at the selected depth, an acceleration sensor arranged to record vertical acceleration housed within the acoustic sensor, a housing to shield the acceleration sensor from acoustic signals, and means to apply the signal from the acceleration sensor to the signal from the acoustic sensor ' to cancel acceleration effects from the acoustic sensor output signal, whereby to form a hydrostatic cancelling hydrophone, -

8. An improved hydrophone correction control according to claim 7 wherein the hydrostatic cancelling hydrophone comprises an acoustically transparent encapsulant housing the acoustic sensor, and in a hollow in the housing the acceleration sensor, whereby the acceleration sensor measures only acceleration and not acoustic pressure.

9. An improved hydrophone correction control according to claim 8 wherein the acoustic sensor comprises at least a piezo-ceramic bender element, and wherein the acceleration sensor is a piezo-ceramic element supported to flex under acceleration forces.

10. An improved hydrophone correction control according to claim 9 wherein the acoustic sensor comprises a pair of piezo-ceramic bender elements positioned to minimize inertial acceleration sensitivity.

11. An improved hydrophone correction control according to claim 8 wherein the acoustically transparent encapsulant houses electronic circuitry to perform amplification and filtering and subtraction to produce an acoustic pressure output signal only.

1 Z n 0

12. A method of infrasonic hydrophone correction control substantially as hereinbefore described.

13. A hydrophone correction control substantially as hereinbefore described with reference to the accompany ing drawing Published 1990 at The Patent Office, State House, 66171 High Holborn, London WC 1R 4TPFurther copies may be obtained from The Patent Mce.

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