AU718492B2

AU718492B2 - Apparatus and method for the reduction of unwanted noise in hydrophones by use of differential displacement cancellation of the end-plugs

Info

Publication number: AU718492B2
Application number: AU40233/95A
Authority: AU
Inventors: Ronald John Wyber
Original assignee: MIDSPAR SYSTEMS Pty Ltd
Current assignee: MIDSPAR SYSTEMS Pty Ltd
Priority date: 1994-12-05
Filing date: 1995-12-05
Publication date: 2000-04-13
Anticipated expiration: 2015-12-05
Also published as: AU4023395A

Description

MIDSPAR SYSTEMS PTY, LTD ACN 002 506621 24 Farrer Place Oyster Bay NSW 2225 Commonwealth of Australia The Patents Act 1990 SPECIFICATION FOR THE INVENTION ENTITLED APPARATUS AND METHOD FOR THE REDUCTION OF UNWANTED NOISE IN HYDROPHONES BY USE OF DIFFERENTIAL DISPLACEMENT CANCELLATION OF THE END-PLUGS APPARATUS AND METHOD FOR THE REDUCTION OF UNWANTED NOISE IN HYDROPHONES BY USE OF DIFFERENTIAL DISPLACEMENT CANCELLATION OF THE END PLUGS A towed array comprises a number of hydrophone sensors which are towed behind a ship. a submarine or a helicopter to detect acoustic signals in the water. The water flowing past the array induces noise into the hydrophone which reduces the performance of the array. Additional noise may be generated by vibration propagating along the array which couples into the hydrophone. This vibration may be generated by motion at the head or tail of the array or induced by the flow of water past the array.

A hydrophone in the array consists of one or more acoustic receiving elements mounted in a tube with end-plugs which is filled with a material which may be a fluid or other material which allows the acoustic signal external to the array to be coupled to the receiving elements.

The present invention relates to a method of reducing the noise at the hydrophones by the use of active transducers and sensors which measure the displacement at the end-plugs and apply a compensating displacement to reduce the noise or vibration caused by this effect.

SThis invention differs from other methods of active noise cancellation in that it uses the differential displacement between the two end-plugs and applies a displacement to one or two end plugs to provide Scancellation of this displacement.

In one aspect this invention discloses a hydrophone sensor for movement through water or other fluid medium to detect acoustic signals theron, said hydrophone sensor comprising an elongated enclosed vessel t with end plugs and an interior compartment, one or more acoustic receiving elements disposed within said interior compartment and sound conveying means within said interior compartment for conveying an acoustic signal external of said hydrophone sensor to said one or more acoustic receiving elements, wherein means are provided for determining the differential displacement arising between said end-plugs as said hydrophone passes through said water or other fluid medium to enable generation of a S: compensating displacement to be applied to said end-plugs to cancel or reduce unwanted noise or vibration caused by said displacement differential.

S In another aspect this invention also discloses a method of reducing unwanted noise induced in said hydrophone by water or other fluid medium flowing around the outside thereof, or vibration excitation, S said method including the steps of: S a. determining the displacement between said end-plugs by subtracting the output of displacement sensors thereon, and b. applying a compensating displacement of opposite sign to one or both end-plugs to provide simultaneous cancellation or reduction of the component of the differential displacement at each end-plug.

This arrangement allows reduction of noise from the following sources: a. Noise induced by pressure fluctuations acting on the array jacket exterior to the hydrophone associated with the turbulent boundary layer generated by the movement of the array through the water.

b. Noise induced by vibration acting on the end-plugs of the hydrophone c. Noise induced by vibration causing constriction of the hydrophone tubing due to differential movement of the end-plugs.

d. Vibration propagating along the array generated by the pressure induced into the hydrophone which drives the end-plugs which are coupled to the array structure.

e. Noise coupled from the adjoining array section which is generated by the turbulent boundary layer pressure acting on that section Different embodiments of the invention are shown in figs. 1, 4 and *e So a a a.

*oo* *oo o In the first embodiment shown in fig. I an active transducer is used at each end of the hydrophone. The active transducers may be coupled to the end-plugs exterior to the hydrophone as shown in fig. 1. A displacement sensor is also placed on each end-plug and the outputs of these arc subtracted and used to drive the active transducers. The displacement sensors may measure the displacement directly or use either the acceleration or velocity differential followed by one or two integrators. Alternatively the output of the displacement sensors may be input to a digital signal processor which calculates the signals for the active transducers. This cancels the differential displacement between the end-plugs so that the movement of endplug B matches that of endplug A. The interconnection of the sensing elements and the active transducers are shown in fig. 2. The alternative interconnection using a digital signal processor is shown in fig. 3 In the second embodiment shown in fig. 4 the active transducers are either internal to the hydrophones adjacent to the end-plugs, integrated with the end-plugs or comprise the end-plugs. This arrangement operates in the same manner as the first embodiment except that the active transducers drive the hydrophone filling material directly instead of through the end-plugs.

In the third embodiment shown in fig. 5 an active transducer is used at only one end-plug. This mail be placed internal to the hydrophone end-plug, external to the hydrophone end-plug.. be integrated with the hydrophonc end-plug or comprise the hydrophone end-plug. Displacement sensors are placed on both endplugs and the outputs of these are subtracted and used to drive the end-plug, The interconnection of the sensing elements and the active transducer using accelerometers is shown in fig. 6. Alternatively interconnections similar to figs 2 and 3 may be used.

Noise Reduction Mechanism 1. Turbulent Boundary Layer Noise The turbulent boundary layer pressure generated by the flow of water along the array may be represented as an ensemble of pressure waves, each of which propagates past the hydrophone with a different velocity.

These couple noise into the interior of the hydrophone with components known as a. The direct coupled term for which the internal pressure in the hydrophone matches the external pressure with the addition of some filtering associated with the hydrophone tubing and array jacket.

b. The in-phase term which may be represented by a pressure source at each end of the hydrophone which is in phase with the external pressure at that point.

c. The out of-phase term which may be represented by a pressure source at each end of the hydrophone which is in phase with the external pressure at one end of the hydrophone and out of S* phase at the other end.

The in phase term may be reduced by cancellation of any differential displacement between the end-plugs.

g: 2. End plug Vibration Energy propagating in the array will drive the end-plugs of the hydrophone generating a pressure in the hydrophone. Cancellation of the differential displacement of the end-plugs will result in the end-plug remote from the vibration generating an out of phase pressure which will tend to cancel noise from this source.

e 3 Jacket Constriction S Differential vibration between the end-plugs will result in a stretching of the hydrophone tubing and also of the array jacket if this is coupled to the end-plugs. This results in a circumferential constriction which generates a pressure in the hydrophone. Cancellation of the differential displacement will tend to cancel noise from this source 4 Vibration Generation As well as generating a pressure source in the hydrophone the in phase and out of phase pressure terms impose a force on the end-plugs This generates a propagating vibration wave which couples noise into the other hydrophones in the array. By cancelling these pressures in the hydrophone this driving force is also cancelled resulting in a reduction in the flow induced vibration noise.

6 Pressure Coupled from Adjoining Array Sections When the external turbulent boundary layer pressure acts on a section of array adjoining a hydrophone a pressure is induced due to the movement of the end plug of the hydrophone. By cancelling the differential displacement of the end plugs a displacement is induced into the other end plug which generates a pressure which is out of phase with this pressure. Provided the hydrophone elements are symmletric this results in cancellation of the effect of the pressure at the two end plugs reducing the noise coupled from the adjoining array sections.

S.*

S*

f *ft ot f *o o **fttf ft, It will be appreciated that this invention at least in the form of the embodiments disclosed provides a novel and unique improvement in hydrophone sensors which substantially improves their performance.

Clearly however the examples described are only the currently preferred forms of the invention and a wide variety of modifications may be made which would be apparent to a person skilled in the art. For example the shape and configuration of the hydrophone vessel and placement of the various components therein may be changed according to application or design preference. In addition the displacement could clearly be determined by measuring the velocity or acceleration rather than by a direct measurement of displacement.

o 9.

9 9 9 9 9 9 9 9 o 9* 9 o* 9 9 9* 9* 0* *9 *9 0* 9.* 9 9 9 9 9

Claims

2. The hydrophone sensor as claimed in claim 1 wherein said means for determining the differential displacement arising between said end plugs includes active transducers disposed one at each end plug and displacement sensors disposed one in association with each end plug whereby the output *eof said displacement sensors can be subtracted and used to drive said active transducers to generate said compensating displacement. S3. The hydrophone sensor as claimed in claim 2 wherein said displacement sensors sense the displacement of said end plugs directly. So 4. The hydrophone sensor as claimed in claim 2 wherein said displacement sensors use either the acceleration or velocity differential of said end plugs followed by one or two integrators. 9*9
5. The hydrophone sensor as claimed in claim 2 wherein the output of said displacement sensors is S* adapted for input to a digital signal processor which calculates signals to drive said active transducers.
6. The hydrophone sensor as claimed in any one of claims 1 to 5 wherein said active transducers are disposed external of said end plugs and said compensating displacement is applied through said end plugs to said sound conveying means. 9
7. The hydrophone sensor as claimed in any one of claims 1 to 5 wherein said compensating o displacement is applied directly to said sound conveying means and said active transducers are disposed inside of said end plugs or are integrated with said end plugs or comprise said end plugs.
8. The hydrophone sensor as claimed in claim 1 wherein the outputs of said displacement sensors are subtracted and used to drive a single active transducer to generate said compensating displacement and said means for determining the differential displacement arising between said end plugs comprises said displacement sensors which are disposed one at each end plug and said single active transducer which is inside of one of said end plugs or is outside of one of said plugs or comprises one of said end plugs.
9. A hydrophone sensor for detecting an acoustic signal in water or other fluid medium through which it is moved, said hydrophonic sensor being substantially as described herein with reference to figures 1 to 3 or figure 4 or figure A towed array which includes at least one hydrophone sensor as claimed in any one of claims I to 9.
11. A method of detecting an acoustic signal in water or other fluid medium which includes the following steps: procuring a hydrophone sensor comprising an elongated vessel which defines an interior compartment between end plugs, one or more acoustic receiving elements disposed within said interior compartment and sound conveying means within said interior compartment for conveying said acoustic signal to said one or more acoustic elements wherein means are provided for determining the differential displacement arising between said end plugs as said hydrophone sensor passes through said water or other fluid medium to enable generation of a compensating displacement to cancel or reduce unwanted noise or vibration in said acoustic signal caused by said differential displacement, rendering said hydrophone sensor operative to detect said acoustic signal. moving said hydrophone sensor through said water or other fluid medium. determining the differential displacement between said end plugs, using the differential displacement to generate said compensating displacement, and applying said compensating displacement to cancel or reduce said unwanted noise or vibration in said acoustic signal caused by said differential displacement. S 12. The method as claimed in claim 11 including the further step of moving said hydrophone sensor through said water or other fluid medium as part of a towed array.
13. The method as claimed in any one of claims 11 or 12 wherein said step of using the differential displacement to generate said compensating displacement includes subtracting and using the ***too output of displacement sensors to drive at least one active transducer.
14. The method as claimed in claim 11 or 12 wherein said step of using the differential displacement to generate said compensating displacement includes using a digital signal processor to calculate signals from the output of displacement sensors to drive at least one active transducer. *ss 0* so