GB2579672A

GB2579672A - A piezo-resistive hydrophone

Info

Publication number: GB2579672A
Application number: GB1820265.5A
Authority: GB
Inventors: Bond Andrew; Elliott Alan
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2018-12-12
Filing date: 2018-12-12
Publication date: 2020-07-01
Anticipated expiration: 2038-12-12
Also published as: GB2579672B; GB201820265D0

Abstract

The sound detecting sensor comprises a piezoresistive substrate 102 having conductive tracks 104'' thereon, wherein the conductive tracks comprises an ink having a plurality of conductive nanoparticles particles within a binder. The nanoparticles may comprise graphene, carbon nanotubes, silver, nickel or copper. The substrate may be covered with an impermeable membrane to allow for underwater use as a hydrophone. The conductive tracks are deposited to traverse the areas of maximum deflection for one or more modal frequencies of the substrate. A variety of track numbers and patterns are discussed.

Description

A Piezo-Resistive Hydrophone

Technical Field of Invention

This invention relates to a sound detecting sensor having conductive ink-based tracks on a substrate. In particular, though not exclusively, the invention relates to an ink-based piezo-resistive hydrophone or microphone.

Background

Hydrophones are microphones can be used in a liquid, typically water. Conventionally, hydrophones rely on piezo-electric or magnetostrictive sensors. These typically require materials with high density which can result in relatively heavy sonar arrays which may not be suitable for some applications. The excess mass may lead to higher power demands on batteries which can reduce operational periods of remote vessels. Further, the fabrication of traditional materials may restrict the possible configurations of sonar arrays, particularly larger arrays. This can limit the locations and positions they can be used in and the applications they can be used for.

The present invention seeks to provide a hydrophone which addresses at least some of the above mentioned problems.

Summary

The present invention provides a hydrophone and method of fabricating a hydrophone according to the appended claims.

The present disclosure provides a sound detecting sensor comprising: a substrate having a conductive track thereon, wherein the conductive track comprises an ink having a plurality of conductive particles within a binder.

The conductive particles may be nano-particles. The conductive particles may comprise one or more of: graphene, carbon nanotubes, silver, nickel and copper.

The sound detecting sensor may comprise a plurality of conductive tracks.

The substrate may have a first side and a second side, and at least one conductive track may be provided on both of the first and the second sides.

The conductive tracks may be arranged to detect the amount of deflection in the substrate. The substrate may have a modal shape having one or more areas of maximum displacements and the conductive tracks are located across at least one area of maximum deflection.

The conductive tracks may be arranged as longitudinal elements having a longitudinal axis, wherein the longitudinal axis extends radially from a peripheral edge of the substrate to a central region of the substrate. The conductive tracks may be straight or meandering. The meandering tracks may comprise portions of straight tracks connected in series to provide a meander.

The substrate may comprise a width, and the radial extent of the conductive track is at least 40% of the width in a single direction.

The sound detecting sensor may comprise at least a pair of conductive tracks. Each of the pair of conductive tracks may be provided on opposite sides of the central region of the substrate. There may be a plurality of pairs of conductive tracks.

The conductive track may comprise a branched conductive track having a central conductive track with one or more supplementary conductive tracks connected in parallel. A branched track may comprise a network of series connected parallel tracks. The conductive tracks may include one or more low frequency track and one or more high frequency track. The low and high frequency tracks may be determined by their placement on the substrate relative to the modal shape for different frequencies. There may be one or more central track and one or more dual line tracks.

The sound detecting sensor may further comprise between two and one hundred and seventy-seven conductive tracks. The sound detecting sensor may comprise eighty-eight sensors on either side of the substrate.

The substrate may be any suitable shape. The substrate may be polygonal and a first conductive track may extend from a corner region of the substrate. A second element may extends from an edge element of the substrate.

The conductive track may include a pair of legs connected in series, each leg extending from respective terminals. The pair of legs may comprise a dual line track. The legs may be arranged in a parallel relation to one another.

The sound detecting sensor may further comprise a plurality of substrates, each substrate may have a conductive track thereon. The substrates may be provided in a stack.

The sound detecting sensor may further comprise a damping mass. The damping mass may be located on or in the substrate. The mass may be located on the or each conductive track.

The substrate may be cantilevered from a support structure.

The substrate may have one or more peripheral edges, each of the peripheral edges being fixedly attached to a support structure so as to provide a constrained plate arrangement.

The conductive track may comprise a piezo-resistive material. The conductive track may have a piezo-resistive gauge factor of between 10 and 200. The upper range of the piezo-resistive gauge factor may be as high as 2000.

The sound detecting sensor may further comprise a water impermeable coating located at least over the conductive tracks such that the sound detecting sensor is operably submersible in water or saline. The water impermeable coating may be polyurethane.

The substrate material may be polyurethane.

Also described herein is a system comprising a sound detecting sensor as described herein and according to the disclosures above. The system may comprise: a resistance measurement system, wherein the or each conductive track is electrically connected to the resistance measurement system.

The resistance measuring system may comprise a wheatstone bridge.

Also described herein is a method of manufacturing a sound detecting sensor comprising: providing a substrate; determining the areas of maximum deflection for one or more modal frequencies and, depositing at least one conductive element on the substrate such that it traverses the areas of maximum deflection.

The method of manufacture may further comprise: coating the substrate in an impermeable membrane.

It will be appreciated that the examples provided within the description and above may be used in conjunction with one another where possible. Thus, for example, the methods of manufacture may include the manufacturing steps required for providing a stack of substrates. Many other combinations of the hydrophone features and methods of manufacture will be possible.

Brief Description of the Drawings

Figures la and 1 b show schematic representations of a hydrophone; Figure 2 shows a flow diagram representing some fabrication steps of a hydrophone; Figure 3 shows a deformation profile of a substrate; Figures 4a to 4j show various representations of hydrophone design; Figure 5 shows a schematic representation of a hydrophone stack; Figure 6 shows a schematic representation of a mass damped hydrophone; Figure 7 shows a schematic representation of a cantilevered hydrophone; Figure 8 shows a circuit layout of a bridge circuit which may be used with a hydrophone; and, Figure 9 shows a schematic representation of a vessel comprising a hydrophone.

Detailed Description

Figures la and 1b provide basic overviews of possible examples of hydrophones 100. In each case the hydrophone includes a substrate having a conductive track thereon.

The conductive track may comprise an ink having a plurality of conductive particles within a binder.

The substrate 102 is flexible and is typically provided by a thin sheet or membrane of suitable material. The substrate may be a plate-like member having opposing facing surfaces of which one will typically provide an incident surface against which the pressure waves are received, in use. The incident surface may be the uppermost surface as viewed in Figure 1. The substrate 102 may comprise any suitable substrate 102 upon which the conductive track 104 can be applied. The substrate 102 may be any particular shape as determined by a particular application, size or shape of the conductive track 104. In the example shown, the substrate 102 is a generally polygonal, e.g. rectangular when viewed in plan and is generally planar. The cross section of the substrate 102 is not shown, but the substrate 102 will typically be thin so as to provide the necessary pressure wave responsive deflection which allows the conductive track 104 to flex and the resistance to change to provide a measure of the change in resistance. It will be appreciated that the substrate 102 may be mounted to or form part of a larger structure in use.

The substrate may be attached to a peripheral support structure which allows the substrate to be suspended above a void. Thus, the substrate may be fixedly attached to a support structure so as to provide a constrained plate arrangement The substrate 102 may comprise any suitable material or combination of materials. The choice of substrate material may be influenced by the, for example, the choice of the conductive track material and the adherence, mechanical or electrical properties. For example, the dielectric properties and differential thermal expansion may be a consideration, amongst other characteristics. The material may be one or more of metallic or a polymer having plastic or elastic properties. The material may be polyimide, polyurethane, silicon or polyethylene terephthalate (PET). Where a metallic substrate is used, it will be appreciated that one or more intermediate layers may be required to insulate the conductive tracks 104 and prevent the substrate 102 from interfering with the changes in resistance. Further, the substrates of any given material may include one or more intermediate layers for various purposes, such as promoting the adhesion of the conductive tracks 104. In other examples, the tracks 104 may be laid down directly onto the surface of the bulk material of the substrate.

The substrate may have a width and height in the order of millimetres to centimetres. The extent of the substrate in any one dimension may be in the range of between 5mm to 10cm. However, a large array of sensors may be in the range of metres in size. The thickness of the substrate will be determined by the material and other dimensions.

Operationally, the substrate must be thin and flexible enough to allow it to deform due to the acoustic pressure wave which is to be sensed. The substrate must also be resilient enough to have the ink applied directly to it. The substrate may be in the range of micrometres to millimetres to provide the required flexibility for pressure wave deformation, whilst a thicker substrate in the range of millimetres to centimetres may be preferable for manufacturing purposes. The substrate may be between 0.5mm and 1mm. In use, the substrate may be applied to a second or backing substrate. The second substrate may be a device or vessel to which the hydrophone is attached for deployment. For example, the second substrate may be the hull of a vessel. In some examples, the substrate may be the provided by the hull of a vessel. That is, the conductive tracks may be applied directly to the hull. If the second substrate comprises acoustic properties which are well matched to the submersion medium (e.g. water), then the substrate could be considerably thicker than the measurements provided above.

The term conductive track(s) may be interchangeable with a piezo-resistive track or track throughout the description. The conductive tracks are located on the substrate 102 in such a way that they can detect the deflection and deformations in the substrate 102 which result from vibrations, in use. The deformation of the substrate 102 will be dependent on the characteristics, e.g. the shape, materials and dimensions of the substrate 102, and the frequency of the vibrations, as is well known in the art. In some exemplary applications, the point of maximum deflection will be located within a central region of the substrate 102. In others, the substrate 102 may have a particular modal response which determines where the point of maximum deflection will be for a particular frequency. It will be appreciated, that although dependant on several characteristics, the skilled addressee will be able to determine the point of maximum deflection for a particular application using techniques known in the art.

The conductive track may be configured to maximise the signal produced by the deformation of the substrate 102, in use. This may include changing the layout or dimensions of the track 104, the positioning of the track relative to the region of maximum deflection, the material of the track, etc. For example, the length, width and thickness may be altered to maximise the resistive response of the conductive track. Generally, longer tracks may be preferable for lower frequency operation, with shorter tracks used for higher frequency operation. The layout of the track may be determined in accordance to providing a particular length of track.

The conductive tracks will generally be relatively thin with thickness measurements typically being below 10microns. Thus, the use of an ink based conductive track is advantageous as it allows the hydrophones to be thinner than conventional devices.

In one example, the substrate may be a square having sides (or a circle having a diameter) of between 10-100mm. In such an example, the tracks may be 50-200 microns in width and have a thickness of approximately 10 microns. The total length of the track will vary depending on the sensor design.

The conductive track may include one or more straight sections which extend across a surface of the substrate 102. The conductive track may comprise a meandering or serpentine layout. Hence, the conductive track may comprise a plurality of interconnected parallel sections. The parallel sections may be connected in series.

Hence, as shown in Figure la, the conductive track(s) 104, 104' may extend between first and second terminals 106, 106' along a meandering path in which a plurality of parallel linear sections are connected by u-bend portions to provide a series connection of the straight sections between the two terminals. The two terminals 106, 106' may provide suitable connection points for connecting input and/or output wires with which the electrical characteristics of the conductive track can be measured. Depending on the layout of the conductive track 104, 104', the terminals may be located proximate and adjacent to one another. The terminals 106, 106' may provide a termination for multiple conductive tracks 104, thereby providing a parallel connection, where desired.

As shown in Figure lb and Figures 4a to 4h, the conductive tracks 104, 104' may comprise single lines or pairs of lines connected by a connecting portion to provide a loop. The connecting portion may be distalmost with respect to the first and/or second terminals. The conductive tracks 104, 104' may extend across the area of maximum deflection. Alternatively, the conductive tracks 104, 104' may extend towards the area of maximum deflection and terminate at or short of the central-most portion of the area of maximum deflection.

Thus, as shown in Figure 1 b, there are a plurality of single line conductive tracks 104" which extend across the substrate in a straight line. The tracks 104" may be distributed across the surface of the substrate so as to be separated from one another. The tracks 104" may be parallel. The positioning of the individual tracks may correspond to regions of large or maximum deflection of particular modal shapes. Thus, the central line may pass through a central region of the substrate which represents the area of maximum deflection during low frequency excitation, whereas the outer lines pass the lateral portions of the substrate which may represent areas of maximum deflection at higher frequency excitations. The actual positioning of the conductive tracks may be application specific. As with Figure la, the conductive tracks 104" provided in Figure 1b terminate at terminals 106 provided at the periphery of the substrate area and the substrate 102 may be of similar construction.

In some examples, conductive tracks 104 may be provided on opposing sides of the substrate 102. One or more conductive tracks 104 may be provided on the first surface and the second surface. The tracks 104 on the first surface and second surface may be have overlapping footprints such that the first and second conductive tracks overlay one another on their respective sides.

The conductive track 104 may comprise a conductive liquid or paste which is applied in a desired track layout before being allowed to dry or cure. The ink may include a mixture of conductive particles and a vehicle for the conductive particles in the form of a binder. The combination of the conductive particles and binder may be any suitable for providing the requisite piezo-resistive properties required for the hydrophone 100.

The conductive particles may be nano-particles and may comprise graphene, carbon nanotubes or other conductive materials such as nickel, copper or silver particles. The particles sizes are preferably nano-particles to help reduce the presence of defects and impurities within the ink once dried and helps increase the piezo-resistive effect. The shape of the particles may be chosen to enhance the piezo-resistive properties. For example, the particles may be platelets.

Various conductive inks which will provide the necessary piezo-resistive properties for the hydrophone are known and commercially available. For example, Haydale Technologies Company PLCprovide a range of conductive inks for various uses. One such ink is HDplasTmIGSCO2002 which may be used for the purposes described herein.

The conductive track may have a piezo-resistive gauge factor of between 10 and 200.

Figure 2 shows a method of manufacturing a hydrophone 100. The first step is the provision of a suitable substrate 202. Once the substrate is provided, the design of the conductive track can be determined. As described herein, the conductive track is designed with a layout which is related to the modal shape of the frequency response and in particular places the conductive tracks 104 over the areas of maximum deflection. Once the conductive track layout is determined and provided 204 The ink is deposited 206 on to the substrate 102. After deposition, the ink can be dried/cured as required per the characteristics of the ink. The method of deposition may include printing. The printing may be determined by the substrate 102 and ink but screen printing is a known suitable method for some conductive inks. Other methods of printing, such as ink jet printing, may be known.

Once printed, the conductive tracks 104 can be connected to suitable terminations which are connectable to the measurement and signal processing circuitry. Prior to or after the connection of the measurement and signal processing circuitry, the hyprophone 100 may be packaged in a suitable housing. The packaging may comprise one or more protective layers which cover and seal 208 the conductive tracks 104 and substrate 102 in a water impermeable membrane to prevent liquid contacting the conductive tracks 104 or other electrical connections. The protective layers/coating(s) may include a polymer. The polymer may be the same or different to of the substrate 102 and may be one of the polymers identified above.

The method of determining the placement of the conductive tracks 104 may include an analysis of the frequency response of the substrate 102. Thus, a method for providing the hydrophone 100 may include analysing the substrate 102 for a particular range of frequencies to determine the modal shapes; determining the areas of greatest deflection for the modal shapes of interest; and, placing conductive tracks on the substrate 102 such that they traverse the areas of greatest deflection.

Once the areas of greatest deflection are determined, the conductive tracks 104 placed and the hydrophone manufactured, the hydrophone may be submerged in a fluid, e.g. water, and used to measure sound within the fluid. The measurement of the sound may be determined using the change in the resistance which is a result of the piezo-resistive properties of the tracks 104. As described in relation to Figure 9 below, hydrophone may be employed on a marine vessel.

It will be appreciated, that the hydrophone may form one of a plurality of hydrophones which are arranged to provide a sonar array, as known in the art. Thus, the method may include assembling a plurality of hydrophones into an array.

As mentioned above, the substrate 102 should be flexible to the extent required to provide a suitable frequency response for a given application. The frequency response of a substrate 300 is shown in Figure 3a. The substrate 300 is provided in the form of a square plate which is simulated as being excited by sonic vibrations having a particular but nominal frequency. The modal shape of the substrate 300 can be represented by the contoured regions 302 which show deformation of the plate in the Z axis, with the plane of the substrate lying in the X-Y directions, as shown.

Generally, the placement of the conductive tracks is related to the maximum displacement of the plate. For low frequency operation this may be in the centre of the plate 300. At higher frequencies, different mode shapes are realised in which the areas of maximum displacement 304 may be offset from centre of the plate 300 to the sides, as shown in Figure 3. Here, there are four high areas which experience the maximum deflection. These areas are located in respective quadrants of the plate and along the diagonal lines which extend between the corners of the plate. Thus, it may be preferable to provide at least one conductive track across the substrate from a corner region towards the centre region of the plate and opposing corner. The tracks may extend fully between opposing corners, or terminate at or before the centre of the plate.

The acoustic frequencies of interest may range from 0Hz to MHz. The acoustic frequency range may be from 0Hz to 100kHz.

Providing multiple tracks may be useful for increasing the sensitivity of the device.

Figures 4a to 4h shows alternative configurations of conductive tracks in plan on a polygonal substrate. The substrate is similar to the plate provided in a Figure 3 comprising a plate-like member in the form of a square. The substrates can be presumed to be the same in each instance.

Figure 4a shows a single track 401 extending from a corner of the substrate 402 and passing directly through the centre of the substrate. There may be more than single track distributed across the surface of the plate. For example, there may be a plurality of single tracks which are spatially distributed across the surface of the plate. The tracks may be parallel to one another for example and traverse between adjacent edges of the plate rather than extending corner to corner.

Figure 4b shows a pair of dual line tracks 403, 404 extending radially inwards from opposing corners of the substrate towards a central region. Each dual line track 403, 404 includes a pair of conductive lines extending in a parallel relation to a distal end where they are connected by a connection portion. The other ends of the lines are attached to respective terminals to provide respective an inputs and outputs. The two opposing dual line tracks are shown as being identical to one another, but this may not be the case. The extent to which the individual conductive tracks crosses the substrate will be dependent on the modal shape of interest. However, the conductive tracks may advantageously extend at least 40%, or 45% or 49% of a width of the substrate.

Figure 4c shows the reverse side of the substrate shown in 4b which includes a further set of tracks 403', 404' located on the underside and opposing second surface of the substrate. The tracks on either side of the substrate are positioned so as to directly overlay each the tracks on the opposing other side but this need not be the case. Providing tracks on both sides of the substrate means that the number of sensors can be doubled for the same sized device. Further it provides some advantages to the data processing aspects such as allow a differential signal to be obtained which may be advantageous if the device is used as an accelerometer. A further advantage is that the resistance of the ink may change with temperature causing errors in the measurement which can be compensated for by having a second sensor attached in a Wheatstone bridge circuit, an example of which is described below.

The pairs of tracks may be advantageously nested within one another. Thus, as shown in Figure 4d there is an example of a nested or coaxial arrangement of dual line tracks in which an inner track 405 is provided within an outer track 406. Each of the dual line tracks 405, 406 are similar in configuration and operation, with the lines of the outer track 406 being spaced further apart to accommodate the inner track 405. The tracks are coaxial in as much as they extend along the same elongate axis. The terminal ends of the lines are connected to terminals as described above. Each line end may be connected to a dedicated terminal, however, the lines may share a terminal if a parallel arrangement of nested tracks is required. Providing nested tracks in this way provides a more convenient arrangement for providing the terminals around the periphery of the substrate.

As stated above in relation to Figure 3, there may be an area of maximum deflection located towards a central portion of the substrate or distributed away from the centre on the diagonals which extends between opposing corners. In order to increase the resistive response of the hydrophone, conductive tracks may be provided in each quadrant of the substrate. Thus, as shown in Figure 4e, the substrate may comprise a radial arrangement of conductive tracks 407 in which at least one track extends from a corner of the substrate towards a central region of the plate. The four tracks shown in Figure 4e are provided in opposing pairs. The tracks may be nested as shown but this need not be the case and individual dual line tracks may be used.

Although the maximum deflections may, in many applications, be located towards the central region or distributed on diagonals therefrom, there are some examples in which it is beneficial to provide intervening or intermediate tracks which lie between adjacent corner/diagonal tracks. Thus, as shown in Figure 4f, there is provided an example of a hydrophone having a plurality of radially extending tracks in which a portion of the tracks extend from corner regions 408 of the substrate, and a portion 408' extend from edge regions of the substrate. As shown, the edge tracks 408' may be wider than the corner tracks such that the individual lines of the dual line tracks are spaced further apart. Although the intermediate tracks may not correspond to the areas of maximum displacement, they may detect secondary local deflections which help provide a more complete data set and resulting analysis of the mode of vibration and sound which is being received. As with the corner tracks 408, the intermediate tracks 408' may or may not be nested pairs of dual line tracks.

Figures 4g and 4h show, respectively, arrangements of forty-four 409 and eighty-eight 410 tracks. The arrangement of 4g provides forty-four single dual line tracks 409. Figure 4h provides an arrangement of forty-four nested dual line tracks 410 to provide eighty-eight. The maximum number of tracks may be greater than this and will ultimately be determined by the dimensions of the tracks. As with the other arrangements, each of the tracks extends radially from a corner region or edge region of the substrate towards a centre of the substrate.

Figures 4i and 4j show yet further examples of conductive track layouts. In Figure 4i there is provided an example of a branched conductive track. A branched conductive track may comprise one or more conductive tracks from which extend other conductive track branches. Additionally or alternatively, the one or more conductive tracks may extend from the terminals to provide a branched conductive track. The conductive tracks may include pressure responsive and non-pressure responsive sections. The pressure responsive sections may be substantially parallel to one another. The pressure responsive sections may be located on selected areas of high displacement for one or more particular frequency.

Thus, as shown in Figure 4i, there is a central piezo-resistive track 412 extending corner to corner across the substrate 402. Supplementary tracks piezo-resistive tracks 412' are arranged adjacent and, in this case, parallel to the central track 412. The lengths of the supplementary tracks 412' are significantly less than the central track 404 and with each end separated from a terminal and edge of the substrate. The supplementary tracks may be connected at either end to the central track or an end terminal by non-pressure responsive portions 412" as indicated by the dotted lines. There may be a plurality of parallel arrangements arranged in series along the central line 412 as shown. There may also be more than one central line 412 with each having one or more parallel arrangements. The parallel arrangement may have more than two supplementary tracks 412' and each supplementary track may comprise a supplementary central track having its own parallel arrangements. The supplementary tracks 412' may be located on areas of high displacement at higher frequencies, with the central track 412 located at positions of high displacement at lower frequencies, or possibly vice versa in some applications.

Figure 4j shows yet a further arrangement in which a conductive track 414 is flanked by to shorter dual line tracks. Thus, there is a central conductive track 414 which extends corner to corner across the substrate 402, with the dual line tracks 414' looping in from a periphery of the substrate 402 extending to and from terminals which are adjacent to the a terminal 406 of the central track 414. The dual line tracks 414' extend radially inwards from the edge of the substrate and terminate part way across the substrate 402 in a similar fashion to those described in Figure 4b. The dual line tracks 414' may be positioned to extend over an area of high frequency high displacement. The dual line tracks may be arranged symmetrically either side of the central line and there may be a corresponding pair of dual line tracks on the opposing corner of the substrate 402.

Providing a distributed arrangement of conductive tracks for different frequencies (mode shapes) allows the hydrophones to be more responsive to a broader range of frequencies. It will be appreciated that the embodiments of Figures 4i and 4j may be combined in some examples so as to provide dual line conductive tracks with branched conductive tracks, and there may be more or less high frequency sensors than shown in Figure 4j.

Each of the hydrophone arrangements shown in Figure 4a to 4j may have conductive tracks on both sides of the substrate. Hence, the total number of tracks on the substrate shown in Figure 4h may be one hundred and seventy six with eighty eight on each side. Further, it will be noticed that many of the arrangements have multiple fold symmetry. For example, Figure 4i is symmetrical about the central conductor and about a line which dissects the substrate at the mid plane between the terminals.

Although the substrates have been described above as being discrete plates having edges and corners, it will be appreciated that the substrate may be a section or region of a larger member or substrate. Thus, the edges and/or corners referred to above may be virtual edges and/or corners.

It will be appreciated that the exact properties of the hydrophone may depend on several factors and a certain amount of trial and error may be required to provide a suitable hydrophone in accordance with this description.

Figure 5 shows an alternative hydrophone arrangement 500 comprising a plurality of individual hydrophones 502 provided in a stack. The individual hydrophones may be may be similar to those described above. Thus, the hydrophone arrangement may comprise two or more substrates 504 each comprising one or more conductive tracks 506 on one or both sides, wherein the substrates 504 are provided in a stack within a flexible casing. The casing 508 may be made from a similar material as the substrates and may provide the water impermeable cover (e.g. membrane) as described above. There may be any suitable number of substrates in the stack, and the substrates may or may not overlay one another when viewed from the incident surface. Thus, the example of Figure 5 has a stack of three hydrophone substrates 504 each with conductive tracks 506 on either side thereof. The substrates 504 are vertically aligned so as to overlay one another and are encased in a material which may be acoustically matched to the substrates. The casing 508 may provide structural integrity and seal the substrates from water ingress. In one example, the substrates and casings may be made from polyurethane. It will be appreciated that the stack as shown is schematic and the individual features not drawn to scale.

Providing a stack of hydrophones in this way allows a directional measurement to be made by comparing the responses from the different layers. In doing so, the stack of hydrophones may be used to measure the gradient of the pressure wave which gives a dipole type response if two devices are used rather than an omni-directional response.

Another technique may be to measure the time delay in the signal between sensors to provide some information on direction. This latter technique may be more application to an array.

Figure 6 shows a damped hydrophone 600. Thus, the hydrophone 600 comprises a mass 602 which is attached to, through or around the substrate 604 to modify the frequency response thereof. Thus, as can be seen, there is a substrate 604 having conductive tracks 606 on either side thereof, with a mass 602 located in a central region of the substrate 604. The shape, size, and density of the mass 602 may be determined or adjusted to provide a desired frequency response and modal shape. The dampening mass 602 may be used to affect the resonance of the substrate and possibly increase the deflection of the substrate 604, thereby increasing the deflection and change in resistance experienced by the conductive tracks 606.

The mass may be positioned in the location which corresponds to the highest displacement or where the highest displacement is desired. In one example, particularly a low frequency example, the mass may be placed in the centre of the substrate. In some examples, the mass may be distributed amongst several locations on the substrate. So for the mode shape provided in Figure 3, the mass may be distributed amongst the four peaks. It will be appreciated that the design may require some optimisation.

Figure 7 shows a cantilever hydrophone 700 in which the substrate 702 has a first end and a second end, in which the first end is attached to a fixed support structure 704 and the second end is a free end. Providing a cantilevered substrate may provide a preferable frequency response for certain applications. Such hydrophones may find applications in which individual sensors are required, rather than arrays. In other ways, the hydrophone may be similar to those described above, and may, for example, include a dampening mass. Due to the more flexible nature of the cantilever structure, the substrate 702 may be made from a stiffer material such as a metal.

Figure 8 shows part of the signal processing arrangement which may be used with the hydrophones described herein. The signal processing arrangement includes a Wheatstone bridge 800 which may be used to help compensate for temperature variations. The changes in resistance of the piezo-resistive ink is likely to be affected to temperature changes, as is the case with graphene. A Wheatstone bridge 800 allows accurate measurements of an unknown resistance Rx when connected to a three known resistances R1, R2, R3 and as such can be used to compensate for the changes in resistance. Figure 8 shows a conventional Wheatstone bridge 800 in which all of the resistors with are conductive tracks, and measuring the potential across points A and B allows the change in resistance of all of the tracks to be calculated, as known in the art. The resistors R1, R2, and R3 could be provided with ordinary resistors as known in the art or other conductive tracks. The Wheatstone bridge may be used in service as part of the method for converting the displacement of the ink into an electrical signal.

Figure 9 shows a schematic representation of a vessel 900 incorporating a hydrophone arrangement 902 and associated signal processing equipment 904. The hydrophone arrangement 902 may be any described herein and the signal processing equipment is largely conventional. The vessel may be an unmanned vessel.

Although the above described embodiments relate to hydrophones, it will be appreciated that the inventions described herein may be applicable to other sounds detecting sensors such as microphones which are operated out of water.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

CLAIMS: 1. A sound detecting sensor comprising: a substrate having a conductive track thereon, wherein the conductive track comprises an ink having a plurality of conductive particles within a binder.
2. A sound detecting sensor as claimed in claim 1, wherein the conductive particles are nano-particles.
3. A sound detecting sensor as claimed in either of claims 1 or 2, wherein the conductive particles comprise one or more of: graphene, carbon nanotubes, silver, nickel and copper.
4. A sound detecting sensor as claimed in any preceding claim, comprising a plurality of conductive tracks.
5. A sound detecting sensor as claimed in any preceding claim, wherein the substrate has a first side and a second side, and at least one conductive track is provided on both of the first and the second sides.
6. A sound detecting sensor as claimed in any preceding claim, wherein the conductive tracks are arranged to detect the amount of deflection in the substrate.
7. A sound detecting sensor as claimed in any preceding claim, wherein the substrate has a modal shape having one or more areas of maximum displacement and the conductive tracks are located across at least one area of maximum deflection.
8. A sound detecting sensor as claimed in any preceding claim, wherein the conductive tracks are arranged as longitudinal elements having a longitudinal axis, wherein the longitudinal axis extends radially from a peripheral edge of the substrate to a central region of the substrate.
9. A sound detecting sensor as claimed in claim 8, wherein the substrate comprises a width, and the radial extent of the conductive track is at least 40% of the width in a single direction.
10. A sound detecting sensor as claimed in either of claims 8 or 9, further comprising at least a pair of conductive tracks and optionally a plurality of pairs of tracks, wherein each conductive tracks of the pair of conductive tracks are provided on opposite sides of the central region of the substrate.
11. A sound detecting sensor as claimed in claim 10, further comprising between two and one hundred and seventy-seven conductive tracks.
12. A sound detecting sensor as claimed in any of claims 8 to 11, wherein the substrate is polygonal and a first conductive track extends from a corner region of the substrate, and a second element extends from an edge element of the substrate.
13. A sound detecting sensor as claimed in any preceding claim, wherein the conductive track includes a pair of legs connected in series, each leg extending from respective terminals.
14. A sound detecting sensor as claimed in any preceding claim, wherein the legs are arranged in a parallel relation to one another.
15. A sound detecting sensor as claimed in any preceding claim, further comprising a plurality of substrates, each substrate having a conductive track thereon, wherein the substrates are provided in a stack.
16. A sound detecting sensor as claimed in any preceding claim further comprising a damping mass, wherein the damping mass is located on or in the substrate and/or on the or each conductive track.
17. A sound detecting sensor as claimed in any preceding claim, wherein the substrate is cantilevered from a support structure.
18. A sound detecting sensor as claimed in any of claims 1 to 17, wherein the substrate has one or more peripheral edges, each of the peripheral edges being fixedly attached to a support structure so as to provide a constrained plate arrangement.
19. A sound detecting sensor as claimed in any preceding claim, wherein the conductive track comprises a piezoresistive material wherein, optionally, the conductive track has a piezoresistive gauge factor of between 10 and 200.
20. A sound detecting sensor as claimed in any preceding claim, wherein the conductive track comprises a branched conductive track having a central conductive track with one or more supplementary conductive tracks connected in parallel.
21. A sound detecting sensor as claimed in any preceding claim, further comprising a water impermeable coating located at least over the conductive tracks such that the sound detecting sensor is operably submersible in water or saline, wherein, optionally, the water impermeable coating is polyurethane.
22. A system comprising a sound detecting sensor as claimed in any of claims 1 to 21, the system comprising: a resistance measurement system, wherein the or each conductive track is electrically connected to the resistance measurement system.
23. A system as claimed in claim 22, wherein the resistance measuring system comprises a Wheatstone bridge.
24. A method of manufacturing a sound detecting sensor according to any of claims 1 to 21 comprising: providing a substrate; determining the areas of maximum deflection for one or more modal frequencies and, depositing the least one conductive track on the substrate such that it traverses the areas of maximum deflection.
25. A method as claimed in claim 24, further comprising: coating the substrate in an impermeable membrane.