Electroacoustic transducer

Abstract

An electroacoustic transducer 400 is described. The electroacoustic transducer 400 comprises an active element 410. The electroacoustic transducer 400 comprises an acoustic coupling layer 430 arranged to acoustically couple, in use, the active element 410 to a transmission medium. The electroacoustic transducer 400 further comprises a cavity 420 arranged between the active element 410 and the acoustic coupling layer 430 to receive a fluid. In this way, acoustic coupling of the electroacoustic transducer 400 and the transmission medium is improved.

Claims

1. An electroacoustic transducer for use in a liquid transmission medium, the electroacoustic transducer array comprising: an active element; an acoustic coupling layer having a porous structure and arranged to acoustically couple, in use, the active element to a liquid transmission medium external to the electroacoustic transducer; and wherein the active element is spaced from the acoustic coupling layer by a cavity arranged between the active element and the acoustic coupling layer, wherein in use the cavity is in fluid communication with the liquid transmission medium via the porous structure, wherein in use the cavity contains part of the liquid transmission medium, and wherein in use the liquid transmission medium provides a fluid reservoir to the cavity; and wherein the electroacoustic transducer has a centre frequency f in a range from 1 kHz to 80 kHz.

2. The transducer according to claim 1, wherein the acoustic coupling layer comprises an acoustic meta-material.

3. The transducer according to claim 1, wherein an acoustic impedance of the acoustic coupling layer changes in one or both of a longitudinal direction away from the cavity and a transverse direction.

4. The transducer according to claim 1, wherein a thickness of the acoustic coupling layer corresponds to either: (2n+1)Λ/4 where Λ is the acoustic wavelength within the acoustic coupling layer, and n is either 0 or a positive integer; or (n+1)Λ/2, where Λ is the acoustic wavelength within the acoustic coupling layer, and n is either 0 or a positive integer.

5. The transducer according to claim 1, wherein the acoustic coupling layer comprises a plurality of acoustic coupling members.

6. The transducer according to claim 1, wherein a porosity of the porous structure is in a range from about 5% to about 90% by volume of the porous structure.

7. The transducer according to claim 6, wherein the porosity changes in a longitudinal direction.

8. The transducer according to claim 1, wherein the acoustic coupling layer comprises a plurality of acoustic coupling members, wherein the plurality of acoustic coupling members taper in the longitudinal direction.

9. The transducer according to claim 1, comprising a housing having a wall arranged to surround, at least in part, the acoustic coupling layer.

10. The transducer according to claim 9, wherein the housing defines a cylindrical housing having a diameter in a range from about 1 to about 2.5 times a diameter of the active element.

11. The transducer according to claim 9, wherein a wall thickness of the wall is in a range from about 4 mm to about 10 mm.

12. The transducer according to claim 9, wherein a fill ratio of the housing by the acoustic coupling layer is in a range of from about 5% to about 90%.

13. The transducer according to claim 9, wherein a shape of the housing is arranged to support longitudinal vibration Eigen-frequency modes.

14. The transducer according to claim 1, wherein the active element comprises a single crystal piezoelectric material.

15. The transducer according to claim 1, further comprising: a layer providing a sound emitting surface and arranged between the active element and the cavity, wherein the layer is mechanically coupled to the active element and wherein the layer comprises an alloy that includes aluminum and beryllium.

16. The transducer according to claim 1, wherein a centre frequency f of the transducer is in a range from about 10 to 60 kHz.

17. The transducer according to claim 1, wherein the transducer is configured as a Tonpilz transducer.

18. The transducer according to claim 1, further comprising an encapsulant.

19. A Tonpilz transducer for use in a liquid transmission medium, the Tonpilz transducer comprising: an active element; and an acoustic coupling layer arranged to acoustically couple, in use, the active element to a liquid transmission medium external to the Tonpilz transducer, wherein the acoustic coupling layer comprises an acoustic meta-material with an array of acoustic coupling members mutually aligned in a longitudinal direction from the acoustic coupling layer and defining open pores between adjacent acoustic coupling members; and a cavity between the active element and the acoustic coupling layer, the cavity in fluid communication with the liquid transmission medium via the open pores, wherein in use the cavity contains part of the liquid transmission medium; wherein the Tonpilz transducer has a centre frequency f in a range from 1 kHz to 80 kHz.

20. An electroacoustic transducer for use in a liquid transmission medium, the electroacoustic transducer comprising: a housing having a hollow cylindrical geometry exending along a central axis from a first end to an open second end; an active element configured to generate and/or receive sound via at least one of a piezoelectric effect, an electromagnetic effect, and an electrical effect; an acoustic coupling layer mechanically supported by the housing and arranged to acoustically couple, in use, the active element to a liquid transmission medium external to the electroacoustic transducer; wherein the active element is spaced from the acoustic coupling layer along the central axis by a cavity arranged between the active element and the acoustic coupling layer, the cavity in fluid communication with the liquid transmission medium via the open second end, and in use the cavity contains part of the liquid transmissin medium; wherein the electroacoustic transducer has a centre frequency f in a range from 1 kHz to 80 kHz.

21. The Tonpilz transducer of claim 19, wherein the centre frequency f is from 10 kHz to 60 kHz.

22. The electroacoustic transducer of claim 20, wherein the centre frequency f is from 20 kHz to 50 kHz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

(2) FIG. 1 depicts a graph of predicted ambient noise spectrum level with sea state and thermal noise versus frequency;

(3) FIG. 2A schematically depicts a conventional Tonpilz transducer;

(4) FIGS. 2B and 2C schematically depict characteristic vibration modes of the Tonpilz transducer of FIG. 2A consisting of a longitudinal mode (2B) and a flapping mode (2C);

(5) FIGS. 3A and 3B schematically depict a published triple resonant structure Tonpilz transducers as a configuration to provide a route to wider frequency bandwidths;

(6) FIG. 4 schematically depicts a transducer according to an exemplary embodiment;

(7) FIGS. 5A and 5B depict calculated electrical insertion loss frequency responses for an electrically tuned Pz27 grade PZT transducer with and without a controlled thickness of an acoustically thin and acoustically impedance mismatched layer between the transducer and the substrate into which sound is being broadcast;

(8) FIGS. 6A and 6B depict photographs of an acoustic meta-material according to an exemplary embodiment and measured reflection coefficients for a transducer comprising the acoustic meta-material, respectively;

(9) FIG. 7 schematically depicts a transducer according to an exemplary embodiment;

(10) FIGS. 8A to 8C depict calculated sound pressure level plots for a conventional transducer;

(11) FIGS. 9A to 9C depict calculated total radiated acoustic power plots for the transducer of FIG. 7; and

(12) FIG. 10 depicts a photograph of an acoustic meta-material according to an exemplary embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

(13) FIG. 4 schematically depicts a transducer 400 according to an exemplary embodiment.

(14) In detail, the electroacoustic transducer 400 comprises an active element 410. The electroacoustic transducer 400 comprises an acoustic coupling layer 430 arranged to acoustically couple, in use, the active element to a transmission medium. The electroacoustic transducer 400 further comprises a cavity 420 arranged between the active element 410 and the acoustic coupling layer 430 to receive a fluid. In this way, acoustic coupling of the electroacoustic transducer and the transmission medium is improved, as described above.

(15) In this example, the active element 410 comprises a piezoelectric material 411. The active element may comprise additionally and/or alternatively other materials as described previously.

(16) FIGS. 5A and 5B depict calculated frequency responses for an electrically tuned Pz27 grade PZT transducer with and without a low acoustic impedance filled layer of controlled thickness between the piezoelectric platelet and the steel substrate into which sound is being broadcast;

(17) In detail, FIG. 5A shows the electrical Insertion Loss Loss responses of a transducer both with an impedance mismatch layer and without an impedance mismatch layer. FIG. 5B shows the Smith Chart plots, corresponding to the Insertion loss plots of FIG. 5A. In this example, the impedance mismatch is provided by a 13 μm adhesive layer comprising a 2 part epoxy resin adhesive layer (MasterBond EP46HT-2), to demonstrate a principle of the impedance mismatch layer, as provided by the cavity comprising the fluid according to the invention.

(18) Particularly, in this transducer, a platelet of PZT is bonded to a steel barrier with an epoxy resin. The acoustic impedance of the epoxy is far smaller than the acoustic impedances of both the steel barrier and PZT ceramic transducer.

(19) The surprising result with this high frequency transducer design is that although the acoustic impedance of the epoxy bond is completely mismatched from the two substrates that it bonds together, the optimal structural design with the widest fractional bandwidth is not obtained by making the adhesive bond-line as thin as possible; although the bond line needs to be thin, the optimal bond line thickness should generally be non-zero in thickness. Modelling of the transducers electrical response shows that reducing the thickness of the adhesive layer from 13 μm to zero reduced the 2:1 VSWR bandwidth of the transducer by approximately ˜50%. The plots show modelled electrical insertion loss responses for the same transducer, using individually optimised electrical matching circuits for each case, with and without a low acoustic impedance adhesive bond layer between the transducer and the steel barrier.

(20) The reflectivity of the epoxy interface between the piezoelectric substrate and the steel substrate to which it is bonded is found to rapidly increase with the bond thickness until subsequent constructive interference effects in the bond layer generate a series of periodic narrow transmission windows with increasing bond thickness. In transducer design the epoxy interface cannot be ignored unless it extremely thin, that is for example between <<λ/10 to <λ/100 at the operational centre frequency. The calculated reflectivity of the 13 μm thick adhesive layer is 30.3%. This is much larger than the reflectivity arising from the acoustic impedance mismatch of the piezoelectric layer and the 17-4 stainless steel substrate of only 2.2%.

(21) The greatest improvement to the fractional bandwidth of the transducer from a controlled thickness bond layer is found to be achieved when the acoustic impedance of piezoelectric layer is smaller than that of the substrate into which the piezoelectric is coupled.

(22) The cavity comprising the fluid according to the invention is expected to behave similarly to the adhesive layer.

(23) FIGS. 6A and 6B depict photographs of an acoustic meta-material according to an exemplary embodiment and measured reflection coefficients for a transducer comprising the acoustic meta-material, respectively.

(24) In detail, FIG. 6A depicts photographs of a binary grating structure comprising a rectangular array of approximately square pillars that were sawn into the surface of the titanium substrate using a wafer saw. This binary grating was designed for a 36° Y cut lithium niobate transducer with a centre frequency of 3.3 MHz and a 2:1 VSWR frequency bandwidth of 1.51 MHz, a fractional bandwidth Δf/fc of 45.5%.

(25) In detail, FIG. 6B depicts results of acoustic reflectivity of a flat interface between titanium and de-ionised water from simple acoustic impedance mismatch theory would be ˜80.6%. The results show that the reflectivity of the grating surface over the frequency range ˜1 to ˜6 MHz varies from a peak reflectivity of 93.4% at 3.557 MHz to less than 50% reflectivity at 5.45 MHz, where the anti-reflection performance of the grating is then compromised by appearance of 1st order diffraction lobes in the water medium from the square pillars of the array whose square pitch was 280 μm.

(26) Hence, for example, relatively simple acoustic meta-structures consisting of a square symmetric array of sub-wavelength (in water) square pillars may markedly change the acoustic reflectivity of a titanium-water interface over a wide frequency band centred on 3.56 MHz.

(27) Hence, for example, relatively complex tapered metal profile acoustic meta-structures that progressively match the impedance of the metal to the fluid may provide further improved performance. These relatively complex acoustic meta-structures will be more practical to fabricate for transducers designed to operate at centre frequencies of 25 kHz to 35 kHz, than at a relatively higher centre frequency of ˜3.6 MHz.

(28) FIG. 7 schematically depicts a transducer 700 according to an exemplary embodiment.

(29) In detail, the electroacoustic transducer 700 comprises an active element 710. The electroacoustic transducer 700 comprises an acoustic coupling layer 730 arranged to acoustically couple, in use, the active element to a transmission medium W. The electroacoustic transducer 700 further comprises a cavity 720 arranged between the active element 710 and the acoustic coupling layer 730 to receive a fluid. In this way, acoustic coupling of the electroacoustic transducer and the transmission medium is improved, as described above.

(30) In detail, the electroacoustic transducer 700 is a Tonpilz transducer comprising a piston head mass 702, a tail mass 703, a piezoelectric stack 711 and a stress rod 705. In this example, the active element 710 comprises the piezoelectric stack 711. The piezoelectric material 711 is in the form of a stack of four annular rings that for piezoelectric materials are optimally poled in adjacent rings in opposite directions. In this way, an issue of a ground electrode being directly next to a high voltage live electrode, and separated from one another by only a thin adhesive layer typically, is avoided. The transmission medium W is water.

(31) The cavity 720 has a thickness of 1 mm in this example and is completely filled with the transmission medium W. The cavity 720 is in fluid communication with the transmission medium W, such that the transmission medium provides a fluid reservoir, in use.

(32) The acoustic coupling layer 730 comprises an acoustic meta-material 731 comprising 37 (i.e. a plurality) acoustic coupling members 732. The acoustic coupling members 732 are in the form of rods or pillars having constantly-shaped circular cross sections. The acoustic coupling members comprise elongate acoustic coupling members, having lengths of 50 mm and diameters of 4 mm and hence aspect ratios of 12.5. A length of an acoustic coupling member of the plurality of acoustic coupling members 732 corresponds to Λ/2 where Λ is the acoustic wavelength within the acoustic coupling layer. The plurality of acoustic coupling members 732 have the same length.

(33) The plurality of acoustic coupling members 732 are arranged mutually aligned in a longitudinal direction. The plurality of acoustic coupling members 732 are substantially mutually adjacent, equispaced apart by a distance 7 mm. The plurality of acoustic coupling members 732 are arranged in a regular square array. Regions between the plurality of acoustic coupling members 732 provide open pores. The open pores in this example are in fluid communication with the transmission medium W, in use, but in other examples the pores would be filled with for example butyl rubber to isolate the acoustic coupling members from seawater corrosion. The open pores are also in fluid communication with the cavity 720. Where the acoustic meta-material is potted with a material such as Butyl rubber, a access channel to the cavity 720 would need to be provided to a fluid reservoir or alternatively to the external fluid medium W.

(34) The transducer 700 comprises a housing 750 having a wall 751 arranged to surround the acoustic coupling layer 730. The housing 750 defines a cylindrical housing having an inner diameter of 52 mm. A wall thickness of the wall 751 is 5 mm. A fill ratio of the housing 750 by the acoustic coupling layer 730 is 78.1%.

(35) The housing 750 has an open end 752, in this example distal the cavity 720 and a closed end 753, proximal the cavity 720. The housing 750 is arranged to support, for example mechanically and/or structurally support, the acoustic coupling layer 730, using a frame (not shown). A transverse thickness of the frame is <<Λ. The housing 750 is arranged to support the acoustic coupling layer 730 spaced apart from the active element 710, thereby providing the cavity 720.

(36) The active element comprises a polycrystalline ceramic piezoelectric material. Lead Zirconate Titanate (PZT-4).

(37) FIGS. 8A to 8C depict calculated sound pressure level plots for a conventional Tonpilz transducer.

(38) FIGS. 9A to 9C depict calculated total radiated acoustic power plots for the Tonpilz transducer of FIG. 7 as a function of frequency

(39) In detail, FIGS. 8A to 8C and FIGS. 9A to 9C depict sound pressure level (dB) plots calculated using COMSOL Multiphysics (RTM) Modeling Software (COMSOL, Inc., USA). FIGS. 8A and 9A are calculated at 30 kHz, FIGS. 8B and 9B are calculated at 35 kHz and FIGS. 8C and 9C are calculated at 40 kHz.

(40) At 35 kHz, the central acoustic beam of the meta-material structure (FIG. 9B) is much more collimated that achieved with a standard Tonpilz transducer.

(41) FIG. 9 shows calculated total transmitted sound power plots for the transducer of FIG. 7 as a function of frequency with and without the acoustic meta-material structure and shroud described above.

(42) FIG. 10 depicts a photograph of an acoustic meta-material AMM according to an exemplary embodiment.

(43) In detail, the acoustic meta-material AMM is formed by 3D printing, providing a mechanically stiff acoustic meta-material structure having interconnected open pores, suitable for an acoustic coupling layer according to an exemplary embodiment. The acoustic meta-material AMM may be mounted directly onto a housing. The acoustic meta-material AMM would be formed from a metal that is then filled for example with a butyl rubber to prevent corrosion issues.

(44) In summary, the invention provides an electroacoustic transducer comprising: an active element; and an acoustic coupling layer arranged to acoustically couple, in use, the active element to a transmission medium; wherein the electroacoustic transducer further comprises a cavity arranged between the active element and the acoustic coupling layer to receive a fluid; whereby acoustic coupling of the electroacoustic transducer and the transmission medium is improved. Also provided is a Tonpilz transducer comprising: an active element; and an acoustic coupling layer arranged to acoustically couple, in use, the active element to a transmission medium; wherein the acoustic coupling layer comprises an acoustic meta-material. Also provided is an array comprising a plurality of such transducers.

(45) Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

(46) All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

(47) Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(48) The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.