GB2120902A - Underwater acoustic devices - Google Patents

Abstract

An underwater acoustic transducer which, for example, is suspended in water from a buoy, comprises a thin gas-pressurised spherical structure (1) composed of polymeric piezoelectric material, preferably polyvinylidene difluoride (PVDF), which has a high piezoelectric stress constant and a low Young's modulus, films of layers (10, 11) of electrically conductive material on the inner and outer surfaces of the structure, and electrical terminal means (12,13) for connecting the films or layers with cables (6,7). The transducer is preferably inflatable with nitrogen through above (9). <IMAGE>

Description

SPECIFICATION Improvements in or relating to underwater acoustic devices The present invention relates to underwater acoustic devices. The invention particularly, though not exclusively, relates to acoustic devices, which, in use, are suspended in water from a buoy or other flotation equipment.

Known underwater acoustic devices or sound transducers employ either a slab of piezoelectric material, a ferroelectric ceramic or a moving coil as their active element. Several such prior art transducers are described in US Naval Research Laboratory Report NRL 7735 entitled "Twenty Years of Underwater Electroacoustic Standards" dated 21 February 1974.

The present invention provides a low power, low frequency acoustic device.

According to the present invention an underwater acoustic device comprises a thin spherical structure composed of polymeric piezoelectric material having a high piezoelectric stress constant and a low Young's modulus, wherein the spherical structure is gas pressurised, a film or layer of electrically conductive material on the inner and outer surfaces of the structure, and electrical terminal means contacting the film or layer on said inner and outer surfaces.

The spherical structure is preferably composed of polyvinylidene fluoride (PVDF).

The spherical structure may have an aperture which receives a closure member which includes a valved inlet through which the structure may be pressurised.

The closure member may further include attachment means for connection to a cable for downwardly suspending, or towing, the structure in water.

The electrical terminal contacting the film or layer on the inner surface may be connected to a power supply line which extends through the closure member.

Preferably the spherical structure is pressurised such that at the working depth there is a pressure differential across the structure wall of 0.2 atmospheres.

An embodiment of the invention will now be described by way of example only with reference to the drawings of which: Figure 1 is a sectional side view of an acoustic device in accordance with the invention.

Figure 2 is a sectional side view on a larger scale of part of the acoustic device of Figure 1.

Figure 1 shows an underwater acoustic device which comprises an apertured spherical shell 1 of 0.6 m external diameter composed of polyvinylidene difluoride (PVDF) and having a wall thickness of 1 mm. The shell has electrically conductive thin metallic coatings which form an outer layer 10 and inner layer 11 as may be seen in Figure 2. The shell 1 has a single aperture which contains a closure member 2.

The closure member is connected to a support cable 5 which has an eye 4 mounted at its lower end, and the eye is linked to the closure member 2 through an aperture 8 in the upper part of the closure member.

A pair of electrical cables 6, 7 are connected to the conductive coatings 10,11 atterminals 12,13 respectively. The cables 6,7 extend through, and are sealed into, holes in the closure member 2 and extended upwardly around the support cable 5 to an electrical power source.

The shell 1 is inflated with the closure member in place after the electrical connections at 12 and 13 have been made, by pumping nitrogen gas through a bore 9 via a valve 3 in the closure member 2 until the correct pressure for the working depth in water is reached.

In operation when the device is immersed to the working depth and an alternating current of 100 H2 is applied to the metallic coatings on the PVDF shell, the shell vibrates, and acts as an underwater sound generator.

The device may be used as an underwater sound detector by processing electrical signals generated in the coatings on the PVDF shell.

1. An underwater acoustic device comprising a thin spherical structure composed of polymeric piezoelectric material having a high piezoelectric stress constant and a low Young's modulus, wherein the spherical structure is gas pressurised, a film or layer of electrically conductive material on the inner end outer surfaces of the structure, and electrical terminal means contacting the film or layer on said inner and outer surfaces.

2. An underwater acoustic device according to claim 1 wherein the spherical structure is composed of polyvinylidene fluoride (pvdf).

3. An underwater acoustic device according to claim 1 or claim 2 wherein the spherical structure has an aperture and a closure member therefore wherein the closure member includes a valved inlet through which the structure may be pressurised.

4. An underwater acoustic device according to any ofthe preceding claims including attachment means for connection to a cable for downwardly suspending, or towing, the structure in water.

5. An underwater acoustic device according to any of the preceding claims wherein the electrical terminal contacting the film or layer on the inner surface is connected to a power supply line which extends through the closure member.

6. An underwater acoustic device according to any of the preceding claims wherein the spherical structure is pressurised such at the working depth there is a pressure differential across the structure wall of about 0.2 atmospheres.

7. An underwater acoustic device substantially as described herein with reference to the drawings.

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to underwater acoustic devices The present invention relates to underwater acoustic devices. The invention particularly, though not exclusively, relates to acoustic devices, which, in use, are suspended in water from a buoy or other flotation equipment. Known underwater acoustic devices or sound transducers employ either a slab of piezoelectric material, a ferroelectric ceramic or a moving coil as their active element. Several such prior art transducers are described in US Naval Research Laboratory Report NRL 7735 entitled "Twenty Years of Underwater Electroacoustic Standards" dated 21 February 1974. The present invention provides a low power, low frequency acoustic device. According to the present invention an underwater acoustic device comprises a thin spherical structure composed of polymeric piezoelectric material having a high piezoelectric stress constant and a low Young's modulus, wherein the spherical structure is gas pressurised, a film or layer of electrically conductive material on the inner and outer surfaces of the structure, and electrical terminal means contacting the film or layer on said inner and outer surfaces. The spherical structure is preferably composed of polyvinylidene fluoride (PVDF). The spherical structure may have an aperture which receives a closure member which includes a valved inlet through which the structure may be pressurised. The closure member may further include attachment means for connection to a cable for downwardly suspending, or towing, the structure in water. The electrical terminal contacting the film or layer on the inner surface may be connected to a power supply line which extends through the closure member. Preferably the spherical structure is pressurised such that at the working depth there is a pressure differential across the structure wall of 0.2 atmospheres. An embodiment of the invention will now be described by way of example only with reference to the drawings of which: Figure 1 is a sectional side view of an acoustic device in accordance with the invention. Figure 2 is a sectional side view on a larger scale of part of the acoustic device of Figure 1. Figure 1 shows an underwater acoustic device which comprises an apertured spherical shell 1 of 0.6 m external diameter composed of polyvinylidene difluoride (PVDF) and having a wall thickness of 1 mm. The shell has electrically conductive thin metallic coatings which form an outer layer 10 and inner layer 11 as may be seen in Figure 2. The shell 1 has a single aperture which contains a closure member 2. The closure member is connected to a support cable 5 which has an eye 4 mounted at its lower end, and the eye is linked to the closure member 2 through an aperture 8 in the upper part of the closure member. A pair of electrical cables 6, 7 are connected to the conductive coatings 10,11 atterminals 12,13 respectively. The cables 6,7 extend through, and are sealed into, holes in the closure member 2 and extended upwardly around the support cable 5 to an electrical power source. The shell 1 is inflated with the closure member in place after the electrical connections at 12 and 13 have been made, by pumping nitrogen gas through a bore 9 via a valve 3 in the closure member 2 until the correct pressure for the working depth in water is reached. In operation when the device is immersed to the working depth and an alternating current of 100 H2 is applied to the metallic coatings on the PVDF shell, the shell vibrates, and acts as an underwater sound generator. The device may be used as an underwater sound detector by processing electrical signals generated in the coatings on the PVDF shell. CLAIMS

1. An underwater acoustic device comprising a thin spherical structure composed of polymeric piezoelectric material having a high piezoelectric stress constant and a low Young's modulus, wherein the spherical structure is gas pressurised, a film or layer of electrically conductive material on the inner end outer surfaces of the structure, and electrical terminal means contacting the film or layer on said inner and outer surfaces.

2. An underwater acoustic device according to claim 1 wherein the spherical structure is composed of polyvinylidene fluoride (pvdf).

3. An underwater acoustic device according to claim 1 or claim 2 wherein the spherical structure has an aperture and a closure member therefore wherein the closure member includes a valved inlet through which the structure may be pressurised.

4. An underwater acoustic device according to any ofthe preceding claims including attachment means for connection to a cable for downwardly suspending, or towing, the structure in water.

5. An underwater acoustic device according to any of the preceding claims wherein the electrical terminal contacting the film or layer on the inner surface is connected to a power supply line which extends through the closure member.

6. An underwater acoustic device according to any of the preceding claims wherein the spherical structure is pressurised such at the working depth there is a pressure differential across the structure wall of about 0.2 atmospheres.

7. An underwater acoustic device substantially as described herein with reference to the drawings.