GB2087687A

GB2087687A - Underwater sound transducer

Info

Publication number: GB2087687A
Application number: GB8131496A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1980-10-21
Filing date: 1981-10-19
Publication date: 1982-05-26
Also published as: GB2087687B

Abstract

The present invention provides an underwater sound transducer intended particularly, though not exclusively, for the generation of low frequency sound for the acoustic testing of passive sonobuoys. The transducer includes a casing having at least one wall 1 which comprises a diaphragm, means for pressurising the casing, and a diaphragm actuator comprising at least one thin strut 9, 9, 9a, 9b, composed of piezo-electrical material, preferably polyvinylidene fluoride, and electrical terminal means 19 contacting opposite faces of the strut, or each of the struts, wherein the strut, or each of the struts, is attached at one end thereof to the diaphragm and is arranged to vibrate the diaphragm by the piezo-electric effect. <IMAGE>

Description

SPECIFICATION Improvements in or relating to underwater sound transducers The present invention relates to underwater sound transducers and particularly to low frequency trans- ducers. The invention is of particular, though not exclusive, relevance to the acoustic testing of passive sonobuoys.

Known underwater sound transducers employ either a slab of piezo-electric materials, 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. Many ofthe transducers described in the NRL report do not operate, or operate inefficiently, at low frequencies.

In addition many prior art transducers intended for low frequency underwater operation tend to be bulky and some have excessively high power input requirements, and are not suitable for towing to simulate a moving low frequency sound source.

The present invention provides a compact sound transducer for low frequency operation particularly in the range 500 Hz to 50 kHz.

According to the present invention an underwater sound transducer includes a casing having at least one wall which comprises a diaphragm, means for pressurising the casing, a diaphragm actuator comprising at least one thin strut composed of piezoelectric material, and electrical terminal means contacting opposite faces of the strut, or each of the struts, wherein the strut or each of the struts is attached at one end thereof to the diaphragm and arranged to vibrate the diaphragm by the piezoelectric effect.

The piezo-electric material is preferably polyvinylidene fluoride (PVDF).

The transducer may have a second diaphragm which also forms a wall of the casing and which faces the other diaphragm, and wherein each thin strut is attached at its opposite ends to the diaphragms.

Preferably the strut or struts, together with the diaphragm, or diaphragms, form a resonant structure within the range of vibration frequencies of the transducer.

The, or each strut, may comprise a thin wall which for example, may be flat or have a tubular, spiral or other curved configuration. The, or each strut, may be shaped or curved to provide a low stiffness member.

Preferably the electrical terminal means for applying an ac signal to the piezo-electric material of the strut or struts includes contacts which are urged against opposite faces of the, or each, strut.

Embodiments of underwater sound transducers in accordance with the invention will now be described by way of example only with reference to the drawings of which: Figure 1 is a plan view of an underwater sound transducer in accordance with the invention, Figure 2 is a sectional view of the transducer of Figure 1, Figures 3 and 4 are schematic plan views of further transducers having different transducer element configurations.

The sound transducer shown in Figures 1 and 2 include identical upper and lower rigid, support rings 1 and 2, respectively, which are held in a coaxial, spaced relationship by means of three spacer tubes 7 and clamping bolts 3 which extend through the tubes and screw threaded bores in the rings 1,2 and hold the rings a fixed distance apart. The support rings 1,2 and the tubes 7 form a housing for upper and lower diaphragms 5, 6 which comprise rigid discs. The diaphragms are each resiliently mounted upper and lower annular foam rubber members 12, 13 respectively which extend circumferentially around the faces of each diaphragm.A rubber cover 4 extends over, and is bonded to, the outer edges of the rubber members 12, and the diaphragms 5, 6 and is also bonded to the lower face of the upper support ring 1 and the upper face of the lower support ring 2, so that the diaphragms and the cover 4 form a water-tight compartment 30. Such a construction permits large linear movements of the diaphragms.

The transducer has a conventional hydrostatic pressure compensator 14 to enable the transducer to operate at depths to about 24 m below the surface.

The compensator comprises an inflatable rubber bag 16 which is inflatable via an air inlet valve 15 and which communicates with the compartment 30 via an inlet pipe 17 bonded in a hole in the lower diaphragm 6.

In the watertight compartment 30 three sets of piezo-eletric assemblies 18a, 18b and 1 & are located in radial planes extending outwardly from the axis of the cylindrical housing, the planes being 120"apart as seen in Figure 1. The piezo-electric assemblies 18a, 18b, 1 & are identical and will be described by reference to assembly 18b which is shown in detail in Figures 1 and 2. The assembly 18b comprises radially outer and inner piezo-electric struts 9a and 9b composed of PVDF sheet having a layer 8 of silver on each face. The assemblies 18a, 18b, 18c are slotted into, and bonded to, radially extending slots 10 in the diaphragms 5 and 6 as can be seen in Figure 2.A pair of electrical contacts 19 is associated with each strut and make electrical contact with the conductive layers 8 on the struts. The electrical contacts are connected via wires 20 to a cable (not shown) which extends through a watertight seal 21 in a hole in the upper diaphragm 5. The cable is connected to an ac electrical generator (not shown).

In the embodiment shown the PVDF struts were each 20 mm x 20 mm x 0.5 mm and the diaphragms 5 and 6 were composed of 'Perspex' (RTM) and were 140 mm dia x 12.7 mm thick. The electrical supply produced 1.5 kV, RMS at 600 Hz, and the output from the transducer was 0.1 Ci BarNolt at 1 m and at 600 The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

Hz. When the transducer is submerged the inflated compensator bag 16 is compressed and increases the air pressure in the compartment 13 to resist col lapse of the compartment.

The embodiments of Figures 3 and 4 have a hous ing 22 and upper and low diaphragms 23 and 24, respectively, which are generally of the same construction as the embodiment of Figure 1.

The embodiment of Figure 3 has a piezo-electric strut 25 having silvered surfaces which is of tubular configuration. Two electrical contacts 26, 27, are held againstthe slivered surfaces of the strut 25 and an ac current applied to the contacts via wires and a cable (not shown) as in the Figure 1 embodiment.

Figure 4shows a transducer which is generally similar to that of Figure 3 wherein a piezo-electric strut having silvered surfaces 28 and of spiral configuration is used as an alternative to the tubular strut of Figure 3.

Claims

1. An underwater sound transducer including a casing having at least one wall which comprises a diaphragm, means for pressurising the casing, and a diaphragm actuator comprising at least one thin strut composed of piezo-electric material, and electrical terminal means contacting opposite faces of the strut, or each of the struts, wherein the strut, or each of the struts, is attached at one end thereof to the diaphragm and is arranged to vibrate the diaphragm by the piezo-electric effect.

2. An underwater sound transducer as claimed in claim 1, including two diaphragms, the one facing the other, and wherein each ofthe said thin struts is attached at its opposite ends to the diaphragms.

3. An underwater sound transducer as claimed in claim 1 or claim 2 wherein the strut or struts together with the diaphragm or diaphragms, form a resonant structure within the range of vibration frequencies of the transducer.

4. An underwatersound transducer as claimed in any previous claim wherein the, or each strut, comprises a thin walled strip.

5. An underwater sound transducer as claimed in claim 4 wherein the thin walled strip is curved to provide a low stiffness member.

6. An underwater sound transducer as claimed in any of claims 1 to claim 3 wherein the, or each strut, comprises a thin walled tube.

7. An underwater sound transducer as claimed in any previous claim wherein the piezo-electric material is polyvinylidene fluoride.

8. An underwater sound transducer as claimed in any previous claim wherein the electrical terminal means comprise spring contacts which are urged agains opposite faces of the, or each, strut.

9. An underwater sound transducer substantially as described herein with reference to the drawings.