GB2144308A

GB2144308A - Electro-acoustic transducer element

Info

Publication number: GB2144308A
Application number: GB08320235A
Authority: GB
Inventors: Lynn W Lipscombe
Original assignee: DB INSTRUMENTATION Ltd
Current assignee: DB INSTRUMENTATION Ltd
Priority date: 1983-07-27
Filing date: 1983-07-27
Publication date: 1985-02-27
Also published as: GB2144308B

Abstract

A hydrophone element 10 is formed from two hemispheres 11, 12 of piezoelectric material. A dome-shaped pair 13, 14 and an annular-shaped pair 15, 16 of electrodes are formed on hemisphere 11 and two similar pairs of electrodes 17, 18 and 19, 20 are formed on hemisphere 12. The electrodes are connected in series between terminals 22 and 23 by leads 24 to 28. Each pair of electrodes is joined to the adjacent pair with the inside electrode of one pair connected to the outside electrode of the other pair. Three alternative configurations for the electrodes are described. <IMAGE>

Description

SPECIFICATION Electro acoustic transducer element This invention relates to an electro-acoustic transducer element and particularly, but not exclusively, to a hydrophone element.

In two presently known hydrophone designs, the hydrophone elements are formed respectively as a cylindrical shell and as a spherical shell. The cylindrical hydrophone element has relatively good sensitivity but poor directional performance whilst the spherical hydrophone element has good directional performance but relatively poor sensitivity.

It is an object of this invention to provide a new electro-acoustic transducer of spherical configuration and having improved sensitivity.

According to this invention there is provided an electro-acoustic transducer element comprising a spherical shell of electro-acoustic material and at least three pairs of electrodes positioned on said shell, the two electrodes of each pair being positioned on the inside and outside surfaces of a respective area of the shell and the pair of electrodes being connected in series.

By providing at least three pairs of electrodes which are connected in series, improved sensitivity may be obtained.

In a preferred arrangement, each electrode is formed symmetrically around a common axis with the pairs of electrodes occupying spaced positions along said common axis.

Also, in the preferred arrangement, each adjacent pair of electrodes is connected together with the inside electrode of one pair being connected to the outside electrode of the other pair.

Each pair of electrodes may be formed on a substantially equal area of the shell.

This invention will now be described in more detail, by way of example with reference to the accompanying drawings, in which: Figure 1 is a view partly in section of a hydrophone element embodying this invention; Figure 2 is a view partly in section of one half of another hydrophone element embodying this invention; and Figures 3 and 4 are views of two further hydrophone elements embodying the invention.

Referring now to Figure 1,there is shown a hydrophone element 10 which is formed from two hemispherical shells 11, 12 of peizoelectric material.

The two shells 11 and 12 are joined together but are illustrated in an exploded position. Two pairs of electrodes 13,14 and 15,16 are formed on the hemisphere 11 and two further pairs of electrodes 17,18 and 19,20 are formed on the hemisphere 12.

The electrodes 13 to 20 may be formed by painting or sputtering metal onto the surfaces of the hemispheres 11,12. In each pair of electrodes, one pair of electrodes is formed on the outer surface of the shell whilst the other electrode is formed on the inside surface. Each electrode is formed symmetrically around a common axis 21 and, as may be observed, the electrodes 13,14, 19 and 20 are formed on polar regions of the shell 11, 12 and so are dome-shaped and electrodes 15, 16, 17 and 18 are annular shaped.

The hydrophone element 10 includes a pair of terminals 22,23. Terminal 22 is connected to electrode 14 by a lead 24. In order to connect the pair of electrodes 13 and 14 to the pair of electrodes 15 and 16, the outside electrode 13 is connected through a lead 25 to the inside electrode 16. Similar connections are made between the remaining pairs of electrodes with leads 26 and 27 and electrode 20 is connected to terminal 23 by lead 28. The leads 24 and 28 pass through a cut-out 29.By connecting adjacent pairs of electrodes together so that the outer electrode of one pair is connected to the inner electrode of the other pair, the potential difference between adjacent electrodes on the inside and outside surfaces of the shell 11,12 isminimised. For example, the potential difference between electrodes 13 and 15 is equaltothe potential difference between the pair of electrodes 13 and 14.

Consequently, loss of sensitivity caused by stray capacitance between adjacent pairs of electrodes such as electrodes 13 and 15 is also minimised.

The hydrophone element 10 has an outside diameter of 14 mm and the shell thickness is 0.8 mm. The dimensions A, B, C and D marked on Figure 1 are respectively 2.75 mm, 1.0 mm, 2.75 mm, and 0.5 mm. With these dimensions, the area of the shell 11,12 enclosed by each pair of electrodes is identical and this serves to make the capacitance between electrodes 22 and 23 as large as possible.

Referring now to Figure 2, there is shown one of two identical hemispheres which form a spherical hydrophone element 40. In the hydrophone element 40, each hemisphere is provided with four pairs of electrodes 41 to 44. In Figure 3 there is shown another spherical hydrophone element 50 and the element 50 has five annular shaped pairs of electrodes 51 to 55. Although not shown, the individual pairs of electrodes of the hydrophone elements 40 and 50 are connected together in the same manner as shown in Figure 1.

In Figure 4, there is shown a spherical hydrophone element 60 having eight quadrant shaped pairs of electrodes which are connected in series. The outer electrodes of four of the pairs are indicated at 61 to 64.

Each of the four hydrophones 10,40, 50 and 60 has an improved sensitivity in comparison with a hydrophone element formed from two parallel connected hemispheres and it is considered that improved sensitivity may be achieved with as few as three pairs of electrodes connected in series.

However, the sensitivity of hydrophone 60 is inferior to the other three hydrophone elements and is substantially less than theoretically expected. It has been found that this loss of sensitivity is caused by the asymmetrical arrangement of the device and the stray capacitance between individual electrodes formed on the outer and inner surfaces. As a result of the asymmetrical arrangement the potential difference between some of the adjacent electrodes or the inside or outside surface is equal to the potential difference across several pairs of electrodes.

Each of the hydrophone elements 10,40,50 and 60 may be formed from a material known under the trade name PZT or other equivalent materials. PZT is an electrostrictive material which comprises lead zirconate to which has been added a small amount of lead titanate and which is polarised to give properties which are analogous to piezoelectric materials. In this specification, the term peizoelectric should be interpreted to include electrostrictive materials such as PZTwhich are polarised. Each of the hydrophone elements 10,40,50 and 60 may be formed into a hydrophone assembly by encapsulation in polyurethane inside an oil-filled 2 mm thick butyl rubber boot.

Although the present invention has been described with reference to a hydrophone element, it is to be appreciated that the invention may also be applied to electro-acoustic receivers operating in other media such as air.

Claims

1. An electro-acoustic transducer element comprising a spherical shell of electro-acoustic material and at leastthree pairs of electrodes positioned on said shell, the two electrodes of each pair being positioned on the inside and outside surface of a respective area of the shell and the pairs of electrodes being connected in series.

2. A transducer element as claimed in claim 1 in which each electrode is formed symmetrically around a common axis with the pairs of electrodes occupying spaced positions along said common axis.

3. A transducer element as claimed in claim 2 in which each adjacent pair of electrodes is connected together with the inside electrode of one pair being connected to the outside electrode of the other pair.

4. A transducer element as claimed in any one of the preceding claims in which each pair of electrodes is formed on a substantially equal area of the shell.

5. An electro-acoustic transducer element substantially as hereinbefore described with reference to and as shown in the accompanying drawings.