GB2190561A

GB2190561A - Acoustic transducer

Info

Publication number: GB2190561A
Application number: GB08611573A
Authority: GB
Inventors: Michael Laurence Henning
Original assignee: GE Healthcare UK Ltd; Plessey Co Ltd
Current assignee: GE Healthcare UK Ltd; Plessey Co Ltd
Priority date: 1986-05-12
Filing date: 1986-05-12
Publication date: 1987-11-18
Also published as: EP0267223A1; GB2190561B; GB8611573D0; AU7395387A; WO1987007069A1

Abstract

A plane-shaped transducer body for detecting an acoustic signal in the presence of noise, the transducer body comprising an array of transducer elements having a non-uniform spatial filter response across its working aperture to attenuate the response to noise lying outside the spatial spectrum of a wave to be detected by the elements, with primary elements (2) mainly for detecting the required acoustic signal and secondary elements (3, 4) mainly for sensing noise in the signal, the elements (2, 3, 4) being spaced apart from one another over the area of the body (1) in a linear arangement such that the body will act as a rejector of noise pressure waves incident from a given direction (6) but as a receptor to signals or noise from an alternative direction (7).

Description

SPECIFICATION Acoustic transducer This invention relates to an acoustic transducer. It relates particularly to a transducer body capable of being used in a passive sonarsystem.

In the design of a transducer body for a passive sonar system there is a requirement to provide a useful transducer surface having a comparatively large area, for example an area offifteen centimetres square. If building such a largeareatransducerbya conventional constructional method, one might choose to form a matrix ofsmaller elements of transducer material so as to create the whole area of sensitive surface that would be required in the finished body. This constructional method could thus enable the required area of sensitivity to be provided, but the constructional cost would be likely to be high.

One advantage ofthis construction, however, is thatthe transducer body obtained is extremely effective in operation since the large area of the transducer can be used to reduce the effects of unwanted noise buy a spatial integration process.

Attempts have been made to reduce the constructional cost by providing a regular spaced arrangement of the elements of transducer material within the large area but then it has been found that the performance of the transducer body as a rejection filter for unwanted spatial frequencies is degraded.

The present invention was devised to provide a transducer body that was capable of being made in a large area construction and at a comparatively low cost, but which would still have a built-in capacity of distinguishing an acoustic beam pattern signal from noise field effects.

According to the invention, there is provided a transducerfor detecting an acoustic wave, the transducer comprising an array oftransducer elements, each transducer element having a spatial filter response and a working aperture for receiving the acoustic wave, at least one transducer element of the array having a non-uniform spatial filter response across its working aperturetherebyto attenuate the response of the transducer to spatial noise lying outside the spatial spectrum of an acoustic wave to be detected by the transducer.

Preferably, at least one of the said transducer elements is shaped with a major and a minor axis of different lengths, the element being aligned within the said array of elements such that the said major axis is located in line with the expected source of the noise pressure wave to be rejected. Two or more transducer elements, each element having a major and a minor axis of different lengths, may be aligned within the said array of elements such that the said major axes of all of said elements are located in line with the expected source ofthe noise pressure wave to be rejected. The said major axes of all of said elements may be positioned on a common line within the array, the said common line being located in line with the expected source of the noise pressure wave to be rejected.

In addition to having elements positioned with theirmajoraxeson afirstcommon line,the transducer may include further elements arranged with their major axes located on a second common line, the said second line being aligned parallel to the first line.

Bywayofexample, an embodimentofthe invention will now be described with reference to the accompanying drawing, in which: Figure lisa plan view of an acoustic transducer according to the invention, and, Figure2 is a side view of the transducer.

As shown in Figure 1, the acoustic transducer had a rectangular-shaped body 1 which was in the form of a thin plane. The body 1 was in the shape of a square having each side fifteen centimetres in length and this was intended to provide a sensitive transducer area for use in a passive sonar system.

The body 1 was formed of an encapsulantmaterial and this surrounded a set of primary transducer elements 2 forming a row, with a first set of secondary transducer elements 3 located above this row and a second set ofsecondarytransducer elements 4 located below the elements 2.

This arrangement of the transducer elements within the body 1 had been designed to preferentially reject a horizontal pressure wave incident on the body from a direction indicated by the arrow 6, since the transducer body had a low rejection performance to any vertical pressure wave incident on the body from the direction of the arrow 7.

In the design of the arrangement of the transducer elements within the area of the body 1, some selectivity over the degree of noise rejection available had been made by locating a large number ofthe elements (2,3 and 4) in a horizontal arrangement within the body with a reduced number ofthe elements in a vertical arrangement. Thus the body 1 had five elements in each horizontal row but onlythree elements in each vertical column.

Afurthercontrol overthe noise rejection behaviourwas achieved by controlling the area of each ofthe transducer elements which was exposed to noise from a vertical direction with respect to the area exposed to noise from a horizontal direction.

The filter response of the transducer elements was thus 'shaded' to reduce the response to noise from the unwanted direction. One possible shapeforsuch an element might thus be an ellipse with a horizontal majoraxisand avertical minoraxis. In the embodimentdepictedin Figure 1,the elements (2,3 and 4) were shaped like lemons with each major axis arranged horizontally.

Itwill be noticed that in the case of the primary elements 2, these have all been positioned with their majoraxes located on a common line which is aligned with the arrow 6 which defines the direction of the expected source of unwanted noise pressure waves to be rejected by the transducer.

Similarly, in the case of the secondary elements 3, these have been positionedwiththeirmajoraxes located on a second common line, the said second line being aligned parallel to thefirst line. The arrangement is the same in the case of the secondary elements 4.

An additional control over the noise rejection performance was achieved by varying the sensitivities of the elements of each set across the full transducer body aperture. This was done by using elements of different areas (even though they were of similar shapes) which were exposed to the acoustic signal. This thus gives a control over the working aperture of each element which was capable of receiving the acoustic wave.

Thus in the embodiment of Figure 1, the set of primarytransducerelements2hacithetwoelements at the ends of the set of rather a small area in size, the two next elements had slightly larger areas, and the element in the middle of the set of five elements had the largest area.

In the cases ofthetwo sets of secondary transducer elements (3,4) the elements at the ends of the set were similarly of rather o small area in size, the two next elements had slightly larger areas, and the element in the middle ofthe set had the largest area.

The secondarytransducer elements were generally of a smaller area than the primary transducer elements.

This provision ensured thattheti ansducer body had a greater sensitivity to the acoustic signal in the middle portion of its area ratherthan on the periphery.

Figure 2 is a side view of the transducer body 1 as seen from below. The elements 4 are seen to be supported centrally within the thickness ofthe body 1.

In the construction ofthe embodiment according to the invention, the actual size of the transducer elements was made to be comparable to the wavelength of the expected noise since this provision was effective to integrate out the noise effects from the received signal . The material used forthe transducer elements was a lead zirconate titanate ceramic composition which was capable of being madebya moulding processintheform of a flat plate having the required lemon shape. In an alternative embodiment, a modified lead titanate or a piezo sensitive plastics material such as a olyvinylidene fluoride composition might be used.

The foregoing description of an embodiment of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, although the acoustictransducer has been described as being of a square shape with a side of fifteen centimetres, it could clearly be made of a different shape if necessary, and have a size which is greater or less than this measurement. A large area could conveniently be built up by mounting the resulting transducer bodies alongside one another like tiles on a wall. In this way, a hydrophone array having a comparatively large surface area could be built up from a regular arrangement ofthetransducer bodies.

Claims

1. Atransducerfor detecting an acoustic wave, the transducer comprising an array oftransducer elements, each transducer element having a spatial filter response and a working aperture for receiving the acoustic wave, at least one transducer element of the array having a non-uniform spatial filter response across its working aperturetherebyto attenuate the response ofthetransducerto spatial noise lying outside the spatial spectrum of an acoustic wave to be detected bythetransducer.

2. Atransduceras claimed in claim 1,in which at least one ofthe said transducer elements is shaped with a major and a minor axis of different lengths, the element being aligned within the said array of elements such that the said major axis is located in line with the expected source of the noise pressure wave to be rejected.

3. Atransducer as claimed in claim 2, in which two or more transducer elements, each element having a major and a minor axis of different lengths, are aligned within the said array of elements such that the said major axes of all of said elements are located in line with the expected source of the noise pressure wave to be rejected.

4. Atransducer as claimed in claim 3, in which the said major axes of all of said elements are positioned on a common line within the array, the said common line being located in line with the expected source of the noise pressure wave to be rejected.

5. Atransduceras claimed in claim 3, in which in addition to having elements positioned with their major axes on a first common line, the transducer includes further elements arranged with their major axes located on a second common line, the said second line being aligned parallel to thefirst line.

6. Atransducer substantially as hereinbefore described, with reference to the accompanying drawing.

7. A hyd rophone comprising an arrangement of transducers as claimed in any one of claims 1 to 6.