WO1987007069A1

WO1987007069A1 - Acoustic transducer

Info

Publication number: WO1987007069A1
Application number: PCT/GB1987/000326
Authority: WO
Inventors: Michael Laurence Henning
Original assignee: Plessey Overseas Limited
Priority date: 1986-05-12
Filing date: 1987-05-12
Publication date: 1987-11-19
Also published as: GB8611573D0; AU7395387A; GB2190561B; EP0267223A1; GB2190561A

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

ACOUSTIC TRANSDUCER

* - This invention relates to an acoustic transducer. It relates particularly to a transducer body capable of being used in a passive sonar system.

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 of fifteen centimetres square. If building such a large area transducer by a conventional constructional method, one might choose to form a matrix of smaller 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 of this construction, however, is that the 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 by 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 transducer for detecting an acoustic wave, the transducer comprising an array of transducer 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 aperture thereby to 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 of the 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 their major axes on a first common 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.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawing, in which:

Figure 1 is a plan view of an acoustic transducer according to the invention, and.

Figure 2 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 encapsulant material and this surrounded a set of primary transducer elements 2 forming a row, with a first set of secondary transducer ele ents 3 located above this row and a second set of secondary transducer 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 of the elements (2, 3 and 4) in a horizontal arrangement within the body with a reduced number of the elements in a vertical arrangement. Thus the body 1 had five elements in each horizontal row but only three elements in each vertical column.

A further control over the noise rejection behaviour was achieved by controlling the area of each of the 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 shape for such an element might thus be an ellipse with a horizontal major axis and a vertical minor axis. In the embodiment depicted in Figure 1, the elements ( 2 , 3 and 4) were shaped like lemons with each major axis arranged horizontally.

It will be noticed that in the case of the primary elements 2, these have all been positioned with their major axes 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 positioned with their major axes located on a second common line, the said second line being aligned parallel to the first 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 primary transducer elements 2 had the two elements 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 of the two sets of secondary transducer elements (3, 4) the elements at the ends of the set were similarly of rather a small area in size, the two next elements had slightly larger areas, and the element in the middle of the set had the largest area.

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

This provision ensured that the transducer body had a greater sensitivity to the acoustic signal in the middle portion of its area rather than 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 of the body 1.

In the construction of the 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 for the transducer elements was a lead zirconate titanate ceramic composition which was capable of being made by a moulding process in the form 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 polyvinyl idene 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 nade without departing from the scope of the invention as defined in the ap eale claims. For instance, although the acoustic transducer has been described as being of a square shape with a side of fifteen centimetres, it could clearly be stade 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 of the transducer bodies.

Claims

CLAI S ;

1. A transducer for detecting an acoustic wave, the transducer comprising an array of transducer 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 aperture thereby to attenuate the response of the transducer to spatial noise lying outside the spatial spectrum of an acoustic wave to be detected by the transducer.

2. A transducer as claimed in claim 1, in which 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.

3. A transducer 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. A transducer 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. A transducer as 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 the first line.

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

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