Ultrasonic transducers

Abstract

 An ultrasonic transducer for generating or receiving compression waves in a fluid, comprising an elongated metal body (22) which terminates in an active surface (24), means (21) for exciting or responding to surface acoustic waves, as hereinbefore defined, in the elongated metal body, and means (23, 26) for acoustically isolating all but the active surface of the body from a fluid in which the compression waves are to be generated or from which they are to be received.
Two forms of the transducer are described, one (fig. 1) intendend for total immersion in a fluid, and the other (fig. accompanying abstract) in the form of a rod for insertion in the fluid.

Description

[0001] The present invention relates to ultrasonic transducers for producing or receiving compression waves in fluids.

[0002] If one face of a strip or plate carrying a surface acoustic wave is immersed in a fluid, then over a distance of several wavelengths of the surface acoustic wave '(10-15 if the strip is made of stainless steel) the surface acoustic wave in the strip is progressively converted into a compression wave in the fluid which travels at an angled to the normal to the strip given .by the relation

sin θ = Velocity of the compression wave in the fluid Phase velocity of the surface acoustic wave in the strip

[0003] The reverse phenomenon also occurs. That is, if a compression wave in a fluid impinges at the appropriate angle on the surface of a strip or plate immersed in the fluid, it will be mode converted into a surface acoustic wave in the strip.

[0004] The term surface acoustic wave describes an acoustic wave which involves the displacement of the surface region of a body in which the wave is travelling over a depth of the same order of magnitude as the wavelength of the wave. Thus for the purposes of this specification, Lamb waves are to be considered as surface acoustic waves in which the wavelength of the waves is comparable with the thickness of the body in which they are travelling.

[0005] According to the present invention there is provided _'an ultrasonic transducer for generating or receiving compression waves in a fluid, comprising an elongated metal body which terminates in an active surface, means for exciting or responding to surface acoustic-waves, as hereinbefore defined, in the elongated metal body, and means for acoustically isolating all but the active surface of the body from a fluid in which the compression waves are to be generated or from which they are to be received.

[0006] According to the invention in one aspect there is provided an ultrasonic transducer for generating or receiving compression waves in a fluid, comprising an enclosed chamber at least a portion of one wall of which constitutes the active surface, a metal strip within the chamber and acoustically coupled to the radiating surface, means for exciting or responding to surface acoustic waves, as hereinbefore defined, in the metal strip, and means for preventing the passage of surface acoustic waves in the surfaces of the chamber other than the active surface.

[0007] According to the invention in another aspect there is provided an ultasonic transducer for generating or receiving compression waves in a fluid, comprising a metal strip, means for generating and responding to surface acoustic waves as hereinbefore defined, a sleeve surrounding the metal strip and arranged to leave a portion of the metal strip exposed to form the active surface, the sleeve being attached to the metal strip in a liquid-tight fashion so as to prevent the access of fluid to the inside of the sleeve and means for preventing physical and acoustic contact between the inactive portion of the metal strip and the sleeve.

[0008] The invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic representation of a transducer embodying the invention.

Figure 2 shows two views of another embodiment of the invention.

[0009] Referring to Figure 1 of the drawings, a transducer 1 for the generation or reception of compression waves in a liquid 2 in which the transducer 1 is immersed consists of a stainless steel box 3 which is closed by means of top and bottom plates which are not apparent in the figure. The major part 4' of one wall 4, of the box 3 is constituted by one limb 5 of a u-shaped stainless steel strip 6. Attached to the free end 7 of the strip 6 is.a piezo electric or electromagnetic acoustic generator 8 of surface acoustic waves in the strip 6 in response to electr-cal signals applied via leads 9. Piezo electric generation would normally be used wherever possible, but electromagnetic/acoustic generation has distinct advantages if the transducer is to be located in a hostile liquid, e.g. radioactive or at a high temperature. As is usual in the art, the generator 8 also will respond to surface acoustic waves in the strip 6 to generate electrical signals, thus enabling the transducer 1 to be used in either generating or receiving modes. The top and bot+om plates of the box 3 and the leads 9 have seals associated with them so that no liquid can penetrate into the inside of the box 3. The surfaces of the box 3, with the exception of the part 4' of the wall 4 are treated so as to render them incapable of transmitting coherently surface acoustic waves. A suitable way of doing this is by roughening or breaking up the smooth surface. Surface acoustic waves produced by the generator 8 and travelling in the metal strip 6 will only appear at the outer surface 4" of the part 4' of the wall 4' of the box 3, which therefore forms an active surface. As has been explained, such surface acoustic waves will be mode converted into a beam 10 of compression waves in the liquid 2 travelling at an angle δ to the normal to the active surface 4". Conversely, compression waves in the liquid 2 which impinge on the active surface 4" at the same angle will be converted into surface acoustic waves which will travel along the strip 6 to the generator 8, there to be converted into electrical signals.

[0010] Figure 2 shows an alternative form of transducer in which the surface acoustic wave generator/receiver is positioned outside the liquid in which compression waves are to be generated. Referring to Figure 2, a piezoelectric crystal 21 is arranged to generate surface acoustic waves in a stainless steel strip 22. One way of doing this, shown in Figure 2 is to use a comb type coupler 28 between the crystal and the metal strip...The strip 22 is surrounded by a sleeve 23 which extends to the bottom of the strip 22 on one side, but terminates somewhat shot of the end on the other side so as to leave a portion of the strip 22 exposed to form an active surface 24. The sleeve 23 is welded to the strip 22 around the periphery of the active surface 24 so as to isolate the remainder of the strip 22 from a liquid 25 in which a compression wave is to be generated. The strip 22 is maintained in a central position in the sleeve 23 by means of a support 26 which makes infrequent contact only with the acoustically quiescent edges of the strip 22. This could be in the form of a specially shaped coil spring, 26, as shown in Figure 2. As in the previous embodiment surface acoustic waves travelling down the strip 22 are mode converted at the active surface 24 into a beam of compression waves 28 in the liquid 25, and vice versa.

Claims

1. An ultrasonic transducer for generating or receiving compression waves in a fluid, comprising an elongated metal body which terminates in an active surface, means for exciting or responding to surface acoustic waves, as hereinbefore defined, in the elongated metal body, and means for acoustically isolating all but the active surface of the body from a fluid in which the compression waves are to be generated or from which they are to be received.

2. An ultrasonic transducer according to Claim 1, wherein there is provided an enclosed chamber at least a portion of one wall of which constitutes the active surface, a metal strip within the chamber and acoustically coupled to the radiating surface, means for exciting or responding to surface acoustic waves, as hereinbefore defined, in the metal strip, and means for preventing the passage of surface acoustic waves in the surfaces of the chamber other than the active surface.

3. An ultrasonic transducer according to Claim 1, wherein there is provided a metal strip, means for generating and responding to surface acoustic waves as hereinbefore defined, a sleeve surrounding the metal strip and arranged to leave a portion of the metal strip exposed to form the active surface, the sleeve being attached to the metal strip in a liquid-tight fashion so as to prevent the access of fluid to the inside of the sleeve and means for preventing physical and acoustic contact between the inactive portion of the metal strip and the sleeve.

4. An ultrasonic transducer according to any of Claims 1 to 3, wherein the means for generating and responding to the said surface acoustic waves comprises a piezo-electric device so shaped and positioned on the elongated metal body as to generate the said surface acoustic waves in the elongated metal body in response to an exciting stimulus.

5. An ultrasonic transducer according to Claim 4, wherein the piezoelectric device comprises a lamina of piezoelectric material and a coupler, the coupler having a plane surface to which the piezoelectric lamina is bonded and an opposite surface having a plurality of regularly spaced slots therein to provide a plurality of regularly spaced contacting surfaces.

6. An ultrasonic transducer according to any of Claims 1 to 3, wherein the means for generating and responding to the said surface acoustic waves in the elongated metal body utilises electromagnetic forces to generate the said surface acoustic waves.

7. An ultrasonic transducer substantially as hereinbefore described with reference to the accompanying drawings.

Drawing