Ultrasonic transducer

Abstract

An ultrasonic transducer 600 comprising: a back mass 601; a front mass 602; an ultrasonic actuator arrangement 607 held between the back mass and front mass; an ultrasonic horn arrangement 603 forward of the front mass; and a blade 611 having a bend. These components are arranged along a longitudinal axis of the transducer. Vibrations generated are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path. One or more of the back mass, front mass; wherein the ultrasonic horn arrangement comprises means for controlling the orientation of motion in a direction along a vibrational energy transfer path. Vibrations may be amplitude amplified by the ultrasonic horn. The front mass and / or ultrasonic horn may be radially asymmetric. One or more of the back mass, front mass and ultrasonic horn arrangement may, have a plurality of openings (409, Fig.4c) towards the longitudinal axis and intersecting the vibrational energy transfer path, or may be made from at least two materials having different densities. This advantageously compensates for asymmetry in the transducer's vibration pattern introduced by the radial asymmetry of the blade.

Description

The present invention primarily, although not exclusively, relates to an ultrasonic transducer and to a method of operation of an ultrasonic transducer. Background to the Invention Ultrasonic transducers are known in the art for a variety of applications. One such application is in ultrasonic surgical devices (either for hard or soft tissue, and for wound cleaning), which adopt ultrasonic vibrations to enhance cutting, sealing and cleaning performance, employing a transducer mounted in a hand-held device. An ultrasonic transducer converts high-frequency electrical signals into mechanical vibrations. In ultrasonic surgical devices, the transducer is designed to operate at a predetermined frequency. The frequency of vibration is typically dependent on the tissue type encountered, such as skin, muscle, artery, or bone. Advantages of using ultrasonic surgical devices include their ability to coagulate vessels within a localised area, thereby minimising damage to surrounding tissue and reducing blood loss. The vibration of the ultrasonic transducer is typically achieved using piezoelectric rings (in a piezoelectric stack) which, when subjected to a high-frequency electrical signal, generate a force along the longitudinal axis of the ultrasonic transducer, inducing the desired vibrational motion. To optimise performance, the transducer typically comprises a front mass and back mass that are designed to resonate at the desired frequency. This resonance condition facilitates the efficient amplification of vibrations in the desired vibrational mode (i.e., longitudinal). A common requirement of ultrasonic surgical devices is for the surgical blade to bend, which improves visibility of the blade in operation. However, this intentional bending introduces asymmetry in the transducer's vibration pattern, which is detrimental to the performance of the ultrasonic transducer. This problem is particularly crucial for small devices operating at half-wavelength, where the nodal plane in the centre of the piezoelectric stack is not symmetric, because this affects the radial stress on the piezoelectric rings. Summary of the Invention There is generally a need for an ultrasonic transducer which addresses one or more of the problems identified above. Further aims and objects of the invention will become apparent from reading the following description. According to a first aspect of the invention, there is provided an ultrasonic transducer, the ultrasonic transducer comprising: a back mass; a front mass; an ultrasonic actuator arrangement held between the back mass and the front mass; an ultrasonic horn arrangement forward of the front mass; and a blade having a bend, wherein at least the back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer, wherein vibrations generated by the ultrasonic actuator arrangement are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path, and wherein one or more of the back mass, front mass and ultrasonic horn arrangement comprises means for controlling the orientation of motion in a direction along the vibrational energy transfer path. The inventors of the present invention have surprisingly found that by including means for controlling the orientation of motion in a direction along the vibrational energy transfer path, the asymmetry in the transducer’s vibration pattern (i.e. asymmetric standing wave with respect to the longitudinal axis) - introduced by the radial asymmetry of the blade -can be compensated for. This compensation is to an extent which is dependent on the specific application of the device. For example, in applications where it is desired to maximise the longitudinal motion of the blade and minimise the bending (transverse) motion of the blade, the asymmetry can be compensated for to a greater extent. If asymmetrical motion of the blade is desired for a different application, then the asymmetry can be compensated for to a lesser extent. The inventors of the present invention have surprisingly found that the ultrasonic transducer can be designed such that the orientation of the motion is either symmetrical or asymmetrical, depending on the specific application of the device. To the best of the Applicant’s knowledge, the prior art does not appreciate the problems associated with one or more of components of the ultrasonic transducer (and preferably the blade) being radially asymmetric with respect to the longitudinal axis, and the prior art does not teach including means to control the orientation of motion and to compensate for said radial asymmetry. Preferably, the vibrations generated by the ultrasonic actuator arrangement (which are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path) are amplitude amplified by the ultrasonic horn arrangement. In some embodiments the front mass, ultrasonic horn arrangement and / or blade are separate components. In some embodiments the front mass and ultrasonic horn arrangement form a unitary structure (i.e. a unitary component). In some embodiments the ultrasonic horn arrangement and blade form a unitary structure. In some embodiments the front mass, ultrasonic horn arrangement and blade form a unitary structure. In forming the unitary structure(s), two or more components may be attached (e.g. welded) together. However, it is preferred that the unitary structure(s) is / are formed as a single structure (for example, through use of computer numerical control techniques). The inventors of the present invention have found that, compared to transducers known in the art, not only is the ultrasonic transducer according to these embodiments cheaper to make, there are also fewer metal interfaces. This improves performance of the ultrasonic transducer, because having fewer interfaces results in fewer air gaps and lower undesirable energy dissipation. Additionally, by forming two or more of the front mass, ultrasonic horn arrangement and blade as a unitary structure, there is a larger mass available for a tightening clamp to hold onto, which facilitates easier assembly. In some embodiments, the front mass is radially asymmetric. In some embodiments, the ultrasonic horn arrangement is radially asymmetric. The ultrasonic transducer preferably comprises a blade. The blade may be asymmetric. The blade may have a bend. In other words, the blade may be inclined with respect to the longitudinal axis. The inventors have found the present invention to be particularly effective at controlling (and compensating for) the asymmetry in the transducer’s vibration pattern introduced by a blade having a bend. The blade may be described as an at least partially bent extrusion. The blade may be described as curved. The blade may preferably be tapered, in that that the cross-sectional area at a proximal end (i.e. proximal to the front mass) is greater than the cross-sectional area at a distal end. The shape of the cross-sectional area at the proximal end may be the same as, or different to, the shape of the cross-sectional area at the proximal end. The shape of the cross-sectional area at the proximal end may be straight circular. The shape of the cross-sectional area at the distal end may be rectangular or square. The blade may have a chamfered edge. In some embodiments, the ultrasonic transducer comprises an asymmetric blade and the front mass is radially asymmetric. In some embodiments, the ultrasonic transducer comprises an asymmetric blade and the ultrasonic horn arrangement is radially asymmetric. In some embodiments, the ultrasonic transducer comprises an asymmetric blade and both the front mass and the ultrasonic horn arrangement are (independently) radially asymmetric. The length of the transducer, measured from the proximal end of the back mass to the distal end of the ultrasonic horn arrangement (with or without blade), along the vibrational energy transfer path, may be not more than 50 mm, or not more than 40 mm. The maximum diameter of the transducer, measured in a direction perpendicular to the length, may be not more than 15 mm. The transducer may be a Langevin transducer. The ultrasonic actuator arrangement may comprise a piezoelectric material element such as a piezoceramic element and / or a piezoelectric crystal. The ultrasonic actuator arrangement preferably comprises a piezoelectric stack. Preferably, the piezoelectric stack comprises one or more electrodes and (at least) two piezoceramic rings of opposing polarity. The two piezoceramic rings preferably sandwich the electrode. A plurality of such piezoelectric material elements may be provided. The electrodes conduct a driving signal to the piezoelectric material element(s). Where the front and / or back mass is formed of an electrically conductive material such as metal, the front and / or back mass may provide ground electrical contact(s) for the piezoelectric material element(s). One or more of the back mass, front mass and ultrasonic horn arrangement may comprise a plurality of openings towards the longitudinal axis and intersecting the vibrational energy transfer path. In some embodiments, the plurality of openings is arranged in a radially asymmetric pattern. In these embodiments, the plurality of openings are the means for controlling the orientation of motion in a direction along the vibrational energy transfer path. The inventors of the present invention have surprisingly found that by arranging the plurality of openings in (one or more) asymmetric patterns (or arrangements or arrays), the openings can compensate for the asymmetry in the transducer’s vibration pattern. As a result, the operating performance is advantageously improved, and the orientation of motion can be tailored for different applications. For the avoidance of doubt, the ultrasonic transducer (or one or more components thereof) being radially asymmetric does not preclude the ultrasonic transducer from having bilateral symmetry. The vibrational energy transfer path in the back mass, front mass and / or ultrasonic horn may include an annular portion. The annular portion may take the form of a hollow cylinder for example, with the wall of the cylinder extending along the vibrational energy transfer path and, in operation, vibrational energy passing along the wall of the cylinder. The plurality of openings intersecting the vibrational energy transfer path may therefore be provided through the wall of the annular portion. Preferably, the front mass includes the plurality of openings. In some embodiments, the plurality of openings is not provided in the ultrasonic horn arrangement. Alternatively, in some embodiments, the plurality of openings is provided in the ultrasonic horn arrangement. Openings may be formed in two or more of the back mass, front mass, and ultrasonic horn arrangement. For the avoidance of doubt, in this specification the terms “opening”, "hole", and "aperture" are used interchangeably and should be construed broadly to cover any geometrical or non-geometrical feature that extends at least partly into the ultrasonic transducer, preferably at least partly into the wall of the above-mentioned annular portion. One or more (or all) of the plurality of openings may be through holes. One or more (or all) of the plurality of openings may be blind holes. The terms “through holes” and “blind holes” are conventional in the art and refer to the depth of the opening. Each opening may have a substantially uniform cross-section along its depth. Considering the depth direction of the opening as the direction parallel to the walls of the opening, the depth direction may be substantially perpendicular to the vibrational energy transfer path. Suitable cross-sectional shapes for the openings include circular, oval, elliptical, round, triangular, quadrilateral, rectangular, square, rhombus, pentagonal, hexagonal, pentagonal, octagonal, etc. Alternatively, one or more (or all) of the plurality of openings may have a non-geometric shape (in other words, an irregular shape). The plurality of openings may be arranged in an axial direction parallel to the longitudinal axis, and / or may be arranged circumferentially. The openings may be longitudinally offset from each other along the longitudinal axis. This longitudinal arrangement may comprise two or more openings. For example, there may be 3 or more, 4 or more, 5 or more, 8 or more, 9 or more, or 10 or more openings which are longitudinally offset from each other. The pattern (or arrangement) of openings may be defined in relation to the geometric centre or centroid of each opening. The geometric centre is the average position of all points along the edge of the opening. The plurality of openings (e.g. the geometric centres) may be arranged such that there is at most one plane of reflective symmetry parallel to and coincident with the longitudinal axis. The plurality of openings may be arranged such that there are no planes of reflective symmetry parallel to and coincident with the longitudinal axis. The plurality of openings may be arranged in a pattern such that the pattern has at most one plane (or no planes) of reflective symmetry parallel to and coincident with the longitudinal axis. Preferably, there is at least one plane parallel to and coincident with the longitudinal axis such that there is a greater number of openings on one side of the plane than the other side. There may be at least one plane parallel to and coincident with the longitudinal axis such that at least about 55% (or at least about 60%, or at least about 65%, or at least about 70%, or at least about 75%, or at least about 80%, or at least about 85%, or at least about 90%, or at least about 95%, or 100%) of the plurality of openings are on one side of the plane. The plurality of openings may be arranged in a radially asymmetric pattern. The plurality of openings may be provided in a reflective asymmetrical array. The plurality of openings may be provided in an achiral array. The plurality of openings may be provided in a chiral array. For a planar cross section taken perpendicular to the longitudinal axis at a position along the longitudinal axis coinciding with a maximum total number of openings intersecting the plane, the openings may be present in a region comprising no more than about 70%, or no more than about 60%, or no more than about 50%, or no more than about 40% of the circumference of the ultrasonic transducer (e.g. the circumference of the back mass, front mass, or ultrasonic horn arrangement). In other words, there may be a continuous region having no openings. In some embodiments, for each and every planar cross section taken perpendicular to the longitudinal axis at a position along the longitudinal axis of the back mass, front mass and ultrasonic horn arrangement, there exists a continuous portion of the circumference of the ultrasonic transducer that comprises no openings. This continuous portion may comprise at least about 5%, or at least about 10%, or at least about 15%, or at least about 20%, or at least about 25%, or at least about 30%, or at least about 40% of the circumference of the ultrasonic transducer. This portion may comprise no more than about 50%, or no more than about 45%, or no more than about 40% of the circumference of the ultrasonic transducer. The openings may be arranged in a plurality of rows longitudinally offset from each other along the longitudinal axis, the rows extending around the circumference of the ultrasonic transducer (i.e. extending circumferentially). The plurality of rows may be substantially parallel to one another. In some embodiments, one or more rows have a radially symmetric pattern of openings, and one or more rows have a radially asymmetric pattern. For example, one or more rows may comprise a radially symmetric pattern of openings that extend around the entirety of the circumference of the ultrasonic transducer, and one or more rows may comprise a radially asymmetric pattern of openings that extend around only a part of the circumference of the ultrasonic transducer. In some embodiments, all of the rows have a radially asymmetric pattern of openings. For example, one or more rows may comprise a radially asymmetric pattern of openings that extend around the entirety of the circumference of the ultrasonic transducer, and one or more rows may comprise a radially asymmetric pattern of openings that extend around only a part of the circumference of the ultrasonic transducer. In some embodiments, the openings each have the same size (e.g. the same diameter if the openings have circular cross-sectional shape, or other characteristic linear dimension(s) for other shapes). The openings may have the same cross-sectional area. In some embodiments, the openings do not all have the same size, and may vary in size. The openings may be selected from a limited number of sizes (e.g. two, three or four). The openings may have the same shape as each other, or may be selected from a limited number of shapes (e.g. two, three or four), or may each have a different shape. The openings may have a cross sectional area of at least about 0.01 mm2, at least about 0.05 mm2, at least about 0.1 mm2, at least about 0.2 mm2, at least about 0.4 mm2, at least about 0.6 mm2, at least about 0.8 mm2, at least about 1 mm2, at least about 1.5 mm2, or at least about 2 mm2. The openings may have a cross sectional area of no more than about 2 mm2, or no more than about 1.5 mm2. The ultrasonic transducer may comprise at least three openings, at least four openings, at least five openings, at least six openings, at least seven openings, at least eight openings, at least nine openings, or at least ten openings. The ultrasonic transducer may comprise no more than twenty openings, or no more than fifteen openings. The openings may be formed by any suitable manufacturing technique, e.g. machining, cutting (e.g. laser cutting) or etching, or by manufacturing the component with net shape or near net shape techniques such as casting, additive manufacture, etc. At least one of the back mass, front mass and ultrasonic horn arrangement may be made from at least two materials: a first material having a first density, and a second material having a second density, the second density being different to the first density. In these embodiments, it is the component(s) made from at least two materials of differing densities which is / are the means for controlling the orientation of motion in a direction along the vibrational energy transfer path. In some embodiments, the density of at least one of the back mass, front mass and ultrasonic horn arrangement varies across a plane parallel to and coincident with the longitudinal axis. In some embodiments, the front mass and / or ultrasonic horn arrangement are made from at least two materials as hereinbefore described. In other words, in some embodiments the back mass is made of a single material, and one (or both) of the front mass and ultrasonic horn arrangement is made from at least two materials as hereinbefore described. In some embodiments, the component (i.e. the back mass, front mass and / or ultrasonic horn arrangement) comprises the first material in an amount of at least 50 wt.% of the component, or at least 60 wt.%, or at least 70 wt.%, or at least 80 wt.%, or at least 90 wt.%. In some embodiments, the component comprises about 75% wt.% of the first material and 25 wt.% of the second material. The ultrasonic transducer may be manufactured by 3D printing, preferably by 3D printing of metal. In other words, the first and / or second material is preferably 3D printed metal. The inventors have found this technique to be particularly effective at making ultrasonic transducer components from at least two materials of differing densities. Two or more components (i.e., back mass, ultrasonic actuator arrangement, front mass, ultrasonic horn arrangement, and blade) of the ultrasonic transducer may be radially asymmetric with respect to the longitudinal axis. For example, in embodiments where the ultrasonic transducer comprises a blade (preferably a blade having a bend, or an asymmetric blade), one or more of the back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement may also be radially asymmetric with respect to the longitudinal axis. Preferably, it is the shape of the component that is radially asymmetric. In some embodiments, the ultrasonic transducer comprises a radially asymmetric blade and a radially asymmetric ultrasonic horn arrangement (i.e. the ultrasonic horn arrangement has a radially asymmetric shape). In these embodiments, the radially asymmetric ultrasonic horn arrangement is the means for controlling the orientation of motion in a direction along the vibrational energy transfer path. In some embodiments, it is a tapered portion of the ultrasonic horn arrangement which is radially asymmetric, and thus the means for controlling the orientation of motion in a direction along the vibrational energy transfer path. The inventors of the present invention have surprisingly found that by having a radially asymmetric tapered portion of the ultrasonic horn arrangement, the orientation of motion in a direction along the vibrational energy transfer path can be controlled. In other words, the radially asymmetric tapered portion of the ultrasonic horn arrangement compensates (to a greater or lesser extent, depending on the specific application) for the asymmetry of the blade. The ultrasonic transducer may be for surgical, therapeutic, and / or diagnostic applications. For the avoidance of doubt, these applications include dentistry. The ultrasonic transducer may be for human and / or veterinary usage. According to a second aspect of the invention, there is provided an ultrasonic transducer, the ultrasonic transducer comprising: a back mass; a unitary component (forward of the back mass), the unitary component comprising a blade having a bend; and an ultrasonic actuator arrangement held between the back mass and the unitary component, wherein at least the back mass, ultrasonic actuator arrangement, and the unitary component are arranged along a longitudinal axis of the transducer, wherein vibrations generated by the ultrasonic actuator arrangement are conducted into the unitary component along a vibrational energy transfer path, and wherein one, or both, of the back mass and the unitary component comprises means for controlling the orientation of motion in a direction along the vibrational energy transfer path. Preferably, the unitary component comprises means for controlling the orientation of motion in a direction along the vibrational energy transfer path. Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or their embodiments, or vice versa. According to a third aspect of the invention, there is provided an ultrasonic transducer, the ultrasonic transducer comprising: a back mass; a front mass; an ultrasonic actuator arrangement held between the back mass and the front mass; an ultrasonic horn arrangement forward of the front mass; and a blade having a bend, wherein at least the back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer, wherein vibrations generated by the ultrasonic actuator arrangement are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path, and wherein one or more of the back mass, front mass and ultrasonic horn arrangement comprises a plurality of openings towards the longitudinal axis and intersecting the vibrational energy transfer path, the plurality of openings arranged in a radially asymmetric pattern to control the orientation of motion in a direction along the vibrational energy transfer path. Embodiments of the third aspect of the invention may include one or more features of the first or second aspects of the invention or their embodiments, or vice versa. According to a fourth aspect of the invention, there is provided an ultrasonic transducer, the ultrasonic transducer comprising: a back mass; a front mass; an ultrasonic actuator arrangement held between the back mass and the front mass; and an ultrasonic horn arrangement forward of the front mass, wherein the back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer, wherein vibrations generated by the ultrasonic actuator arrangement are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path, wherein one or more of the back mass, front mass and ultrasonic horn arrangement comprises a plurality of openings towards the longitudinal axis and intersecting the vibrational energy transfer path, and wherein there is a plane parallel to and coincident with the longitudinal axis such that there is a greater number of openings on one side of the plane than the other side. Embodiments of the fourth aspect of the invention may include one or more features of the first or third aspects of the invention or their embodiments, or vice versa. According to a fifth aspect of the invention, there is provided an ultrasonic transducer, the ultrasonic transducer comprising: a back mass; a front mass; an ultrasonic actuator arrangement held between the back mass and the front mass; and an ultrasonic horn arrangement forward of the front mass, wherein the back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer, wherein vibrations generated by the ultrasonic actuator arrangement are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path, wherein at least one of the back mass, front mass and ultrasonic horn arrangement is made from at least two materials: a first material having a first density, and a second material having a second density, the second density being different to the first density. Embodiments of the fifth aspect of the invention may include one or more features of the first to fourth aspects of the invention or their embodiments, or vice versa. According to a sixth aspect of the invention, there is provided an ultrasonic transducer, the ultrasonic transducer comprising: a back mass; a front mass; an ultrasonic actuator arrangement held between the back mass and the front mass; an ultrasonic horn arrangement forward of the front mass; and a blade having a bend, wherein at least the back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer, wherein vibrations generated by the ultrasonic actuator arrangement are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path, and wherein one or more of the back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement is radially asymmetric with respect to the longitudinal axis. Embodiments of the sixth aspect of the invention may include one or more features of the first to fifth aspects of the invention or their embodiments, or vice versa. According to a seventh aspect of the invention, there is provided an ultrasonic transducer, the ultrasonic transducer comprising: a back mass; a front mass; an ultrasonic actuator arrangement held between the back mass and the front mass; an ultrasonic horn arrangement forward of the front mass; and wherein at least the back mass, ultrasonic actuator arrangement, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer, wherein vibrations generated by the ultrasonic actuator arrangement are conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path, and wherein one or more of the back mass, front mass and ultrasonic horn arrangement comprises means for controlling the orientation of motion in a direction along the vibrational energy transfer path. The ultrasonic transducer according to the seventh aspect of the invention preferably comprises a blade. The blade may be symmetric. The blade may be straight. In embodiments where the blade is symmetric (i.e. straight), the means for controlling the orientation of motion in a direction along the vibrational energy transfer path can advantageously introduce asymmetric motion. For example, this is particularly advantageous where application requirements necessitate a straight blade but the motion to include a transverse (i.e. asymmetric) component. The blade may be asymmetric. The blade may have a bend. Embodiments of the seventh aspect of the invention may include one or more features of the first to sixth aspects of the invention or their embodiments, or vice versa. According to an eighth aspect of the invention, there is provided a kit of parts comprising parts operable to be assembled into an ultrasonic transducer according to any one of the first to seventh aspects of the present invention. Embodiments of the eighth aspect of the invention may include one or more features of the first to seventh aspects of the invention or their embodiments, or vice versa. According to a nineth aspect of the invention, there is provided a surgical tool comprising an ultrasonic transducer according to any one of the first to seventh aspects of the present invention. The surgical tool may further comprise one or more of a casing, a clamping jaw, and a mechanism for actuating the clamping jaw. The surgical tool may be operated by a human or by a programmable machine, such as a robot. Embodiments of the nineth aspect of the invention may include one or more features of the first to eighth aspects of the invention or their embodiments, or vice versa. According to a tenth aspect of the invention, there is provided a method of operation of an ultrasonic transducer according to any one of the first to seventh aspects of the present invention. The method preferably comprises applying an electrical signal to the ultrasonic actuator arrangement to generate vibrations to be conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path and amplitude amplified by the ultrasonic horn arrangement. The ultrasonic transducer may be operated at a power density in the range of from about 10 to about 1000 W cm-2. The ultrasonic transducer may be operated at a power in the range of from about 1 to about 1000 W. Embodiments of the tenth aspect of the invention may include one or more features of the first to nineth aspects of the invention or their embodiments, or vice versa. According to an eleventh aspect of the invention, there is provided a radially asymmetric horn for sound propagation (optionally for sound amplification), the horn having a longitudinal axis, wherein the horn comprises means for controlling the asymmetry in the horn’s acoustic vibration pattern introduced by the radial asymmetry of the horn. The inventors of the present invention have found that the inventive concept applies not just to ultrasonic transducers, but also more generally to asymmetric sound transmission through a horn. In some embodiments, the means for controlling the asymmetry comprises a plurality of openings towards the longitudinal axis and arranged in a radially asymmetric pattern. The plurality of openings compensate for the asymmetry in the horn’s acoustic vibration pattern introduced by the radial asymmetry of the horn. For the avoidance of doubt, the term “radially asymmetric” does not preclude bilateral symmetry. The plurality of openings may be arranged as described in any preceding aspect of the invention. In preferred embodiments, the radially asymmetric horn is for ultrasonic propagation (optionally for ultrasonic amplification). In preferred embodiments, the radially asymmetric horn is for use in a transducer, optionally an ultrasonic transducer, optionally a Langevin transducer. Embodiments of the eleventh aspect of the invention may include one or more features of the first to tenth aspects of the invention or their embodiments, or vice versa. According to a twelfth aspect of the invention, there is provided a radially asymmetric transducer, the transducer having a longitudinal axis, wherein the transducer comprises means for controlling the asymmetry in the transducer’s vibration pattern introduced by the radial asymmetry of the transducer. In other words, when vibrating at the resonant frequency, a standing wave having a nodal plane is generated in the radially asymmetric transducer. The transducer comprises means for controlling the angle of the nodal plane relative to the normal to the longitudinal axis of the transducer. This angle can be termed the nodal plane deflection angle (6). In some embodiments, the means for controlling the asymmetry comprises a plurality of openings towards the longitudinal axis and arranged in a radially asymmetric pattern. The plurality of openings compensate for the asymmetry in the transducer’s vibration pattern introduced by the radial asymmetry of the transducer. The plurality of openings may be arranged as described in any preceding aspect of the invention. The transducer may be an electroacoustic transducer, optionally an ultrasonic transducer. Embodiments of the twelfth aspect of the invention may include one or more features of the first to eleventh aspects of the invention or their embodiments, or vice versa. Brief Description of the Drawings There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which: Figure 1 is a side perspective view of a prior art ultrasonic transducer; Figure 2 shows a blade having a bend for use in an embodiment of the present invention, in side view (Figure 2A), and in side perspective view (Figure 2B); Figure 3 is a side view of an ultrasonic transducer, illustrating the nodal plane deflection angle 5; Figure 4 shows a front mass, ultrasonic horn arrangement and blade of an ultrasonic transducer in accordance with an embodiment of the present invention, in perspective view (Figure 4A), in front view (Figure 4B), and in bottom perspective view (Figure 4C); Figure 5 is a side view of a front mass, ultrasonic horn arrangement and blade of an ultrasonic transducer in accordance with an embodiment of the present invention; Figure 6A is a side view of a comparative ultrasonic transducer, and Figure 6B is a side view of an ultrasonic transducer in accordance with an embodiment of the present invention; and Figure 7A is a side view of a comparative ultrasonic transducer, and Figure 7B is a side view of an ultrasonic transducer in accordance with an embodiment of the present invention. Detailed Description of the Preferred Embodiments An explanation of the present invention will now be described with reference to Figures 1 to 7. Structure of Ultrasonic Transducer The ultrasonic transducer may have any suitable configuration known in the art, including those disclosed in WO / 2023 / 007013 and the Applicant’s pending GB application number GB2308810.7, both of which are incorporated herein by reference. As shown in Figure 1, a prior art transducer 100 (disclosed in WO / 2023 / 007013) has a back mass 101 and a front mass 102 with an ultrasonic horn arrangement 103 located forward of the front mass 102. Two piezoceramic rings 107 of opposing polarity sandwich an electrode 106 to form a piezoelectric stack 108 (also termed an ultrasonic actuator arrangement). The piezoelectric stack 108 is held between the back mass 101 and the front mass 102 by a prestressing bolt 104 and nut 105. The prestressing bolt 104 applies and distributes prestress within the piezoelectric stack 108. The front mass 102 has an annular portion which takes the form of an outer cylindrical wall circumscribing a longitudinal axis A. The front mass 102 has a plurality of openings (i.e. holes or apertures) 109 formed through it. The openings 109 open towards the longitudinal axis A and intersect the vibrational energy transfer path. The openings 109 all have a circular cross-sectional shape, and are arranged in three radially symmetric rows around the circumference of the front mass 102. The front mass 102 has a proximal portion 102a, intermediate portion 102b and distal portion 102c. The proximal portion 102a is in contact with the piezoelectric stack 108. The distal portion 102c is in contact with the ultrasonic horn arrangement 103. In operation, a driving signal is applied to electrode 106 and the front 102 and back 101 masses are earthed, causing oscillation of the piezoelectric rings 107. The back mass 101, piezoelectric stack 108, the front mass 102 and ultrasonic horn arrangement 103 are arranged along the longitudinal axis A of the transducer. Vibrations generated by the piezoceramic rings 107 are conducted into the front mass 102 and into the ultrasonic horn arrangement 103 along a vibrational energy transfer path. The vibrations are then amplitude amplified by the ultrasonic horn arrangement 103. In Figure 1, each of the front mass, ultrasonic horn arrangement and blade are separate components. However, in some embodiments (e.g. Figures 4 and 5), the front mass, ultrasonic horn arrangement and blade form a unitary structure. In forming the unitary structure, the three components may be attached (e.g. welded) together. However, it is preferred that the unitary structure is formed as a single structure (for example, through use of computer numerical control techniques). In alternative embodiments, just the front mass and ultrasonic horn arrangement form a unitary structure, or just the ultrasonic horn arrangement and blade form a unitary structure. It will be appreciated that various modifications can be made to the structure of the ultrasonic transducer. For example, different fastening means (instead of the nut and prestressing bolt as hereinbefore described) can be used. Blade Shape As can be seen in Figure 1, the prior art transducer 100 comprises a straight blade 110. However, in many embodiments of the present invention, and referring to Figure 2, the blade 211 has a curved (or bent) profile. It is the asymmetry of the blade 211 which typically introduces asymmetry to the ultrasonic transducer. The blade 211 can also be considered to have a tapered profile, in that that the cross-sectional area at the proximal end (i.e. proximal to the front mass) is greater than the cross-sectional area at the distal end. The shape of the cross-sectional area at the proximal end is preferably straight circular, to provide continuity with the front mass. The shape of the cross-sectional area at the distal end is preferably rectangular, to provide a flat surface for a clamping mechanism (e.g. a clamping jaw). As previously noted, an ultrasonic transducer blade is typically bent to improve visibility during operation. However, the blade 211 is usually not bent along its entire length. Instead, the blade 211 comprises a straight portion 211a and a bent portion 211b. The straight portion 211a is at the proximal end (i.e. proximal to the front mass), and the bent portion 211b is at the distal end. In operation, the bent blade 211 vibrates between a resting state and a fully extended (i.e. tensioned) state. Nodal Plane Position & Deflection Angle Each resonant mode has locations of minimal amplitude, known as nodes, and locations of maximum amplitude, known as anti-nodes, which constitute a nodal plane. To maximise vibration amplitude along the longitudinal axis A, the nodal plane should be perpendicular to the longitudinal axis A. The nodal plane deflection angle (6) is defined to be the angle between the nodal plane (denoted by 321 in Figure 3) and the axis perpendicular to the longitudinal axis A (denoted by 322 in Figure 3). A lower value of 5 (i.e. close to zero) indicates a more efficient conversion of the force generated by the piezoelectric stack 108 into longitudinal vibrations, and thus maximised longitudinal motion. It is also desirable (and, in some applications, necessary) for the position of the nodal plane to be aligned with the fixing point on the ultrasonic transducer 300. The fixing point is defined as the position along the longitudinal axis A where the fastening (e.g. the prestressing bolt) couples to the front mass or to the ultrasonic horn arrangement (depending on the specific configuration of the ultrasonic transducer). The nodal plane position (n) is defined as the distance of the nodal plane relative to the fixing point along the longitudinal axis A. A positive value of n indicates the nodal plane is positioned forward of the fixing point in the direction of the blade 311, and a negative value of n indicates the nodal plane is positioned backward of the fixing point in the direction of the back mass. The inventors of the present invention have found that use of a blade having a bend introduces a bending component to the shape of vibration. This rotates the nodal plane and increases the nodal plane deflection angle (5). This has the effect of decreasing the longitudinal motion of the blade and increasing the bending (transverse) motion of the blade. Transducer with Asymmetric Openings Figures 4A, 4B and 4C show part of a transducer 400 according to an embodiment of the present invention. In these figures, the front mass 402, ultrasonic horn arrangement 403, and blade 411 having a bend are visible. Not visible is the back mass, piezoelectric stack and fastening means (e.g. a nut and prestressing bolt). The general structure and operation of the transducer 400 is like that of the prior art transducer 100 shown in Figure 1 and described above. A key difference is that the blade 411 has a bend (as shown in Figure 2 and described above), rather than the straight bend of the prior art transducer 100. This intentional bending introduces asymmetry in the transducer's vibration pattern. A further key difference is in the size, shape and distribution of the plurality of openings. Like the prior art transducer 100, transducer 400 also has a plurality of openings 409 formed in the front mass 402. The openings 409 open towards longitudinal axis A and intersect the vibrational energy transfer path. However, unlike in Figure 1, the openings do not all have a circular cross-sectional shape and are not arranged in three radially symmetric rows. The plurality of openings 409 instead are arranged in two rows around the circumference of the front mass 402. Importantly, a greater proportion of the openings 409 are located on one “side” of the front mass 402 compared to the other (appreciating that the front mass is substantially cylindrical and thus does not have true “sides”). The inventors of the present invention have surprisingly found that the pattern (i.e., distribution or arrangement or array) of the plurality of openings in the front mass affects the nodal plane position (n) and the nodal plane deflection angle (5). Without wishing to be bound by theory, it is believed that the openings create non-uniform volumes of higher elastic compliance within the front mass, which pull the nodal plane towards it. Consequently, arranging the plurality of openings asymmetrically on the front mass pulls the nodal plane only on one side of the front mass, causing the nodal plane to rotate and change the nodal plane deflection angle (3). There are several ways in which the asymmetry of the plurality of openings 409 can be defined. In some instances, it is helpful to define the pattern of the plurality of openings 409 with respect to the geometric centre or centroid of each opening. The geometric centre is the average position of all points along the edge of the opening. As can be seen in Figure 4, the plurality of openings 409 are arranged in a pattern such that the pattern has at most one plane of reflective symmetry parallel to and coincident with the longitudinal axis A. In this embodiment, there is one plane of reflective symmetry; the plane is parallel to the bend direction of the blade. However, it will be appreciated that in alternative embodiments there may be zero planes of reflective symmetry parallel to and coincident with the longitudinal axis A. As can also be seen in Figure 4, there is at least one plane parallel to and coincident with the longitudinal axis A such that there is a greater number of openings 409 on one side of the plane than the other side. Referring to Figure 4B, one such plane is perpendicular to the page along the line A. The “left” side of the plane has a greater number of openings than the “right” side of the plane. The plurality of openings 409 in Figure 4 are provided in an achiral array. In other words, the array of openings 409 is superimposable onto a mirror image of itself. The openings 409 are arranged in two rows longitudinally offset from each other along the longitudinal axis A, the rows extending around the circumference of the front mass 402. The two rows are substantially parallel to one another (i.e. the geometric centre of the openings define rows which are substantially parallel to one another). One row comprises a radially asymmetric pattern of openings that extend around the entirety of the circumference of the front mass 402. The other row comprises a radially asymmetric pattern of openings that extend around only a part of the circumference of the front mass 402. In alternative embodiments, one or more of the rows (but not all) may have a radially symmetric pattern of openings. While the above has focused on the spatial distribution of the plurality of openings, it will also be appreciated that the size and shape of the openings will also contribute to the radially asymmetric pattern of the openings. The openings need not all have the same size, and / or need not all have the shape. As can be seen in Figures 4A and 4B, some of the openings have a circular cross-sectional shape, whereas some of the openings have an elliptical cross-sectional shape. Other suitable cross-sectional shapes for the openings include oval, round, triangular, quadrilateral, rectangular, square, rhombus, pentagonal, hexagonal, pentagonal, octagonal, etc. Alternatively, one or more (or all) of the plurality of openings may have a non-geometric shape (in other words, an irregular shape). This is illustrated in Figure 5. The plurality of openings 509a in a first region of the ultrasonic transducer 500 have either a circular cross-sectional shape or an elliptical cross-sectional shape (i.e. a geometric shape). On the other hand, many of the plurality of openings 509b in a second region of the ultrasonic transducer 500 have an irregular shape. By having irregular shapes, the radial asymmetry of the plurality of openings 509 can be precisely tuned. Alternative Embodiments In alternative embodiments, for each and every planar cross section taken perpendicular to the longitudinal axis at a position along the longitudinal axis of the back mass, front mass and ultrasonic horn arrangement, there exists a continuous portion of the circumference of an ultrasonic transducer that comprises no openings. In other words, using the front mass as an example, there is a cylindrical sector of the front mass that comprises no openings. While the plurality of openings in Figure 4 are provided in an achiral array, it will be appreciated that the plurality of openings could be provided in a chiral array, such that the array of openings is not superimposable onto a mirror image of itself. Experimental Data By way of an example, two ultrasonic transducers were designed. The two ultrasonic transducers were almost identical, and both comprised one row of radially symmetric openings (i.e. apertures). The sole difference between transducer A and transducer B is that transducer B further comprised additional 1.2 mm x 1.4 mm x 0.7 mm openings on only one “side” of the front mass, such that transducer B had a pattern of radially asymmetric openings. The nodal plane deflection angle (6) was computationally calculated using a finite element analysis (FEA) software package. In this example, Abaqus™ was used as the FEA software. However, it will be appreciated that there are alternative software packages that can be used. The software is used to find the resonant modes of vibration of the simulated ultrasonic transducer. The software calculates the shapes and frequencies of vibration. The nodal plane deflection angle (6) is measured from the shapes of vibration provided by the software. The simulation results can be experimentally verified using 3D laser vibrometry techniques (which are known in the art) from physical (i.e. real world) ultrasonic transducers. It was found that transducer A had a nodal plane deflection angle (5) of 18.4°, whereas transducer B had a nodal plane deflection angle (6) of 12.8°. Thus, it can be concluded that having a radially asymmetric pattern of openings in the front mass advantageously reduced the nodal plane deflection angle. Components with Varying Density Figure 6A shows a comparative ultrasonic transducer 600, and Figure 6B shows an ultrasonic transducer 600' according to an embodiment of the present invention. The transducers 600, 600’ have a back mass 601 and a front mass 602 with an ultrasonic horn arrangement 603 located forward of the front mass 602. Two piezoceramic rings 607 of opposing polarity sandwich an electrode 606 to form a piezoelectric stack 608 (also termed an ultrasonic actuator arrangement). The piezoelectric stack 608 is held between the back mass 601 and the front mass 602. The general structure and operation of the transducer 600 is like that of the prior art transducer 100 shown in Figure 1 and described above. A key difference is that the blade 611 has a bend (as shown in Figure 2 and described above), rather than the straight bend of the prior art transducer 100. This intentional bending introduces asymmetry in the transducer's vibration pattern. As can be seen in Figure 6A, the front mass 602 of comparative ultrasonic transducer 600 is made of a single material (e.g. metal), and the ultrasonic horn arrangement 603 is made of a single material. On the other hand, the front mass 602’ of the ultrasonic transducer 600’ of the present invention is made from two materials: a first material 602a having a first density, and a second material 602b having a second density, the second density being different to the first density. The first material 602a and second material 602b are preferably both metals. Similarly, the ultrasonic horn arrangement 603’ of the ultrasonic transducer 600’ of the present invention is made from two materials: a first material 603a having a first density, and a second material 603b having a second density, the second density being different to the first density. The first material 603a and second material 603b are preferably both metals. As discussed above, the blade 611 having a bend introduces asymmetry in the transducer's vibration pattern. However, as the front mass 602’ and ultrasonic horn arrangement 603’ are also radially asymmetric (by virtue of being made of two materials, rather than a single material), these components also introduce asymmetry in the transducer's vibration pattern. By controlling the densities of the two materials, and the relative proportions of the two materials, the asymmetry in the transducer's vibration pattern can be controlled. Thus, the blade asymmetry can be compensated for, to an extent which is dependent on the specific application of the device (i.e. whether symmetrical or asymmetrical motion is desired). Front mass 602’ comprises about 75% wt.% of a first material 602a and 25 wt.% of a second material 602b. Similarly, ultrasonic horn arrangement 603’ comprises about 75% wt.% of a first material 603a and 25 wt.% of a second material 603b. However, these quantities can be changed to control the transducer's vibration pattern and thus the orientation of motion in a direction along the vibrational energy transfer path. The front mass 602’ and ultrasonic horn arrangement 603’ shown in Figure 6B are made by 3D printing of metal, but it will be appreciated that any suitable manufacturing technique (that is, where a component can be made from at least two materials of different densities) can be used. Experimental Data By way of an example, two ultrasonic transducers were designed. Transducer C is as shown in Figure 6A, and transducer D is as shown in Figure 6B. Thus, the two ultrasonic transducers were almost identical, with the sole difference between transducer C and transducer D being that transducer D comprises a section the front mass and ultrasonic horn arrangement that is made of a more porous metal (i.e. having a different density) than the remainer of the front mass and ultrasonic horn arrangement. The nodal plane deflection angle (6) was computationally calculated as described above. It was found that transducer C had a nodal plane deflection angle (5) of 10.1°, whereas transducer D had a nodal plane deflection angle (5) of 3.2°. Thus, it can be concluded that changing the density of a portion of the front mass and / or ultrasonic horn arrangement advantageously reduced the nodal plane deflection angle. Multiple Asymmetric Components Figure 7A shows a comparative ultrasonic transducer 600, and Figure 7B shows an ultrasonic transducer 700' according to an embodiment of the present invention. The transducers 700, 700’ have a back mass 701 and a front mass 702 with an ultrasonic horn arrangement 703 located forward of the front mass 702. Two piezoceramic rings 707 of opposing polarity sandwich an electrode 706 to form a piezoelectric stack 708 (also termed an ultrasonic actuator arrangement). The piezoelectric stack 708 is held between the back mass 701 and the front mass 702. The general structure and operation of the transducer 700 is like that of the prior art transducer 100 shown in Figure 1 and described above. A key difference is that the blade 711 has a bend (as shown in Figure 2 and described above), rather than the straight bend of the prior art transducer 100. This intentional bending introduces asymmetry in the transducer's vibration pattern. As can be seen in Figure 7A, the ultrasonic horn arrangement 703 is tapered in a generally frustoconical shape. Thus, the ultrasonic horn arrangement 703 is radially symmetric and the ultrasonic transducer 700 has only one radially asymmetric component (i.e. the blade 711). On the other hand, while the ultrasonic horn arrangement 703’ of the ultrasonic transducer 600’ of the present invention is also tapered, the taper is straight on one side of the ultrasonic horn arrangement 703’ (the left-hand side) and curved on the other side of the ultrasonic horn arrangement 703’ (the right-hand side). Thus, the ultrasonic horn arrangement 703' is radially asymmetric and the ultrasonic transducer 700 has two radially asymmetric components (i.e. the blade 711’ and the ultrasonic horn arrangement 703’). As discussed above, the blade 711 having a bend introduces asymmetry in the transducer's vibration pattern. However, as the ultrasonic horn arrangement 703’ is also radially asymmetric, the ultrasonic horn arrangement 703’ also introduces asymmetry in the transducer's vibration pattern. By controlling the shape (and thus asymmetry) of the ultrasonic horn arrangement 703’, the asymmetry in the transducer's vibration pattern can be controlled. Thus, the blade asymmetry can be compensated for, to an extent which is dependent on the specific application of the device (i.e. whether symmetrical or asymmetrical motion is desired). Experimental Data By way of an example, two ultrasonic transducers were designed. Transducer E is as shown in Figure 7A, and transducer F is as shown in Figure 7B. Thus, the two ultrasonic transducers were almost identical, with the sole difference between transducer E and transducer F being that transducer F has an asymmetric horn, rather than a symmetric horn. The nodal plane deflection angle (6) was computationally calculated as described above. It was found that transducer E had a nodal plane deflection angle (5) of 11.3°, whereas transducer F had a nodal plane deflection angle (6) of 1.5°. Thus, it can be concluded that introducing asymmetry into the horn can compensate for the asymmetry of the blade, and advantageously reduced the nodal plane deflection angle. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Other Applications While the inventors of the present invention have found the inventive concept to be particularly effective with ultrasonic transducers, the present invention is not limited as such. The inventors of the present invention have surprisingly found that the inventive concept can also be used in other applications, primarily those which relate to asymmetric sound transmission through a horn. Such envisaged (but non-limiting) applications include: • Steering of a sound beam in underwater acoustics, with use particularly in navigation and fishing; • Sound transmission in asymmetric auditoriums; • Directional car park sensors; • Ultrasonic welding; and • Ultrasonic food cutters. Ultrasonic transducers are disclosed. In one aspect, the ultrasonic transducer comprises a back mass; a front mass; an ultrasonic actuator arrangement; an ultrasonic horn arrangement; and a blade having a bend. These components are arranged along a longitudinal axis of the transducer. One or more of the back mass, front mass and ultrasonic horn arrangement comprises means for controlling the orientation of motion in a direction along a vibrational energy transfer path. This advantageously compensates for asymmetry in the transducer’s vibration pattern introduced by the radial asymmetry of the blade. This compensation is to an extent which is dependent on the specific application of the device. The ultrasonic transducer is preferably for surgical, therapeutic, and / or diagnostic applications. Throughout the specification, unless the context demands otherwise, the terms “comprise” or “include”, or variations such as “comprises” or “comprising”, “includes” or “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers. Furthermore, unless the context clearly demands otherwise, the term “or” will be interpreted as being inclusive not exclusive. The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best 1 explain the principles of the invention and its practical application to thereby enable others 2 skilled in the art to best utilise the invention in various embodiments and with various 3 modifications as are suited to the particular use contemplated. Therefore, further 4 modifications or improvements may be incorporated without departing from the scope of 5 the invention as defined by the appended claims.

Claims

ms:23 1. An ultrasonic transducer, the ultrasonic transducer comprising:4 a back mass;5 a front mass;6 an ultrasonic actuator arrangement held between the back mass and the front7 mass;8 an ultrasonic horn arrangement forward of the front mass; and9 a blade having a bend,10 wherein at least the back mass, ultrasonic actuator arrangement, front mass and ultrasonic11 horn arrangement are arranged along a longitudinal axis of the transducer,12 wherein vibrations generated by the ultrasonic actuator arrangement are conducted into13 the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer14 path, and15 wherein one or more of the back mass, front mass and ultrasonic horn arrangement16 comprises means for controlling the orientation of motion in a direction along the17 vibrational energy transfer path to compensate for an asymmetry in the ultrasonic18 transducer’s vibration pattern introduced by a radial asymmetry of the blade.1920 2. The ultrasonic transducer according to claim 1, wherein controlling the orientation21 of motion comprises controlling the longitudinal motion.2223 3. The ultrasonic transducer according to claim 1 or claim 2, wherein when vibrating24 at a resonant frequency, a standing wave having a nodal plane is generated in the25 ultrasonic transducer, and wherein the ultrasonic transducer comprises means for26 controlling an angle of the nodal plane relative to the normal to the longitudinal axis of the27 ultrasonic transducer.2829 4. The ultrasonic transducer according to any one of claims 1 to 3, wherein the30 vibrations generated by the ultrasonic actuator arrangement are conducted into the front31 mass and into the ultrasonic horn arrangement along a vibrational energy transfer path,32 and are amplitude amplified by the ultrasonic horn arrangement.3334 5. The ultrasonic transducer according to any one of claims 1 to 4, wherein the front35 mass, ultrasonic horn arrangement and blade form a unitary structure.19 05 2512 6. The ultrasonic transducer according to claim 5, wherein the front mass, ultrasonic3 horn arrangement and blade are formed as a single structure.45 7. The ultrasonic transducer according to any one of claims 1 to 6, wherein the front6 mass and / or the ultrasonic horn arrangement is radially asymmetric.78 8. The ultrasonic transducer according to any one of claims 1 to 7, wherein one or9 more of the back mass, front mass and ultrasonic horn arrangement comprises a plurality10 of openings towards the longitudinal axis and intersecting the vibrational energy transfer11 path.1213 9. The ultrasonic transducer according to claim 8, wherein one or more of the plurality14 of openings has a non-geometric shape.1516 10. The ultrasonic transducer according to claim 8 or claim 9, wherein the plurality of17 openings are arranged in a radially asymmetric pattern.1819 11. The ultrasonic transducer according to any one of claims 8 to 10, wherein the20 plurality of openings are arranged such that there is at most one plane, optionally no21 planes, of reflective symmetry parallel to and coincident with the longitudinal axis.2223 12. The ultrasonic transducer according to any one of claims 8 to 11, wherein there is a24 plane parallel to and coincident with the longitudinal axis such that there is a greater25 number of openings on one side of the plane than the other side.2627 13. The ultrasonic transducer according to any one of claims 8 to 12, wherein the28 ultrasonic transducer comprises no more than twenty openings.2930 14. The ultrasonic transducer according to any one of claims 1 to 13, wherein at least31 one of the back mass, front mass and ultrasonic horn arrangement is made from at least32 two materials: a first material having a first density, and a second material having a second33 density, the second density being different to the first density.3419 05 251 15. The ultrasonic transducer according to claim 14, wherein the density of at least one2 of the back mass, front mass and ultrasonic horn arrangement varies across a plane3 parallel to and coincident with the longitudinal axis.45 16. The ultrasonic transducer according to any one of claims 1 to 15, wherein one or6 more of the back mass, ultrasonic actuator arrangement, front mass, and ultrasonic horn7 arrangement has a radially asymmetric shape with respect to the longitudinal axis.89 17. The ultrasonic transducer according to claim 16, wherein the ultrasonic horn10 arrangement has a radially asymmetric shape.1112 18. The ultrasonic transducer according to claim 17, wherein the ultrasonic horn13 arrangement has a radially asymmetric tapered portion.1415 19. A kit of parts comprising parts operable to be assembled into an ultrasonic16 transducer according to any one of claims 1 to 18.1718 20. A surgical tool comprising an ultrasonic transducer according to any one of claims19 1 to 18.2021 21. A method of operation of an ultrasonic transducer according to any one of claims 122 to 18, comprising the step of applying an electrical signal to the ultrasonic actuator23 arrangement to generate vibrations to be conducted into the front mass and into the24 ultrasonic horn arrangement along a vibrational energy transfer path and amplitude25 amplified by the ultrasonic horn arrangement.

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