a diverging lens 18 the ultrasonic rays 19 emitted by the transducer are reflected on the inner wall of the hollow cylinder 16 all the ultrasonic rays reflected from an inner peripheral circle intersect at a point along the cylinder axis the annular transducer shown in fig3 represents another possibility of producing a rotation symmetrical and convergent sonic field this transducer comprises an annular housing 21 of substantially u shaped cross section the axis of symmetry of the cross sectional surface being inclined to the ring axis and intersecting the same at a given distance for example 12 cm the open side of the u shaped cross section faces this point of intersection and hence the ring axis an annular damping block 24 of epoxy resin tungsten which is acoustically insulated from the housing by cork panels 22 is situated in the annular recess of the housing on the surface of the damping block 24 facing the open side of the housing there are disposed a plurality of in this case four flat segments 23 of a piezoelectric material which together form a ring and which are covered by a layer 25 of epoxy resin suitable for resistance to water the layer 25 also provides mechanical protection for the piezoelectric oscillator formed by the segments 23 an electrical lead 26 runs to each of the segments from the back through suitable bores in the housing 21 and in the damping block 24 the ultrasonic waves emitted by the piezoelectric oscillator form an annular bunch which focuses in a focal line on the ring axis at a certain distance from the ultrasonic transducer the length of the focal line determines the depth over which.