Clamp-on ultrasonic sensor for use with an ultrasonic flowmeter to measure the flow velocity or the volume flow of media in a measuring tube, comprising: at least a transducer element designed to generate and/or detect ultrasonic signals; and at least an acoustic transformer, wherein the acoustic transformer has a geometric structure on a side facing towards the measuring tube, said structure being designed to convert an ultrasonic signal in the transformer to an ultrasonic signal in the measuring tube wall and/or convert an ultrasonic signal in the measuring tube wall to an ultrasonic signal in the transformer wherein the acoustic transformer has at least an ultrasonic signal conductor and at least a coupling element, wherein the ultrasonic signal conductor and the coupling element are mechanically connected in a detachable manner, and wherein the ultrasonic signal conductor is acoustically coupled to the transducer element, and wherein the coupling element has the geometric structure and is designed to be acoustically coupled to the measuring tube, wherein the geometric structure has at least two projections, wherein adjacent projections in the direction of a measuring tube axis are spaced at a distance to one another, wherein the coupling element is coupled with the ultrasonic signal conductor via a side opposite the projections, characterized in that the distance between the adjacent projections is optimized with regard to the wavelength range of the ultrasonic signals and/or with regard to the measuring tube geometry and/or the material characteristics of the measuring tube for the purpose of improving the coupling properties of the coupling element, and in that the acoustic transformer is designed to transmit the ultrasonic signals between a measuring tube wall and a transducer element in such a way that an ultrasonic signal is formed in the measuring tube wall at least as a Lamb wave mode. at least a transducer element designed to generate and/or detect ultrasonic signals; and at least an acoustic transformer, wherein the acoustic transformer has a geometric structure on a side facing towards the measuring tube, said structure being designed to convert an ultrasonic signal in the transformer to an ultrasonic signal in the measuring tube wall and/or convert an ultrasonic signal in the measuring tube wall to an ultrasonic signal in the transformer wherein the acoustic transformer has at least an ultrasonic signal conductor and at least a coupling element, wherein the ultrasonic signal conductor and the coupling element are mechanically connected in a detachable manner, and wherein the ultrasonic signal conductor is acoustically coupled to the transducer element, and wherein the coupling element has the geometric structure and is designed to be acoustically coupled to the measuring tube, wherein the geometric structure has at least two projections, wherein adjacent projections in the direction of a measuring tube axis are spaced at a distance to one another, wherein the coupling element is coupled with the ultrasonic signal conductor via a side opposite the projections, characterized in that the distance between the adjacent projections is optimized with regard to the wavelength range of the ultrasonic signals and/or with regard to the measuring tube geometry and/or the material characteristics of the measuring tube for the purpose of improving the coupling properties of the coupling element, and in that the acoustic transformer is designed to transmit the ultrasonic signals between a measuring tube wall and a transducer element in such a way that an ultrasonic signal is formed in the measuring tube wall at least as a Lamb wave mode.