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. <EOS>