integration electrode 2 and the transducers 3 and 4 are disposed on the surface of a substrate 5 of piezoelectric material illustrated only in section fashion with z and u00b1 x axial directions of the plane coinciding with the surface of the substrate 5 are indicated fig2 shows a section of fig1 namely the right end 21 of the integration electrode 2 and the input transducer 3 opposite the end 21 the illustration of fig2 is substantially enlarged in relation to fig1 and is supplemented by additional details just like the transducer 4 the illustrated input transducer 3 in the case of the invention is a dispersive transducer in which the spacings of the fingers 11 from one another measured on the axis z are non equidistant the use of different spacings to create a dispersive transducer is of course known as discussed above with respect to publication no 1 the spacings are dimensioned of varying size corresponding to the respective wavelength range of the dispersion these varying finger spacings for a dispersive transducer are not taken into account in fig2 particularly for reasons of clarity since the spacing deviations are small in relation to the drawing fig2 as a whole as is apparent from fig2 the transducer 3 has fingers referenced 11 which together with adjacent fingers 11 overlap for at least a portion of the length of said fingers this overlapping length is that particular respective length proportioned in the case of two adjacent fingers.