A process for producing a fleece having metallic wire filaments, includes at least the following steps:a) forming a layer including wire filaments;b) producing first cohesive connections between at least some of the metallic wire filaments in a first joining process; andc) producing second cohesive connections between metallic wire filaments in a second joining process. This piezoelectric resonator has a plurality of transducers stacked in a predetermined direction, and a propagation area of the bulk acoustic wave varies in a propagation direction of the bulk acoustic wave. are analyzed by electronic circuits. Separated regions of pixels within the confidence image are identified which have a confidence value greater than a threshold confidence value. % and a 2,2\u2032-MDI content of 0.