A sonotrode comprising: a head defining a sealing surface elongated along a first direction; and a first and a second drive unit distinct from one another, each comprising at least one piezo-electric element; wherein first and second drive unit may be electrically fed by an unique generator to cause oscillation of head; the height of head measured along a second direction transversal to sealing surface being substantially equal to half the wavelength of wave oscillating in second direction; at least one slot extending through head transversally to first and second direction; a finite number of nodal points at which amplitude of oscillation of head is substantially equal to zero; and a plane on which at least some nodal points lie and which divides head in a first portion and second portion; the slot extends completely within one only between first and second portion and at a certain distance by plane.