There is described a sonotrode for sealing an opening device onto a hole of a packaging material destined to from a sealed package, comprising: a sealing surface adapted to cooperate with packaging material in order to seal the opening device to the packaging material; a cavity bounded by an opening of sealing surface and adapted to receive the opening device and a ring made in an elastomeric material, partially protruding inside the cavity, and adapted to center a first axis of the opening device on a second axis of the sonotrode.