A rotary sonotrode enabling welding to be carried out continuously over a large width. A sonotrode (S) more particularly of revolution, made of a first material for propagation of ultrasonic waves, more particularly metal, comprising a axial bar (2) and an active part (4) connected to the axial part (2), the bar extending uninterruptedly over the entire axial extent of the sonotrode, and a chamber (8), made of a second material which does not allow the transmission of ultrasonic waves from the first material to the second, the second material being more particularly air or avoid, is interposed between the bar (2) and the active part (4).