The invention relates to a method for operating a device (1) for treatment of a tissue (6) of a living being and such a device (1) including a transducer (2) comprising at least two sub-transducers (2', 2"), each for emitting a sub-beam (8") of ultrasound waves, preferably high intensity focused ultrasound waves, for irradiating the tissue (6), each of the sub-beams (8") being focused on or focusable onto a focal point (F). In order to reduce skin burns, the intersection Area (Asi) of a skin surface (7) with each sub-beam (8") is evaluated and a power (Psi) and/or duration (tsi) of the irradiation for every sub-transducer (2' , 2") as a rate of the total power (Ptotal) respectively duration (ttotal) depending from the evaluated intersection area (Asi) is determined.
