A method of 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 subtransducers (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″) is focused on or focusable onto a focal point (F). In order to reduce skin burns, the intersection Area (Asi) of a skin surface (7) for each sub-beam (8″) is evaluated and a power (Psi) and/or a duration (tsi) of the irradiation, for every sub-transducer (2′, 2″), is determined as a rate of the total power (Ptotal) or duration (ttotal) depending from the evaluated intersection area (Asi).
