A medical instrument (900, 1000) comprising: - a high intensity focused ultrasound system comprising an ultrasound transducer (102, 104, 202, 204, 302, 407, 508) with an adjustable sonication frequency for focusing ultrasound into a sonication volume, wherein the ultrasound transducer comprises a first array and a second array of transducer elements (102, 104, 202, 204, 302, 407, 508); - a processor for controlling the medical instrument; - a memory for storing machine executable instructions (930, 933, 934), wherein execution of the instructions causes the processor to: - receive (700, 800) a treatment plan descriptive of a target zone within a subject; - determine (702, 802) a traversal distance through the subject to the target zone using the treatment plan, wherein the traversal distance is descriptive of the traversal of ultrasound from the ultrasound transducer to the target zone; - determine (704, 804) a sonication frequency using the traversal distance for focusing the sonication volume onto the target zone; and - sonicate (706, 806) the target zone using the high intensity focused ultrasound system at the sonication frequency; wherein each of the ultrasound transducer elements has a separately controllable sonication frequency, wherein execution of the instructions causes the processor to determine the sonication frequency for each of the ultrasound transducer elements, wherein execution of the instructions causes the processor to determine a first sonication frequency using the traversal distance for focusing the sonication volume into the target zone, wherein execution of the instructions causes the processor to determine a second sonication frequency using the traversal distance for focusing the sonication volume into the target zone, characterised in that the first array and the second array of transducer elements comprise capacitive micromachined transducer elements, wherein execution of the instructions causes the processor to sonicate the target zone using the high intensity focused ultrasound system using the first sonication frequency for the first array of capacitive micromachined transducer elements and using the second sonication frequency for the second array of capacitive micromachined transducer elements, wherein the first sonication frequency and the second sonication frequency are not hamonics of or identical to each other to create an independent superposition of the ultrasound from the first array and second array - a high intensity focused ultrasound system comprising an ultrasound transducer (102, 104, 202, 204, 302, 407, 508) with an adjustable sonication frequency for focusing ultrasound into a sonication volume, wherein the ultrasound transducer comprises a first array and a second array of transducer elements (102, 104, 202, 204, 302, 407, 508); - a processor for controlling the medical instrument; - a memory for storing machine executable instructions (930, 933, 934), wherein execution of the instructions causes the processor to: - receive (700, 800) a treatment plan descriptive of a target zone within a subject; - determine (702, 802) a traversal distance through the subject to the target zone using the treatment plan, wherein the traversal distance is descriptive of the traversal of ultrasound from the ultrasound transducer to the target zone; - determine (704, 804) a sonication frequency using the traversal distance for focusing the sonication volume onto the target zone; and - sonicate (706, 806) the target zone using the high intensity focused ultrasound system at the sonication frequency; wherein each of the ultrasound transducer elements has a separately controllable sonication frequency, wherein execution of the instructions causes the processor to determine the sonication frequency for each of the ultrasound transducer elements, wherein execution of the instructions causes the processor to determine a first sonication frequency using the traversal distance for focusing the sonication volume into the target zone, wherein execution of the instructions causes the processor to determine a second sonication frequency using the traversal distance for focusing the sonication volume into the target zone, characterised in that the first array and the second array of transducer elements comprise capacitive micromachined transducer elements, wherein execution of the instructions causes the processor to sonicate the target zone using the high intensity focused ultrasound system using the first sonication frequency for the first array of capacitive micromachined transducer elements and using the second sonication frequency for the second array of capacitive micromachined transducer elements, wherein the first sonication frequency and the second sonication frequency are not hamonics of or identical to each other to create an independent superposition of the ultrasound from the first array and second array - receive (700, 800) a treatment plan descriptive of a target zone within a subject; - determine (702, 802) a traversal distance through the subject to the target zone using the treatment plan, wherein the traversal distance is descriptive of the traversal of ultrasound from the ultrasound transducer to the target zone; - determine (704, 804) a sonication frequency using the traversal distance for focusing the sonication volume onto the target zone; and - sonicate (706, 806) the target zone using the high intensity focused ultrasound system at the sonication frequency;