An ultrasound transducer assembly includes an acoustic focusing lens and a therapy transducer mounted to a holder member so that the lens is movable relative to the transducer. The lens and the transducer are mounted to the holder member so that the lens is spaced a predetermined distance from the transducer element. A liquid layer having a thickness of the predetermined distance is provided between the lens and the transducer element. A solid backing member is disposed on a side of the transducer element opposite the lens. The backing member is spaced by an additional liquid layer of a predetermined thickness from the transducer element. The focusing depth of the lens-transducer assembly is controllable by transducer operating frequency.