Variable-frequency stacked annular self-gathering ultrasonic transducer

The invention relates to the technical field of ultrasonic tumor treatment systems, in particular to a variable-frequency stacked annular self-aggregation ultrasonic transducer which comprises a backing and a piezoelectric wafer, and the backing is of a double-layer structure; a hollow air cavity is formed between layers of the backing; the focusing surface of the inner wall of the backing is in a concave spherical table shape; the wall thickness of the inner layer of the backing is an integral multiple of 1/2 of the wavelength lambda in the backing material; the piezoelectric wafer consists of a plurality of groups of independent chip wafers; the chip-type wafer is glued on the focusing surface of the inner wall of the backing; the chip wafers are assembled on the backing to form an outer ring piezoelectric wafer and an inner ring piezoelectric wafer; the inner spherical surface is divided into the inner ring and the outer ring and can transmit power respectively, so that the strength required during treatme