An ultrasound system comprising: a probe including an array of capacitive micromachined ultrasound transducer, CMUT, cells, each cell comprising: - a substrate carrying a first electrode of an electrode arrangement; - a flexible membrane including a second electrode of said electrode arrangement; and - a mass element; the substrate being spatially separated from the flexible membrane by a gap, the flexible membrane comprising the mass element in a central region (17, 17') thereof, the flexible membrane comprising a first material and the mass element comprising a second material having a higher density than the first material, the mass element being distinct from the second electrode, the mass element being either: formed on the membrane; or integrated in the flexible membrane, said central region of the membrane having first thickness less than the thickness of the remainder of the membrane; and a voltage supply coupled to said probe and adapted to, in a transmission mode of the ultrasound system, provide the respective electrode arrangements of at least some of the CMUT cells with a voltage including: - a bias voltage component configured to drive the at least some of the CMUT cells into a collapsed state in which a central part of the flexible membrane contacts the substrate, said central part including the central region; and - a stimulus component having a set frequency configured to resonate the respective flexible membranes of the at least some of the CMUT cells in said collapsed state, wherein an outline, dimensions and a density of the mass element of each of the at least some CMUT cells are selected so that said mass element is configured to force at least the central region of the flexible membrane of said cell to remain in contact with the substrate during said resonating, and to permit release of said central region from the substrate upon reduction or removal of said bias voltage component. a probe including an array of capacitive micromachined ultrasound transducer, CMUT, cells, each cell comprising: - a substrate carrying a first electrode of an electrode arrangement; - a flexible membrane including a second electrode of said electrode arrangement; and - a mass element; - a substrate carrying a first electrode of an electrode arrangement; - a flexible membrane including a second electrode of said electrode arrangement; and - a mass element; the substrate being spatially separated from the flexible membrane by a gap, the flexible membrane comprising the mass element in a central region (17, 17') thereof, the flexible membrane comprising a first material and the mass element comprising a second material having a higher density than the first material, the mass element being distinct from the second electrode, the mass element being either: formed on the membrane; or integrated in the flexible membrane, said central region of the membrane having first thickness less than the thickness of the remainder of the membrane; and a voltage supply coupled to said probe and adapted to, in a transmission mode of the ultrasound system, provide the respective electrode arrangements of at least some of the CMUT cells with a voltage including: - a bias voltage component configured to drive the at least some of the CMUT cells into a collapsed state in which a central part of the flexible membrane contacts the substrate, said central part including the central region; and - a stimulus component having a set frequency configured to resonate the respective flexible membranes of the at least some of the CMUT cells in said collapsed state, wherein an outline, dimensions and a density of the mass element of each of the at least some CMUT cells are selected so that said mass element is configured to force at least the central region of the flexible membrane of said cell to remain in contact with the substrate during said resonating, and to permit release of said central region from the substrate upon reduction or removal of said bias voltage component. formed on the membrane; or integrated in the flexible membrane, said central region of the membrane having first thickness less than the thickness of the remainder of the membrane; and a voltage supply coupled to said probe and adapted to, in a transmission mode of the ultrasound system, provide the respective electrode arrangements of at least some of the CMUT cells with a voltage including: - a bias voltage component configured to drive the at least some of the CMUT cells into a collapsed state in which a central part of the flexible membrane contacts the substrate, said central part including the central region; and - a stimulus component having a set frequency configured to resonate the respective flexible membranes of the at least some of the CMUT cells in said collapsed state, - a bias voltage component configured to drive the at least some of the CMUT cells into a collapsed state in which a central part of the flexible membrane contacts the substrate, said central part including the central region; and - a stimulus component having a set frequency configured to resonate the respective flexible membranes of the at least some of the CMUT cells in said collapsed state, wherein an outline, dimensions and a density of the mass element of each of the at least some CMUT cells are selected so that said mass element is configured to force at least the central region of the flexible membrane of said cell to remain in contact with the substrate during said resonating, and to permit release of said central region from the substrate upon reduction or removal of said bias voltage component.