A transducer array comprising a plurality of CMUT cells (100, 100', 100"), suitable for a collapsed operation mode, each CMUT cell comprising a first electrode supported by a substrate and a second electrode supported by a membrane suspended over a cavity between the first electrode and the second electrode, the plurality of CMUT cells comprising: - a first group of CMUT cells each having a membrane comprising a first layer stack ; and - a second group of CMUT cells each having membrane comprising a second layer stack , the second layer stack including a layer of a material having a higher density than any of the layers in the first layer stack, wherein the membranes of the first group of CMUT cells have a first spring constant and the membranes of the second group of CMUT cells have a second spring constant, characterized in that the second spring constant is no more than 20% different to the first spring constant; and in that the plurality of CMUT cells are arranged in a staggered array comprising: - a first column of spaced CMUT cells or transducer elements on at least one silicon island; - a second column of spaced CMUT cells or transducer elements on at least one further silicon island, the second column being staggered in alignment with the first column such that cells of the second column are partially located in spaces between successive cells of the first column, the first column and the second column being spaced apart by a gap; and - a flexible foil retaining the respective silicon islands, the flexible foil comprising conductive interconnects. - a first group of CMUT cells each having a membrane comprising a first layer stack ; and - a second group of CMUT cells each having membrane comprising a second layer stack , the second layer stack including a layer of a material having a higher density than any of the layers in the first layer stack, wherein the membranes of the first group of CMUT cells have a first spring constant and the membranes of the second group of CMUT cells have a second spring constant, characterized in that the second spring constant is no more than 20% different to the first spring constant; and in that the plurality of CMUT cells are arranged in a staggered array comprising: - a first column of spaced CMUT cells or transducer elements on at least one silicon island; - a second column of spaced CMUT cells or transducer elements on at least one further silicon island, the second column being staggered in alignment with the first column such that cells of the second column are partially located in spaces between successive cells of the first column, the first column and the second column being spaced apart by a gap; and - a flexible foil retaining the respective silicon islands, the flexible foil comprising conductive interconnects. - a first column of spaced CMUT cells or transducer elements on at least one silicon island; - a second column of spaced CMUT cells or transducer elements on at least one further silicon island, the second column being staggered in alignment with the first column such that cells of the second column are partially located in spaces between successive cells of the first column, the first column and the second column being spaced apart by a gap; and - a flexible foil retaining the respective silicon islands, the flexible foil comprising conductive interconnects. <EOS>