An apparatus comprising: a piezoelectric micromachined ultrasonic transducer,pMUT, array comprising: a plurality of piezoelectric transducer element populations disposed over a substrate , for each of the piezoelectric transducer element populations , a size of a piezoelectric membrane of a first transducer element of the element population is different than a size of a piezoelectric membrane of a second transducer element of the element population , and each transducer element having a drive/sense electrode coupled to a piezoelectric membrane; characterized in that the pMUT array further comprises: a plurality of sets of electrode rails (110', 110", 120' , 120") disposed over the substrate, wherein each set of electrode rails is coupled to only a respective one of the transducer element populations, wherein a first electrode rail of the set of electrode rails (110', 110") is electrically coupled to the drive/sense electrode of the first transducer element of the respective one of the transducer element populations and a second electrode rail (110") of the set of electrode rails is electrically coupled to the drive/sense electrode of the second transducer element of the respective one of the transducer element populations; wherein the plurality of sets of electrode rails (110', 110", 120' , 120") form a linear array across a first dimension (x) of the substrate and each of the piezoelectric transducer element populations comprises a linear array across a second dimension (y) of the substrate, or form a two-dimensional array of electrode rails (110A', 110A", 110B', 110B", 120A', 120A", 120B', 120B") along a first (x) and second (y) dimensions of the substrate, wherein the piezoelectric membranes of a first element population are circular or spheroidal with differing diameters, wherein each drive/sense electrode rail is electrically addressable independently from any other drive/sense electrode rails (110", 120'); and at least one signal generator coupled to the pMUT array to apply a first electrical drive signal on the first electrode rail and a second electrical drive signal on the second electrode rail . a plurality of sets of electrode rails (110', 110", 120' , 120") disposed over the substrate, wherein each set of electrode rails is coupled to only a respective one of the transducer element populations, wherein a first electrode rail of the set of electrode rails (110', 110") is electrically coupled to the drive/sense electrode of the first transducer element of the respective one of the transducer element populations and a second electrode rail (110") of the set of electrode rails is electrically coupled to the drive/sense electrode of the second transducer element of the respective one of the transducer element populations; wherein the plurality of sets of electrode rails (110', 110", 120' , 120") form a linear array across a first dimension (x) of the substrate and each of the piezoelectric transducer element populations comprises a linear array across a second dimension (y) of the substrate, or form a two-dimensional array of electrode rails (110A', 110A", 110B', 110B", 120A', 120A", 120B', 120B") along a first (x) and second (y) dimensions of the substrate, wherein the piezoelectric membranes of a first element population are circular or spheroidal with differing diameters, wherein each drive/sense electrode rail is electrically addressable independently from any other drive/sense electrode rails (110", 120'); and at least one signal generator coupled to the pMUT array to apply a first electrical drive signal on the first electrode rail and a second electrical drive signal on the second electrode rail . <EOS>