operating a sensing device, the sensing device comprising:

a first element and a second element;

each of said first element and the second element including:

a first electrode;

a second electrode;

a piezoelectric layer between and connected to the first electrode and the second electrode;

wherein the first electrode, the second electrode and the piezoelectric layer of each element form a pressure wave transducer, and wherein the first element and the second element are oriented such that the second electrode of the first element faces the second electrode of the second element, and said first element and said second element are displaced from one another to leave a space between the first element and the second element and such the second electrodes of said elements form the plates of the capacitor; and

the device being located with the elements of said device disposed on opposite sides of a region of interest;

the method further including:

applying, via a first and second electrode of a first element, a first electric field across a piezoelectric layer of said element such that the piezoelectric layer is made to expand or contract, thereby generating a pressure wave;

detecting the pressure wave via the expansion or contraction of the piezoelectric layer of a second element and consequential generation of a second electric field in a first and second electrode of the second element; and

measuring, via the second electrode of the first element and the second electrode of the second element, capacitance of the region between the second electrodes.