Method and apparatus for determining that a pressure field is applied on a structure. A plurality of acoustic waves are generated within the structure using at least one wave generator and a plurality of measurements of the acoustic waves is taken using at least one wave sensor. A pressure field applied to a surface of the structure is determined by processing at least two of the plurality of measurements. The wave generator and the wave sensor may be piezoelectric elements, which may alternate between acting as the wave generator and the wave sensor. Processing the measurements may comprise obtaining a differential measurement value and comparing the value to a threshold. Determining that the pressure field is applied may comprise processing the measurements using a model based on acoustic wave propagation or experimental results. The processing may provide a mapping of the pressure field of an object on the structure.