Abstract: A sensor apparatus is provided for measuring characteristics of a wall of a structure and/or a medium in contact with the structure, wherein the sensor apparatus includes a transducer arrangement disposed at least partially around a planar or curved surface of a wall of the structure, or disposed over a region of a planar or curved surface of a wall of the structure. The sensor apparatus includes a transducer waveguide including at least one free distal end whereat one or more driver and/or receiver elements are mounted on one or more sides of the at least one free distal end. Moreover, the transducer arrangement is operable, when interrogating the structure to perform at least one of: switching between selected acoustic wave modes present in an acoustic wave propagation, steering an acoustic propagation direction of the acoustic wave propagation.