A sensor apparatus 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 (200, 300, 510, 540) disposed in operation at least partially around a planar or curved surface of the wall of the structure, or disposed over a region of a planar or curved surface of the wall of the structure, wherein the sensor apparatus includes a transducer waveguide including at least one free distal end whereat one or more driver elements are mounted on the at least one free distal end, the at least one free distal end includes a neck region for coupling the free distal end with a remainder of the transducer waveguide in contact with the wall of the structure, wherein the transducer waveguide has a substantially similar thickness to the wall of the structure, and is fabricated from a material which is substantially similar to that of the wall of the structure, wherein the transducer waveguide has similar dispersion characteristics as the wall of the structure, wherein the transducer arrangement (200, 300, 510, 540) is operable, when interrogating the structure to perform at least one of: switching between selected acoustic wave modes present in an acoustic wave propagation, and steering an acoustic propagation direction of the acoustic wave propagation; wherein the one or more driver elements are operable to excite an acoustic Lamb wave propagation within the wall of the structure, for providing information indicative of properties of the wall and/or material present in the vicinity of the wall which interacts with the acoustic Lamb wave propagation in the wall of the structure, wherein the one or more driver elements of the transducer waveguide are operable to select one or more specific Lamb modes of acoustic radiation propagation by exciting a predominantly through-thickness stress/ displacement signature for the one or more specific Lamb modes, wherein the one or more driver elements are configurable by selectively exciting them to switch between the Lamb modes and wherein the switching between the acoustic radiation modes is achieved by selectively switching between in-phase and out-of-phase excitation of elements on upper and lower faces of one or more distal ends of the transducer waveguide, thereby enabling switching between symmetrical and anti-symmetrical acoustic radiation modes in the transducer waveguide. the sensor apparatus includes a transducer waveguide including at least one free distal end whereat one or more driver elements are mounted on the at least one free distal end, the at least one free distal end includes a neck region for coupling the free distal end with a remainder of the transducer waveguide in contact with the wall of the structure, wherein the transducer waveguide has a substantially similar thickness to the wall of the structure, and is fabricated from a material which is substantially similar to that of the wall of the structure, wherein the transducer waveguide has similar dispersion characteristics as the wall of the structure, wherein the transducer arrangement (200, 300, 510, 540) is operable, when interrogating the structure to perform at least one of: switching between selected acoustic wave modes present in an acoustic wave propagation, and steering an acoustic propagation direction of the acoustic wave propagation; wherein the one or more driver elements are operable to excite an acoustic Lamb wave propagation within the wall of the structure, for providing information indicative of properties of the wall and/or material present in the vicinity of the wall which interacts with the acoustic Lamb wave propagation in the wall of the structure, wherein the one or more driver elements of the transducer waveguide are operable to select one or more specific Lamb modes of acoustic radiation propagation by exciting a predominantly through-thickness stress/ displacement signature for the one or more specific Lamb modes, wherein the one or more driver elements are configurable by selectively exciting them to switch between the Lamb modes and wherein the switching between the acoustic radiation modes is achieved by selectively switching between in-phase and out-of-phase excitation of elements on upper and lower faces of one or more distal ends of the transducer waveguide, thereby enabling switching between symmetrical and anti-symmetrical acoustic radiation modes in the transducer waveguide.