A method for determining physical, chemical and/or biological properties of a medium situated in an interior of a waveguide with the aid of at least one acoustic wave which has propagated at least in part through the medium, wherein a. at least one surface wave is excited by means of a transmitter at an outer surface of a first wall portion of the waveguide that bounds the interior, said surface wave propagating at the first wall portion along a first propagation direction (-x), b. the at least one surface wave excites an acoustic wave propagating in the medium at an inner surface of the first wall portion facing the medium, the propagation direction of said acoustic wave comprising a directional component pointing away from the first wall portion, c. the acoustic wave propagating in the medium at least partly couples into an inner surface of a second wall portion of the waveguide, lying opposite the first wall portion, as a second surface wave, which propagates at the second wall portion along a second propagation direction (-x), and d. the first wall portion and the second wall portion are connected to one another by way of a connecting piece of the waveguide in such a way that a second surface wave propagates at least in part over the connecting piece to the first wall portion, e. at least one reflective element is provided at one of the wall portions (31a, 31b) and/or at the connecting piece, at least part of the at least one first surface wave being reflected at said reflective element as a third surface wave (OW1'), and f. physical, chemical and/or biological properties of the medium are determined on the basis of the received surface waves (OW1', OW2), which can be at least partly traced back to the acoustic waves which have propagated in the medium, wherein for this second and third surface waves (OW2, OW1') are received by means of a receiver at the first wall portion, wherein the waveguide is arranged in a cavity of a housing part of the apparatus. a. at least one surface wave is excited by means of a transmitter at an outer surface of a first wall portion of the waveguide that bounds the interior, said surface wave propagating at the first wall portion along a first propagation direction (-x), b. the at least one surface wave excites an acoustic wave propagating in the medium at an inner surface of the first wall portion facing the medium, the propagation direction of said acoustic wave comprising a directional component pointing away from the first wall portion, c. the acoustic wave propagating in the medium at least partly couples into an inner surface of a second wall portion of the waveguide, lying opposite the first wall portion, as a second surface wave, which propagates at the second wall portion along a second propagation direction (-x), and d. the first wall portion and the second wall portion are connected to one another by way of a connecting piece of the waveguide in such a way that a second surface wave propagates at least in part over the connecting piece to the first wall portion, e. at least one reflective element is provided at one of the wall portions (31a, 31b) and/or at the connecting piece, at least part of the at least one first surface wave being reflected at said reflective element as a third surface wave (OW1'), and f. physical, chemical and/or biological properties of the medium are determined on the basis of the received surface waves (OW1', OW2), which can be at least partly traced back to the acoustic waves which have propagated in the medium, wherein for this second and third surface waves (OW2, OW1') are received by means of a receiver at the first wall portion,