The invention relates in particular to a method for determining physical, chemical, and/or biological properties of a medium (M) located in the interior (30) of a waveguide (3) using at least one acoustic wave which has propagated at least partly through the medium (M). According to the invention, a first wall section (31a) and a second wall section (31b) of the waveguide (3) are connected together via a connection piece (31c) such that a second surface wave (OW2) propagates to the first wall section (31a) at least partly via the connection piece (31c). One of the wall sections (31a, 31b) and/or the connection piece (31c) is provided with at least one reflective element (4) on which at least one part of at least one first surface wave (OW1) that is excited on the first wall section (31a) by means of a transmitter (SE) is reflected as a third surface wave (OW1'). A receiver (SE) is used to receive second and third surface waves (OW2, OW1') on the first wall section (31a), and the secondand third surface waves are used to determine physical, chemical, and/or biological properties of the medium (M).