A method for determining characteristics of a medium in form of a liquid or a soft material, the method comprising the following steps: a) providing an acoustic waveguide defining an inner surface (11, 21) and an outer surface (12, 22) and delimiting an interior space to be filled with the medium (L, L1, L2, LE) to be measured such that the outer surface (12, 22) faces away from the interior space and the inner surface (11, 21) of the waveguide forms an interface with the medium, b) providing a sender for feeding acoustic wave energy into the waveguide, c) providing a receiver for receiving surface acoustic waves propagating along the waveguide, d) exciting surface acoustic waves in the waveguide by means of the sender, e) converting at least a part of the energy associated with the surface acoustic waves into volume sound waves of the medium (L, L1, L2, LE), f) propagating both surface acoustic waves (S1, S2) and volume sound waves along a direction of extension of the waveguide, g) reconverting at least a part of the energy associated with the volume sound waves into acoustic wave energy of the waveguide thereby generating surface acoustic waves (S1, S2), h) determining characteristics of the medium (L, L1, L2, LE) from a signal generated by the receiver upon receipt of surface acoustic waves, characterized in that - a thickness (d) of the acoustic waveguide, defined as a distance between the inner surface (11, 21) and the outer surface (12, 22) of the acoustic waveguide, is substantially smaller than the wavelength of surface acoustic waves excited in the waveguide so that surface acoustic waves in the form of Lamb waves are excited in the waveguide and concurrent Lamb waves are propagating on both the inner and the outer surface (11, 21; 12, 22) of the acoustic waveguide with essentially identical amplitudes on both surfaces (11, 21; 12, 22) - wherein a length of the waveguide along a direction of extension is such that a volume sound wave propagating in the medium (L, L1, L2, LE) interacts with the inner surface (11, 21) of the waveguide at a plurality of interaction sites (11, I2, I2') which are spaced from each other and where a conversion of the volume sound wave into a surface acoustic wave (S1, S2) may occur, so that the receiver receives waves propagated through the medium (L, L1, L2, LE) along different paths (P1, P2,...) and a propagation delay involved with the different paths (P1, P2,...) between sender and receiver is measured in order to determine the characteristics of the medium (L, L1, L2, LE). a) providing an acoustic waveguide defining an inner surface (11, 21) and an outer surface (12, 22) and delimiting an interior space to be filled with the medium (L, L1, L2, LE) to be measured such that the outer surface (12, 22) faces away from the interior space and the inner surface (11, 21) of the waveguide forms an interface with the medium, b) providing a sender for feeding acoustic wave energy into the waveguide, c) providing a receiver for receiving surface acoustic waves propagating along the waveguide, d) exciting surface acoustic waves in the waveguide by means of the sender, e) converting at least a part of the energy associated with the surface acoustic waves into volume sound waves of the medium (L, L1, L2, LE), f) propagating both surface acoustic waves (S1, S2) and volume sound waves along a direction of extension of the waveguide, g) reconverting at least a part of the energy associated with the volume sound waves into acoustic wave energy of the waveguide thereby generating surface acoustic waves (S1, S2), h) determining characteristics of the medium (L, L1, L2, LE) from a signal generated by the receiver upon receipt of surface acoustic waves, - a thickness (d) of the acoustic waveguide, defined as a distance between the inner surface (11, 21) and the outer surface (12, 22) of the acoustic waveguide, is substantially smaller than the wavelength of surface acoustic waves excited in the waveguide so that surface acoustic waves in the form of Lamb waves are excited in the waveguide and concurrent Lamb waves are propagating on both the inner and the outer surface (11, 21; 12, 22) of the acoustic waveguide with essentially identical amplitudes on both surfaces (11, 21; 12, 22) - wherein a length of the waveguide along a direction of extension is such that a volume sound wave propagating in the medium (L, L1, L2, LE) interacts with the inner surface (11, 21) of the waveguide at a plurality of interaction sites (11, I2, I2') which are spaced from each other and where a conversion of the volume sound wave into a surface acoustic wave (S1, S2) may occur, so that the receiver receives waves propagated through the medium (L, L1, L2, LE) along different paths (P1, P2,...) and a propagation delay involved with the different paths (P1, P2,...) between sender and receiver is measured in order to determine the characteristics of the medium (L, L1, L2, LE).