It is provided an apparatus for determining at least one of physical, chemical and biological properties of a medium, comprising an acoustic waveguide, which has a conduction element with an inner side that faces the medium and an outer side that lies opposite this inner side. The inner side facing the medium is curved in concave fashion and the outer side is curved in convex fashion and the waveguide with the conduction element curved in that way is configured such that a second surface wave coupling-in at the concave inner side propagates along a propagation direction to the receiver. At least one damping element lies downstream of the receiver in the propagation direction of the second surface wave, said damping element being arranged and configured to prevent surface waves propagating counter to the propagation direction of the at least one second surface wave from reaching the receiver.
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