A robotic cleaning appliance includes a housing to which is coupled a surface treatment item and a sensor assembly with first and second transducers and an acoustic interface. The first sonic transducer transmits sonic signals through an acoustic interface and out of a first acoustic opening toward a surface beneath the robotic cleaning appliance. The sonic signals reflect from the surface as corresponding returned signals received by the second sonic transducer via a second acoustic opening port of the acoustic interface. A first plurality of annular rings is defined in the external surface around the first acoustic opening port and a second plurality of annular rings is defined in the external surface around the second acoustic opening port. The pluralities of annular rings attenuate direct path echoes from a subset of the transmitted sonic signals which attempt to travel across the external surface to the second acoustic opening port.