Non-destructive testing of the civil engineering infrastructure for diagnosis, residual service life estimation and/or structural health monitoring, is increasing in importance and need. For example, post-tensioned concrete bridges may be subject to sudden collapse due to tendon breakage. In France, tendon ducts are currently investigated with gamma ray radiometry, but alternative non ionizing techniques are currently sought. Since the mid-nineties, the impact echo method has been proposed to detect voids in tendon ducts, where a void indicates a possible location for tendon corrosion and rupture. The impact echo method is currently used in civil engineering to determine thicknesses or depths by measuring the resonance frequency of the S1 Lamb mode associated to its zero group velocity (ZGV) frequency. A downward shift of the ZGV frequency is commonly associated with the presence of an internal void, but evidence of such phenomena in case of fully filled ducts calls for deeper physical insight. In this paper we show impact-echo results obtained with a laser interferometer on a 0.25 m thick concrete test wall containing filled and partially or fully empty ducts, with both thin and thick duct walls. The data corroborate ZGV frequency shift to the modification of the local stiffness of the wall.