1. Laboratory for Acoustics and Thermal Physics, Department of Physics and Astronomy, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
2. PolyLab−CNR and Dipartimento di Fisica, Università di Pisa, Largo. B. Pontecorvo 3, 56127 Pisa, Italy