1. Research Associate, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge CB2 1PZ, U.K. (corresponding author).
2. Serra Húnter Fellow, Acoustical and Mechanical Engineering Laboratory, Universitat Politècnica de Catalunya, Terrassa, 08222 Barcelona, Spain.
3. Professor, Acoustical and Mechanical Engineering Laboratory, Universitat Politècnica de Catalunya, Terrassa, 08222 Barcelona, Spain.