1. School of Engineering Science, Lappeenranta University of Technology, P.O. Box 20, FI-53851 Lappeenranta, Finland
2. Sonotecc Oy, Laululinnuntie 18A, 00780 Helsinki, Finland
3. VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Espoo, Finland