Modes of Vibration. By L. J. P OLDERVAART , A. P. J. W IJNANDS , I. H. A. M. V AN M OLL and E. J. V AN V OORTHUISEN . 1974. 16mm, silent, black and white, 10min. 180 Hfl. In English. Sound Pulse-Boundary Layer Interaction Studies. By L. J. P OLDERVAART , A. P. J. W IJNANDS and I. B RONKHORST . 1974. 16mm, optional sound, black and white, 20min. 375 Hfl. In English.