Uwe J. Hansen
Dept. of Phys., Indiana State Univ., Terre Haute, IN 47809
Physikalisch-Technische Bundesanstalt, 3300 Braunschweig, Germany
The study of normal modes of a vibrating structure using modal analysis involves the phase relationship between a large number of coherent signal pairs obtained over a predetermined geometrical grid covering the structure. From a set of transfer functions, resonance frequencies are obtained and an animated picture of the motion at the resonance frequency is displayed on a computer screen. Extending this technique to standing and traveling waves in air involves using an excitation source as reference and forming transfer functions between that reference source and sound pressure level measurements at predetermined locations in the sound field. The technique will be illustrated with animations of air modes in a flute and with animations of sound fields radiated by a piano at several frequencies [I. Bork, Acustica 75, 154--167 (1991)].