M. Lagier G. Vanderborck
Thomson Sintra ASM, 525, Route des Dolines, BP157, 06903 Sophia Antipolis Cedex, France
F. Mortessagne O. Legrand D. Sornette
CNRS URA 190, Faculte des Sciences, 06108 Nice Cedex 02, France and X-RS, Parc-Club, 91893 Orsay Cedex, France
The experimental measurements of high-order mechanical resonances of metallic structures in the high-frequency domain is generally difficult and unreliable, due to the methods of excitations and analysis. Here, an original technique associating a nondestructive percussive excitation together with an acoustic determination of the near field created by the vibrations of the structure in air are proposed. The excitation is obtained using a steel ball magnetically activated, which creates a calibrated impulse. The observation is made using a large bandwidth microphone. The experimentations have been carried out on ``gongs'' (aluminum plates of 1-mm thickness with other dimensions of the order of the meter) possessing noncanonical shape (stadium, sarcophagus...). With this high signal-to-noise ratio obtained in an anechoic room, it is possible to obtain several hundred eigenfrequencies with a resolution of the order or better than 1/10th the difference between neighboring frequencies. This was attained without any particular treatment other than the spectral LMS analysis. The signal dynamics is around 80 dB, which allows one to obtain the pole and zeros of the transfer function, implying that the amount of information thus obtained is considerable. The results were then compared with those predicted within the theoretical framework proposed previously, in terms of quantum chaos methods applied to high-frequency vibrations of structures, in order to characterize the nature of the spectrum and the vibration eigenfunctions.