Experimental modal analysis of a cavity using a calibrated acoustic actuator G. D. Rossetto, J. R. F. Arruda, B. N. Huallpa Departamento de Mecnica Computacional, Universidade Estadual de Campinas, C.P. 6122, 13083-970, Camp- inas, SP, Brazil e-mail : dalben@fem.unicamp.br Abstract In this paper, the acoustical modal analysis of a rectangular shallow cavity is performed. It is shown that a suitable choice for the acoustical excitation is the volume acceleration and, for the acoustical response, the sound pressure. When using a Finite Element model and an analytical model, it is shown that the computed Frequency Response Functions must be multiplied by the mass density of the air to yield units of Pascal per unit volume acceleration ( ), which are straightforward to obtain experimentally. In the experiments, two types of excitation devices were used. The first utilizes a shaker-driven piston which thrusts against a thin rubber membrane stretched flush to one of the cavity side walls, covering a cylindrical hole. The other actuator was built based upon a research report developed in an EEC project (Brite-EuRam II: PIANO). This acoustic actuator has a high impedance (higher than any practical surrounding impedance) so that the impedance of the cavity does not need to be considered in the calibration factor relating the microphone signal and the source strength. A good agreement was obtained in the comparisons between experimental, analytical and numerical Frequency Response Functions and modes. 1. Introduction The practical importance of acoustical modal analy- sis has increased in recent years. In the experimental domain, some difficulties are still to be solved and the present work addresses some of them, namely the unit corrections which are necessary to allow the compar- ison between analytical, numerical and experimen- tal results, the acoustic excitation realization, and the mode shape visualization. In the current literature on acoustical systems, lit- tle attention is paid to defining excitation and re- sponse acoustic variables such that an experimental modal analysis is feasible. Augusztinovicz and Sas [5] have addressed this problem. They have proposed a formulation where volume acceleration is the input variable and pressure the response variable in the dy- namic equations of the acoustical system. Pressure may be easily measured with microphone, while vol- ume acceleration can be produced by calibrated sound sources such as loudspeakers in specially designed configurations. Nieter and Singh in [6] developed a methodology for acoustical modal analysis where the same tools applied in solid mechanics (Fourier analyzers, modal parameter extraction methods, etc) are used. The technique faces no problem with the pressure mea- surement, which can be done with microphones or very sensitive pressure transducers, but has to deal with the acoustic input (volume acceleration of the fluid) that has no direct measurement, due to the lack of a particle velocity transducer. The solution found was the use of a piston driven by a shaker with an ac- celerometer attached. Later work by Singh and Kung [7] proposes another solution, based on a horn-drive loudspeaker, where the volume velocity is monitored by a microphone mounted on a small enclosure of known volume at the back of the driver. The acoustic driver was mounted directly at the point of excitation in the acoustic system being tested. The problem related to the acoustic mode visual- ization is treated by Whear and Morrey in [8], where a probe with three aligned microphones gives, using a second-order finite difference calculation, the second- order derivative of the pressure relative to the space. Given that the first-order derivative is related (Eu- ler’s equation) to the particle acceleration, the second- order derivative, while being directional, will still ex- hibit the same nodes and antinodes as the pressure distribution. The disadvantage of this method is the noise amplification effect of differentiating twice the measured pressure field. Another approach is given