Measurements were made of the natural frequencies corresponding to the first six modes of three uniform rectangular-cross-section sandwich beams supported to simulate free-free conditions. Data were also obtained concerning the-change of the natural frequencies corresponding to the first five modes of one of the three beams when the density of the air surrounding the beam was varied.nComparison of the measured natural frequencies with those predicted by the classical theory of beam vibrations indicated large differences, particularly for the higher modes. Correlation of the measured natural frequencies with those pre¬dicted by the Timoshenko beam theory, which includes the effect of transverse shear and rotary inertia, was excellent for all six modes. For all beams tested, the effect of rotary inertia was found to be negligible. The natural frequencies of a lightweight sandwich beam were found to increase as the density of the air -surrounding the vibrating beam was decreased.
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