In this paper low frequency sound transmission of stiffened panels is studied. The panels are rectangular plates with lattice stiffeners, and boundary of the plates is clamped. The sound transmission of the panels is measured using a mini reverberation chamber. The specimen is MDF board of 3 mm thickness, and size is 1.2 m × 1.0 m. Two different lattice size is tested: 205 × 255 mm for 4 × 4 cells and 280 × 280 mm for 3 × 3 cells, where stiffener is made of wooden rod whose cross section is 30 × 30 mm or 35 × 50 mm. Insertion loss is measured when impact is applied to the base panel with and without stiffened panel, where microphone position is fixed in the receiving room. It is found that insertion loss shows a dip around 220 Hz, while insertion loss becomes larger as frequency decreases. It is important that Young's modulus of the stiffener must be sufficiently larger than that of the plate so that unit cell may show modes of clamped plate.
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