Abstract: The broad-band absorbers for anechoic tank have been developed. Vermiculite flakes were used as filler to increase the loss and to improve the compressibility of butyl rubber, a sound absorbing material with high loss was thus obtained. By adding air bubbles in vermiculite flakes, the bulk compressive strain in rubber is changed into shear strain. Therefore, the effective loss factor of material increases with increase of the volume ratio of the bubble.
Rib structures with parallel channel with total lengths of 20 cm and 15 cm （transition part being 15 cm and 10 cm respectively） have been used for the purpose of economy of the testing material. The channel width is 3 cm The reflection coefficient is less than 10% in the frequency range of 5-80kHz.