The absorption character in low frequency can be improved by adding cavum. The affect factor of absorption coefficient was studied, by three type of acoustic system model of stealth structure. Using the pulse tube system, the resonance peak and peak frequency of samples with cavum, different distance, shapes, and depth, were analyzed. The results show that the cavum enhance absorption in low frequency, and the first resonance peak is higher, peak frequency in lower in smaller cavum distance in air backing, and the result is opposite in water backing, and the first resonance peak is lower, peak frequency in lower in smaller cavum volume.