Abstract: The investigation on the characteristics of diameter-expanded silencers was carried out experimentally and some rules were obtained: 1. silencers are effective only in a limited; pneumatic pressure range and have maximum of noise reduction when the condition （P₀/P₁,v）/（d_A²/d_S²）≈0.81 is fulfilled; 2. silencers also have certain expansion selection, i.e. d_s/d_A must be larger than 1.5 and smaller than 2.5; 3. the length of silencers has no marked effect on their characteristics in the effective pressure range; 4. the efflux noise of silencers in the effective pressure range can be calculated according to the pressure dependence of turbulent noise and the correction to the ratio of expansion. Finally, two intereesting phenomena, which are valuable for further study, are put forward.