The investigation on the characteristics of diameter-expanded silencers was carried out experi-mentally and some results were obtained:1.silencers are effective only in a limited plenum chamber(stagnation)pressure range and have maximum of noisc reduction when the condition(P_0/P_1,v)/(d_A~1/d_S~2)=0.81 is fulfilled;2.silencers also have certain expansion selection,i.e.d_S/d_A must belarger than 1.5 and smaller than 2.5;3.the length of silencers has no marked effect on their charac-teristics in the effective pressure range;4.the efflux noise of silencers in the effective pressure rangecan be calculated according to the pressure dependence of turbulent noise and the correction to theexpansion ratio.Finally,two notable phenomena,which are valuable for further study,are putforward. The investigation on the characteristics of diameter-expanded silencers was carried out experi-mentally and some results were: 1.silencers are effective only in a limited plenum chamber (stagnation) pressure range and have maximum of noisc reduction when the condition (P_0 / P_, v) / (d_A ~ 1 / d_S ~ 2) = 0.81 is fulfilled; 2.silencers also have certain expansion selection, ied_S / d_A must belarger than 1.5 and smaller than 2.5; 3.the length of silencers has no marked effect on their charac-teristics in the effective pressure range; 4. the efflux noise of silencers in the effective pressure rangecan be calculated under the pressure dependence of turbulent noise and the correction to the expansion ratio. Finally, two notable phenomena, which are valuable for further study, are putforward.