Abstract: In the present work, the flow rates and noise radiations of micropore and micropore muffler at different chamber pressures have been investigated. When the number of micro-pore of a micropore muffler is small （the total geometrical area of micropores is, say, less than 1.5 times the cross section area of the exhaust pipe） and the flow is choked at the micropores, the flow rate and noise radiation are proportional to the number of pores and the back pressure is very low, around 0.1 atmosphere pressure or so. The flow rate through each micropore is proportional to the chamber pressure or the stagnation pressure of micro-jet), but the effective area of micropore is only about 60 percent of its geometrical area. When the total geometrical area of pores is near 1.5 times the cross section area of the muffler inlet pipe, the increase of flow rate with the increase of the number of micropores will slow down and finally the total flow reaches a limit value equal to that of the bare exhaust pipe. The noise radiation of each micropore is calculated through the formula for a single small jet.
Knowing the directivity of a single microjet, the average noise level can be found around the jet in its meridian plane. Corrected by the number of micropores this average level should be equal to the noise level around the micropore muffler, when the pores are drilled around the exhaust pipe. Evperiments confirm this. The effect of the separation space between pores on noise reduction is also discussed.