Abstract: In the present work, the flow rates and noise radiations of expansion micropore mufflers at different upstream stagnation pressures have been investigated theoretically and experimentally. We find that there is a critical expansion ratio which determines flow and noise characteristics. Below the critical, flow only can be choked at micropores. At any upstream stagnation pressure flow Mach number in muffler is lower than that at micropores so that the noise produced in the muffler is not important in comparison with that radiated from micropores. Above the critical, flow first chokes at the exit of exhaust and then at micropores. At low upstream stagnation pressure noise produced in the muffler can not be neglected and effects the nois radiation of the muffler, particularly at the pressure at which flow is choked at the exit of exhaust only.
Flow rate is closely related to the effective area defined in the paper, and upstream stagnation pressure. The effective area approaches the total micropores area as the later decreases to zero, and approaches the exhaust area as the total micropore area increases. Experimental results agree well with theoretical calculations.
The directivity of noise radiation varies considerably as the number of micropores increases. The noise reduction of expansion micropore muffler measured by average A-weighted sound pressure level is different from that of theoretical calculation due to the effect of noise produced in expansion and expected to approaching theoretical calculation at high upstream stagnation pressure.
There is no doubt from the experiment and calculation at high upstream stagnation pressure that noise reduction of expansion micropore muffler is slight higher than the reduction obtained from the frequency shift of micro-jets.