Sound Attenuation of Slit-like Duct Expansions Placed Side by Side in the Axial Direction

Abstract

A slit, which is a duct expansion with very short axial length, acts as a resonator muffler with resonant frequency almost depended on the slit depth. Hence, using some slits with different diameter one another, placed side by side in the axial direction, it is expected that the superposed effect of each slit is obtained. In this study, transmission coefficients of such slits were measured with various slit's size and with the plane wave or the (1, 1) mode wave incidence. Also the structures of sound fields in a duct with such slits were numerically analyzed with the finite differece method. As a general result, superposed effects were obtained, but in the case that each slit's reasonance effect was strong and each resonant frequency was close to some extent, interaction between one and next slit occurred. In this case, effective frequency range on sound attenuation became somewhat narrow, but attenuation effect was enough strong in this range. As the case may be, such slits seem to be useful as a sound attenuator.