Abstract:
The role of the perforated pipe in a simple elliptical expansion chamber, especially its ability to improve noise reduction by decreasing the level of resonance caused by traverse waves, is presented. The characteristics of the perforated pipe have been studied by means of impedance which depends on the shape and porosity of a pipe. The following phenomena regarding sound-pressure distribution when the perforated pipe is located at the center of a simple elliptical expansion chamber have been observed. (1) The resonance frequencies of higher-order modes shifted to the lower frequency ranges in comparison with those when a pipe was not attached, and (2) the resonance frequencies inside a perforated pipe are similar to the ones outside; however, the resonance level inside is higher than the one outside. In addition, an arrangement of the output pipe is also studied based on the distribution of higher-order modes in a chamber. The experimental results which tend to prove the effectiveness of the perforated pipe are concretized by a mathematical formula in the paper.