Characteristics of whistle noise from mufflers with perforated pipes

Abstract

The purpose of this study is to improve understanding of whistle noise generation mechanism in mufflers with perforated pipes. First, we measured sound pressure of whistle noise radiating from a straight-through-type sub-muffler with a perforated pipe with steady flow. Results show that the frequency of predominant whistle noise became higher stepwisely with increasing the flow velocity and was higher with smaller hole diameter. Next, we measured the sound pressure of whistle noise radiating from an expansion-cavity-type main muffler with a perforated pipe. Predominant Strouhal number based on the hole diameter and flow velocity existed within a certain range while the hole diameter and flow velocity were varied. The frequency of predominant flow velocity fluctuation near holes of the perforated pipe was close to the frequency of predominant whistle noise.Under the same hole diameter and flow velocity condition, the straight-through-type muffler tended to generate whistle noise more than the expansion-cavity-type muffler. Comparison with experiments without the outer shell or an open window in the outer shell also show that the outer shell of muffler enhanced the whistle noise generation.