A note on the Strouhal number dependence of the relative importance of internal and external flow noise sources in IC engine exhaust systems

This note discusses issues related to the estimation of flow noise emission from rear mufflers in IC engine exhaust systems, through the analysis of measurements performed in a steady cold flow bench. First, the net acoustic power transmitted along the outlet pipe is obtained from in-duct pressure measurements. The in-duct power is then compared with noise measurements carried out in a semianechoic chamber, in order to distinguish between flow noise contributions associated with internal generation (muffler and tailpipe) and flow noise produced in the discharge process (interaction between the outgoing flow and the outside atmosphere). The ratio between the tailpipe in-duct acoustic power and the radiated acoustic power is analysed as a function of the Strouhal number. The results provide some information about the relative importance of internal and external sources.