Experimental Two-Port Characterization of the Aeroacoustic Transmission Properties of a Truck's Exhaust System

Maximizing the acoustic attenuation is one of the important design criteria of automotive exhaust systems. Although both analytical and numerical approaches exist to evaluate the acoustic transmission properties of exhaust systems, they are, at present, insufficient to model the full geometrical complexity and to accurately assess the influence of thermal and aerodynamic phenomena onto the acoustic attenuation characteristics. For this reason, an experimental test campaign is often still indispensable to evaluate the aeroacoustic performance of exhaust systems. One of the most commonly used experimental characterization techniques for flow duct systems is the two-port characterization. Although this measurement approach is limited to low frequencies, below the traversal cut-on frequency of the inlet and outlet ducts, it allows making an in-depth analysis of the acoustic transmission properties of duct systems, including flow and thermal effects with an arbitrary geometrical complexity. Furthermore, using a two-port characterization allows combining various duct elements, which are measured and/or modeled independently from each other, in a network approach, enabling a straightforward acoustic evaluation of various flow-duct system design alternatives. In this paper, an experimental two-port determination technique is used to assess the influence of aerodynamic effects on the acoustic transmission properties of a part of a truck's exhaust system, containing four different components: an inlet bent duct; a flexible, corrugated, connection pipe; a complex muffler configuration; and a tailpipe. The experimental methodology is based on a multiple microphone calibration technique to determine the various environmental parameters. In this way, the final accuracy is increased and the need of accurate instrumental assessment of these parameters is avoided. The aeroacoustic performance of the various flow duct elements is determined and analyzed for each component and the use of a network approach, combining the various subcomponents is evaluated. It is shown that an experimental two-port characterization technique provides useful insight in the noise attenuation principles of flow duct systems and is ideally suited to be adopted in a network approach.