A rapid publicity growth has led to an extensive application of micro-perforated (MP) acoustic elements for broadband sound absorption in the exhaust systems of the internal combustion engine. Most typically, the MPs are exposed to grazing flow conditions, studied thoroughly by various authors in the past decades and represented by adequate acoustic models by now. However, in certain exhaust system designs implemented in the fibreless silencers of modern ground vehicles, an alternative layout for the tubular flow duct MP elements - the flow plug condition has been proven to be useful. In this type of MP’s application, the propagating gas flow is entirely guided through the micro-perforated sections upstream and downstream of the rigid plug, typically increasing the flow resistivity and the viscous damping of the sound in duct. Acoustic studies on such type of MP’s operating condition are scarce.
This paper focuses on the noise control properties of a silencer incorporating tubular MP elements operating in flow plug configuration. Hereby the flow resistivity as one of the key parameters affecting internal combustion engine’s performance and fuel consumption is also treated. A series of acoustic experiments have been performed on a variety of the MP flow plug sections. Respectively, the transmission loss (TL) has been measured with and without a mean flow. The test samples include an applicable range of perforation ratios representing commonly used rectangular laser cut apertures provided for a performance comparison. Additionally, the back pressure (BP) data of the selected silencer elements has been determined experimentally in a usable flow speed range.