Comparison of experimental transmission loss of expansion chambers models with extended ducts and microperforated panel

The use of microperforated panels in mufflers has presented as one of the ways to increase sound absorption properties and replace the use of fibrous materials in some applications. In expansion chamber mufflers, the combination of MPP with other geometrical changes, such as extensions of the inlet/outlet ducts, can provide different amplitudes of transmission loss, mainly in low-frequency bands. To demonstrate this performance four models of expansion chamber mufflers with microperforated panels have been proposed to operate in frequency ranges from 250 to 350Hz. These models combine the MPP position and the geometrical changes: an outlet extend duct inserted in the main chamber, internal partitions added to the MPP air cavity depth, and simultaneous inlet/outlet extend ducts. The acoustic performance of these mufflers was compared using the experimental TL via the Two Load Method (TLM). To validate the experimental results test bench was constructed and validated by the analytical method Transfer Matrix Method (TMM) and via the Finite Element Method (FEM) in the ANSYS software. In all models, the models presented TL with good performance in low and medium frequency ranges. The comparisons show that the addition of duct extensions at the inlet/outlet of the muffler was the principal influence on different TL amplitudes, indicating that the combination between the MPP and simultaneous extend ducts inside the chamber has a great potential for improving the TL of these mufflers, essentially to act at low frequencies.