This paper proposes a muffler with simple geometry to effectively reduce low-frequency noise in ductwork systems. A muffler named infinity tube with an expansion chamber (ITEC) is developed from the infinity tube (IT). Theoretical and numerical analyses of wave propagation in the ITEC have been conducted in this paper. The transfer matrix method is adopted to predict transmission loss theoretically. The theoretical results are validated by the finite element method simulation. The comparison of the transmission loss between the IT and ITEC illustrates that the ITEC has an advantage in low-frequency noise reduction. The transmission loss results of the ITEC are compared with the Helmholtz resonator system to assess the potential for industrial application. Finally, the geometric parameters of the proposed ITEC on its noise attenuation performance have been analyzed. The proposed ITEC can effectively reduce low-frequency noise, and it is suitable for ductwork systems in constrained spaces.

Helmholtz resonators (HR) are widely used in aero-engine systems for noise reduction. By connecting a pair of HRs in series (neck-cavity-neck-cavity), a dual HRs system is formed. This study investigated the influence of neck length, cavity volume and flow Mach number on the noise attenuation performance of a dual HRs system. A three-dimensional numerical simulation was performed to calculate the transmission loss results. The transmission loss (TL) results indicated that the second neck length can influence the second resonance frequency and TL max. Changing the cavity volume significantly influences the noise attenuation ability under lower flow rate conditions compared to higher flow rate conditions. The flow Mach number had a more significant impact on the first TL peak than on the second TL peak. This study shows the relationship between the geometric parameters, grazing flow and noise attenuation performance of a dual HRs system and could provide guidance in designing suitable dual HRs for aero-engine systems.