Study on the arrangement of acoustic liner structures in the inlet silencer of civil aircraft auxiliary power unit

The inlet noise of auxiliary power unit (APU) for the civil aircraft, as a major noise source of apron noise, has been widely discussed and concerned. Given its high-frequency dominant characteristics, this paper distinguishes itself from traditional uniform honeycomb core depth liners by optimizing four types of acoustic liners: single-degree-of-freedom (SDOF) liners, double-degree-of-freedom (DDOF) liners, parallel SDOF liners, and parallel DDOF liners within their respective design spaces. The goal is to achieve a broader absorption bandwidth and higher absorption coefficients. The study identifies the optimal structural parameters and corresponding transmission loss for each type and compares their manufacturing difficulties. The results indicate that in terms of acoustic transmission loss, a parallel configuration can further enhance transmission loss. The parallel SDOF liner exhibits the best performance, while the DDOF liner performs the worst. Regarding manufacturing ease, traditional SDOF liners are the easiest to produce, followed by DDOF liners, with parallel liners being the most challenging. From an engineering application perspective, a comprehensive consideration of the design requirements and noise reduction needs is essential for the flexible selection between these liner types.