Microperforated panel absorbers (MPP) are acoustic absorbers that consist of a perforated panel fitted in front of a rigid wall and air cavity. The absorption frequency range of MPP is limited in the vicinity of the resonance frequencies due to the sound absorption mechanism. The sound absorption frequency band of MPP can be expanded by adding resonance structures such as parallel arrangement and multilayer arrangement, and optimal design of structural parameters. Unfortunately, the approaches increase the volume of the whole absorber. It is important to minimize the volume of the absorber for compactness while widening the absorption frequency range. In this paper, multi-objective optimization of the MPP for widening the bandwidth of the sound absorption and minimizing the volume is performed. The normal sound absorption coefficient of double layer MPP is evaluated by the transfer matrix method, and sequential approximate optimization using radial basis function network is adopted to identify the trade-off relationship between the broadband sound absorption and the volume. Based on the numerical result, the experiment is also carried out to examine the validity of the proposed approach.

To reduce CO₂ emissions, the adoption of renewable energy has accelerated in recent years, increasing the need for energy storage technologies to stabilize electricity supply and demand. The authors proposed a Compressed Air Energy Storage (CAES) system using pneumatic equipment already installed in many factories. In small-scale CAES, the amount of stored air is limited, requiring operation under low flow rate and high pressure. Reciprocating expanders are well-suited to such conditions due to their compatibility with small flow rates and high pressures. However, improving their energy conversion efficiency remains a key challenge. This study presents a control method for the expansion process, where the transition from isobaric to adiabatic expansion is dynamically adjusted based on inlet pressure. Inlet valve timing is regulated using high-speed solenoid valves, allowing efficient operation even under fluctuating pressure conditions. The effectiveness of the method was verified experimentally. Compared with a conventional small expander, the proposed approach demonstrated a clear improvement in expansion efficiency. This proposal offers a practical and adaptable solution for improving the performance of small-scale CAES systems under real-world operating conditions.