Modified Helmholtz resonator-based tunable acoustic metamaterial for ventilation and low-frequency sound absorption

Low-frequency noise control in ventilation ducts remains challenging due to the combined requirements of compact size and unobstructed airflow. This study proposes a tunable ventilation muffler based on modified Helmholtz resonators (HRs) that achieves low-frequency sound absorption without reducing the duct cross-sectional area. The modified HR's resonance frequency is tuned by modulating its acoustic inertance through inserting an annular plate into the HR cavity. The inserted plate forms air passage that connected to the HR's neck and increase acoustic inertance. Therefore, it enables a significant downward shift of the operating frequency without increasing the overall cavity volume. A temporal coupled mode theory (TCMT) model is employed to interpret the absorber–reflector coupling mechanism. Numerical simulations and impedance tube experiments are conducted to evaluate the acoustic performance. The results demonstrate that, by adjusting the insertion depth from 1 to 48 mm, the absorption peak frequency can be tuned from 635 to 325 Hz, with a maximum absorption coefficient exceeding 88%. The proposed design provides a compact and mechanically simple solution for tunable low-frequency noise control in ventilation duct applications.