Improving low-frequency attenuation of passive earplugs using Helmholtz resonators

Earplugs are widely used in occupational and recreational settings to prevent noise-induced hearing loss. However, their effectiveness in real-world conditions is often limited by inconsistent use, imperfect fit, and intrinsic acoustic limitations, particularly in attenuating low-frequency sounds. Improving low-frequency attenuation is critical not only for increasing overall hearing protection but also for achieving a more balanced attenuation profile, which can enhance perceived sound quality and speech intelligibility. In this study, we investigate how the reflection coefficient at the earplug's medial surface facing the ear canal cavity influences low-frequency noise reduction (NR) in passive earplugs and how passive design can optimize it. From an analytical NR model, we derive the exact condition for maximum attenuation: a reflected wave in anti-phase with the incident wave, producing destructive interference within the occluded ear canal. Using meta-earplugs incorporating three Helmholtz resonators, we demonstrate, on both an acoustic test fixture and human participants, that inducing near anti-phase or quadrature-phase reflections increases low-frequency attenuation by up to 15 dB below 1 kHz. In addition, this passive strategy remains effective despite moderate acoustic leakage. Originally designed to reduce the occlusion effect, these meta-earplugs also show strong potential for enhancing low-frequency attenuation, thereby enabling more efficient and robust hearing protection.