Enhancing acoustic comfort for earplug users: objective and subjective evaluation of bone-conducted sound with meta-earplugs incorporating Helmholtz resonators

Passive earplugs are commonly used to protect workers from excessive noise exposure, but they often result in discomfort. The occlusion effect (OE) is a major discomfort that corresponds to an increased perception of bone-conducted sound at low frequencies. Objectively, the OE is associated with an increase in the sound pressure level generated in the earcanal under bone-conducted stimulation. Inspired by metamaterials, “meta-earplugs” incorporating Helmholtz resonators have been developed to minimize this phenomenon, and their effectiveness has been validated using artificial ears in the authors’ prior work. In this study, 34 participants evaluated the effectiveness of meta-earplugs in reducing the OE. Three configurations of the meta-earplug were tested alongside a commercial foam earplug. Objective measurements of both OE and sound attenuation were conducted. Participants also completed a questionnaire evaluating their perception of low-frequency sound amplification and the judgement of the naturalness of their own voice while speaking with the earplugs. On average, the results demonstrate that meta-earplugs reduced the objective OE by up to 20 dB below 1 kHz. Additionally, the perception of low-frequency sound amplification decreased by 2 points, while voice naturalness judgement increased by 2 points, both assessed on a 7-point Likert scale. Using linear mixed-effects models, it was found that the perception of low-frequency sound amplification was primarily driven by the objective OE at 125 Hz, while voice naturalness was also significantly influenced by the objective OE at 4 kHz and the psychosocial characteristic of familiarity with the experimenter. Overall, meta-earplugs were preferred by 85 % of the participants.