Characterization of Auricle-EAM Transitional Structure and Its Expansion

Intrusive ear-related wearables rely heavily on the mechanical behavior of the anatomical region connecting the auricle and the external acoustic meatus (EAM), yet this transitional structure has received little systematic investigation. This chapter defines the auricle-EAM transitional structure as an integrated unit consisting of the medial tragus, the anterior cartilage of the medial concha, and the anterior and posterior margins of the EAM entrance, and examines its deformation characteristics and comfort implications under intrusive fitting conditions. A novel angular variable, the angle between tragus, entrance, and medial concha (ATEMC), is proposed to characterize its shape-structure relationship. Using large-scale anthropometric data and controlled expansion experiments, the study demonstrates that ATEMC shows substantial interindividual variability and follows a phasic age-growth pattern, with apparent sex differences largely explained by body-size covariates. To standardize deformation across individuals, the tragus expansion angle (TEA) and the tragus expansion index (TEI) are introduced and linked to subjective comfort ratings. Results reveal a staged comfort-perception process in which functional comfort dominates at low expansion levels, mixed functional-physical perception occurs near the optimal range, and physical discomfort governs responses at higher expansion levels. Quantitative comfort thresholds are identified, providing actionable design criteria for in-ear devices. Overall, the findings establish a robust ergonomic framework that bridges anthropometric measurement and comfort perception, offering practical guidance for the design of stable, comfortable intrusive ear-related wearables.