Description of the ear canal’s geometry is important for understanding peripheral sound flow, yet anatomical measurements along the entire human canal are sparse. Recent measurements from the lateral end of the canal showed both larger variations in area than previously described and an unexpected increase in canal area with increasing subject age (Voss et al., 2020). Substantial changes in area along the length of the canal have been documented in cadaver ears (Stinson and Lawton, 1989), but such variations have have not been incorporated into auditory models, which generally assume either a cylindrical (e.g., Keefe et al., 2015; Rabinowitz, 1981) or horn-shaped canal (e.g., Keefe, 2020; Rasetshwane and Neely, 2011b). Clinically, tympanometry can be used to measure the canal’s volume between an inserted probe and the tympanic membrane in order to assess individual variability in canal volume, and wideband reflectance measures at higher frequencies can assess middle-ear function under the assumption that reflectance is only weakly influenced by variations in canal geometry from a cylindrical shape (e.g., Stinson et al., 1982). However, as the measurements presented here demonstrate, canal geometry can vary substantially (1) within an individual ear with both increases and decreases in area along the canal and (2) across individuals with areas that differ by factors of more than 3 at the location of the canal’s first bend. Additionally, we are not aware of any published descriptions of pediatric canal geometry. Because canal geometry influences the sound field during all auditory measurements, a clear understanding of its variability will inform the development of new and better measurement methods within the general areas of basic science, audiology, and medicine.

We have identified only three publications that report physically-measured ear-canal areas along a substantial length of the canal: Johansen (1975), Stinson and Lawton (1989), and Egolf et al. (1993). In all three cases, the published areas are from a small number of cadaver ears, and in no cases do the publications offer clear and consistent definitions for important anatomical features of the canal, making it impossible to directly compare across studies. For example, these publications offer results for the length of the canal but all three lack an objective definition for the entrance of the canal. Additionally, neither the location nor the area of the first bend are reported, even though the first bend is likely where canal probes sit during canal-based measurements (Voss et al., 2020).

This work presents a novel method to measure canal geometry and develops objective definitions for relevant canal features. In particular, we report (1) A method that utilizes high-resolution CT scans to define, identify, and measure the canal’s termination, the canal’s central axis, the canal’s cross-sectional area along its length, the canal’s entrance, and the location of and area at the canal’s first bend; (2) Repeated measurements and statistical analysis from three subjects (six ears) that demonstrate a high level of repeatability of the method across three independent investigators; and (3) Measurements and corresponding analysis of canal geometry of 66 ears from 47 adult subjects.