Systematic Comparison of Ear Temperature Probing Positions for Continuous Wearable Vital Sign Monitoring

In recent years, research on sensory ear-worn devices ("earables") explored measuring body temperature at different locations in and around the ears. While the tympanic membrane (eardrum) is a well-established medical gold-standard position, earables face challenges integrating body temperature sensors because of the limited space available, the diversity of earphone designs, and the obstruction of the eardrum by the audio hardware components. Therefore, to understand the trade-offs in accuracy between different sensor positions in and around the ears, we contribute the first systematic comparison of locations around the ear. Based on related work, existing earable form factors, and a pre-study with thermal ear images from four subjects, we selected five positions to compare to tympanic temperature: concha, ear canal, and three positions spread behind the ear. Subsequently, we developed a custom earable with six optical temperature sensors that can measure all positions simultaneously. In a study with 12 participants at room temperature and settled isothermal conditions, we find that compared to the tympanic membrane, the mean temperature difference was 0.30°C colder at the concha (sufficient according to the American Society for Testing and Materials) and ear canal, and 0.6°C colder at positions behind the ear. Exposing participants to varying environmental conditions and physical movements resulted in unreliable measurements which could not be calibrated.