Eartip modifications for more accurate acoustic length estimations of ear canal models or calibration cavities

The cross-sectional discontinuity between a probe eartip and an ear canal (EC) causes the latter to “acoustically” appear longer than it is “geometrically”. In this study, the idea of whether modifications within the eartip geometry can reduce this length overestimation was investigated: (i) upon extending one of the connecting (sound or microphone) tubes and (ii) upon sinking the entire probe tube assembly into the eartip. Finite element models of the eartip modifications were created, and validated by measurements on rigid EC models using eartip prototypes. Whereas extending the sound tube yielded no considerable effect, extending the microphone tube by 2 mm counterbalanced the discontinuity effect for a 12 mm diameter EC model. Alternatively, sinking the tube assembly 2 mm into the eartip allowed for radially symmetric sound radiation at the discontinuity, which was then described by a series inductance in the lumped-element equivalent circuit. Whereas this elaborate modification with the recess is more appropriate for calibration purposes (where the exact geometry of a calibration cavity or EC simulator is known), a microphone extension is more practical when a rough length estimate of large-diameter ECs is required. In most practical applications, discontinuity effects can be accounted for by using modified eartips.