<title>Three-dimensional laser scanning and reconstruction of ear canal impressions for optimal design of hearing aid shells</title>

Abstract

The hearing aid shell (or earmold) couples the hearing aid with the user ear. Proper fitting of the earmold to the subject ear canal is required to achieve satisfactory wearing comfort, reduction in acoustic feedback, and unwanted changes in the electroacoustic characteristics of the aid. To date, the hearing aid shell manufacturing process is fully manual: the shell is fabricated as a replica of the impression of the subject ear canal. The typical post-impression processes made on the ear impression modify the physical dimensions and the shape of the final shell thus affecting the overall performance of the hearing aid. In the proposed approach, the surface of the original ear impression is 3D laser scanned by a prototype equipment consisting of a pair of CCD cameras and a commercial He-Ne laser. The digitized surface is reconstructed by means of iterative deformations of a geometrical model of simple and regular shape. The triangular mesh thus obtained is smoothed by a non-shrinking low-pass spatial filter. With this approach, post-impression processes are no more needed because the digitally reconstructed impression can be directly fed to rapid prototyping equipments, thus achieving a better accuracy in obtaining an exact replica of the ear impression. Furthermore, digital reconstruction of the impression allows for simple and reliable storage and transmission of the model without handling a physical object.