Hearing aid shells (or earmolds) must couple the hearing aid with the user's ear. Earmolds have to fit the subject's outer ear canal properly to ensure a good performance of the aid. Because of the great variability in the anatomical pattern of the ear, earmolds are custom made. At present, an impression of the subject's ear canal is taken and used to fabricate the silicon-made mold. The postimpression activities that typically are performed during the fabrication process modify the physical dimensions of the resulting earmold and thus affect the fit of the product. A novel system for 3-D laser scanning and mesh reconstruction of the surface of ear canal impressions is presented. The reconstructed impression can be digitally stored and passed directly to dedicated CAD 3-D printing machines to model the silicon earmold and thus achieve the best possible fit. The proposed system is based on a couple of cameras and a commercial laser for the surface digitization and on a straightforward algorithm, based on the deformation of a geometric model, for the reconstruction of the acquired surface. Measurements on objects of well-known geometric features and dimensions are performed to assess the accuracy and repeatability levels of this 3-D acquisition system. Robustness to noise of the proposed reconstruction algorithm is determined by simulations with a synthetic test surface. Finally, the first measurements (acquisition+reconstruction) of closed surfaces from ear canal impressions are reported.
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.