The present subject matter provides apparatus and methods for physically fitting a hearing assistance device, including mathematically characterizing a persons ear canal. The apparatus includes an expandable balloon for positioning in the ear canal, the balloon including a hollow interior channel. A flexible catheter extends through the interior portion of the balloon, and the flexible catheter includes a track. A platform is movably coupled to the track. An ultrasonic transducer and a microelectromechanical systems (MEMS) sensor are coupled to the platform. The ultrasonic transducer scans the ear canal and the MEMS sensor senses motion and position of the transducer. According to various embodiments, a mathematical representation of the ear canal is adapted to be computed using information from the transducer and sensor.
