1. Introduction
The sense of balance is affected by complicated interaction between the sensing organs such as the inner ears, the eyes, the force receptors, and the central nerves. If even one of these organs sends inappropriate signals due to something wrong on the user, the sense of balance becomes to be in disorder and tottery motion of the user's body gets unusual. A measurement on the position of the center of gravity (COG) of the user's body, which is called stabilometry, was reported to diagnose some symptoms such as Meniere's disease (dizziness) related to otolaryngology [1][2][3][4]. Referring to the idea of stabilometry, in which the force distribution onto a solid platform is determined while the user is standing still on the platform, we showed a possibility of a wearable headphones solution utilizing a high-sensitivity 2-axis accelerometer in [5][6]. In the preliminary results, we found that some incidents such as lack of sleep, catching a cold, and being suffered from physical or emotional stress, might affect the motion of the user's body. The motion measurement needed to be taken place at the top of the user's head to maximize the amplitude of the faint acceleration signals as well as not to be seriously affected by breathing motions. An ad-hoc evaluation parameter called the acceleration trace length (ATL) was introduced to enumerate the degree of the sense of balance. The absolute value of the time-differential acceleration in 2-D within 10 milliseconds was time-integrated for 30 seconds, which was specified from the stabilometry diagnosis procedure established under Japanese National Health Insurance System. As the user only needs to be standing still for 30 seconds while wearing an ordinary headphones device, the sense of balance diagnosis can easily be performed anytime anywhere.