Improvement of Occupational Noise-induced Temporary Threshold Shift by Active Noise Control Earmuff and Bone Conduction Microphone

Among the hazardous factors in the workplace, noise is one of the most difficult to eliminate because enclosure or reduction of its source is often infeasible. This may underlie the fact that many workers are exposed to noise and develop hearing problems. For instance, in Japan, in the year 2000 alone, at least 208,000 workers were exposed to noise and more than 36,000 (17%) of them were found to have abnormalities on audiometric examination. Furthermore, once a worker develops hearing loss, there is no effective medical treatment. Therefore the primary prevention is of the utmost importance. To achieve this, the most practical method seems to be the proper use of hearing protectors such as earplugs or earmuffs. Nevertheless, in actual workplaces, workers would not always use protectors because they feel that it causes difficulty in detecting irregularities in machine operation or in communicating with colleagues . There seem to have been two important techniques that are applicable to hearing protectors of a reasonable price. One is a real time active noise control system that suppresses noise by generating out-of-phase signals. Another is a bone-conduction microphone that detects vibration of the skull caused by the speaker’s voice. Due to rapid advances in digital signal processing, both techniques have become widely available at modest cost. Meanwhile, a number of studies have indicated the importance of field measurement for evaluating the effectiveness of hearing protectors . Therefore, this study was performed to devise a new earmuff combining these new techniques and to evaluate its effectiveness by comparing it with a conventional hearing protector subjectively and objectively in an actual working environment.