Development of active noise control and voice communication systems for personal hearing protectors

This thesis discusses the development of active noise control and voice communication systems for personal hearing protectors. The main focus is on implementing active noise cancellation (ANC) techniques for a personal earcup-type hearing protector. A personal passive hearing protector has two drawbacks that can be eliminated with electronic systems. These are poor attenuation of low frequency noise and blocking of speech and other important sounds. It is well-known that ANC methods can improve the poor low frequency performance of a passive hearing protector. Despite the apparently simple principle, the practical implementation of an ANC hearing protector is not as straightforward as might at first appear. It is, in fact, a rather complex mechatronical problem. This study will consider some of the reasons it has taken so long to progress from Paul Lueg s patent of 1936 to the practical implementation of an ANC system. To understand the basics needed to design ANC systems, the function of the ear and the operation of a passive hearing protector are reviewed. The modeling of a passive hearing protector is also discussed. In addition, the operation of a closed-box mounted loudspeaker element is analyzed, and some previously implemented ANC hearing protectors are also reviewed. This review provides the basis for combining existing theories and models to develop a more complete model for a hearing protector with an internal closed-box mounted loudspeaker. Performance and usability of hearing protectors in actual situations are both important factors, though there is a trade-off between them. A major challenge, therefore, is attaining good active attenuation performance with reliable operation in demanding environments, while at the same time retaining the original performance and comfort of the passive hearing protector. To achieve this, the electronic, mechanical, and acoustic designs of the hearing protector must be in balance with each other. One specific challenge is to achieve adequate dynamic range. In addition to the development of an ANC hearing protector, two methods are presented for enabling communication while wearing a hearing protector. First, short-range communication is made possible by means of an active sound transmission (AST) hearing protector. An AST hearing protector transmits speech and other useful sounds through the earcup while also limiting the sound pressure of the transmitted sound to a safe level. Secondly, long-range communication is made possible by a wireless Bluetooth radio link. Bluetooth itself is a shortrange radio link, but here its purpose is to establish a link between the hearing protector and a cellular phone. As a result, a cellular phone can be used while wearing a hearing protector without the need for inconvenient cables between the hearing protector and the cellular phone. As a result of this research, a series of electronic hearing protectors is developed through several prototypes. The improved low frequency noise attenuation is valuable in very loud environments: it reduces the risk of hearing loss and prevents the development of noise-induced stress. In addition, communication is made possible in conditions where hearing protection is necessary. The developed AST hearing protector outperformed several commercial equivalents on the market in 2000. The developed ANC hearing protector performs well in practical situations and is comfortable to wear since there are none of the external battery packs or controller boxes that are fitted to the few commercially available ANC hearing protectors. Earlier theories are successfully combined and applied in a practical solution. The developed ANC hearing protector was tested with authentic noise samples in the laboratory and under actual noisy conditions. The system has been tested with both wideband and tonal noise. The results show that a maximum active attenuation of 18 dB can be achieved while also retaining passive performance and the comfort of the passive hearing protector. Thus, prior knowledge and previously developed theories were successfully combined to implement a prototype which provides significant improvement to the low frequency performance of a passive hearing protector