Bone conducted sound transmission for communication systems

Licentiate thesis, 2006

Bone conducted (BC) sound is vibrations in the skull bone that are transmitted to the cochlea and perceived as sound, originating from a sound field or a vibration transducer. This transmission path can be advantageous for communication systems in some situations, e.g. extremely noisy environments, where it is required to protect the hearing with hearing protection devices in the ear canals; such devices may prevent the use of ordinary communication systems. Moreover, if a BC microphone that records one's own voice at the skull bone is used, an advantageous signal-to-noise situation can be achieved compared with an ordinary AC microphone in front of the mouth. The requirements and possibilities of using BC communication systems are investigated in this thesis. It was found that the possible improvement in signal-to-noise ratio of using a BC microphone, instead of an ordinary AC microphone, could be as much as 40 to 60 dB. Furthermore, due to the occlusion effect, the perception of the BC sound is affected at low frequencies when using hearing protection devices, such as earplugs and earmuffs. This reduces the effectiveness of hearing protection devices at low frequencies. By devising an acoustical model of the occlusion effect, the low-frequency increase from occluding the ear canal could be estimated. The estimations were compared with measured occlusion effect data allowing fitting of the model parameters. The model is able to predict occlusion effects of different types and at different positions with a typical error of less than 5 dB. Also, perception data was compared with ear canal sound pressures when the BC stimulation was at three different positions: ipsilateral mastoid, forehead, and contralateral mastoid. This provided information on the amount of sound that reaches the cochleae from the stimulation position. The results also showed that the ear canal sound pressure is a good proxy of the relative BC perception at frequencies above 800 Hz.