Protecting beyond the bone-conduction limit

Workers operating in extreme noise may need levels of hearing protection that are beyond that possible with double hearing protection comprising earplugs and circumaural hearing protectors. In such noise fields, sound conducted along bone-conduction transmission paths that bypass the ear canal can be sufficient to cause hearing damage. To provide sufficient protection beyond this bone-conduction limit, hearing protectors must attenuate sound that is transmitted to the cochlea via bypass mechanisms. The design of a passive hearing protection helmet that provides protection beyond the bone-conduction limit is described here. The helmet was developed for Navy aircraft carrier deck crews who are exposed to extreme sound levels of 150 dB during aircraft launch operations. The helmet design was based on extensive measurements with human subjects and with a human head simulator built to measure bone-conducted sound. These measurements demonstrated that a helmet shell fitted with an edge seal that created an acou...