Hearing Protection for Bone-Conducted Sound

The high-noise environments in and around modern military jet aircraft can impair voice communications and cause permanent damage to the hearing of pilots and maintenance crews if adequate hearing protection is not worn. With external noise levels of some aircraft approaching 150 dBA, adequate hearing protection must provide better than 50 dB of attenuation. In order to reach such a high level of attenuation, soundtransmission routes that bypass the ear canal must be attenuated in addition to the ear canal transmission routes. These boneand tissue-conduction paths limit the maximum attenuation that can be achieved, even if all air-conducted sound in the ear canal were eliminated. An investigation of bypass sound transmission mechanisms is presented. The aim of this investigation was to characterise the transmission levels of bypass sound to the cochlea and to determine the effectiveness of different hearing protection components in attenuating this sound. Measurements were made with a head simulator as well as with human subjects. In order to make detailed measurements of the vibration of skull bones in an intense sound field, we developed a head simulator. The simulator was built from a human skull, with silicon gel used to model internal organs and silicon and latex used to model the skin. Accelerometers attached to the skull bones were used to measure the skull vibration response. Mechanical point impedance measurements on the simulator were compared to results reported in the literature for humans, cadavers, and skulls. Reasonable agreement with these results served to validate the simulation. The relationship between skull vibration measured on the skull simulator and bone-conducted sound at the cochlea of a human subject was determined by comparing mechanical point impedance measurements recorded from the skull simulator and from human subjects, with the latter including psycho-acoustic responses. The effect of different hearing protection components on skull vibration and on the attenuation perceived by human subjects was also measured. Finally, the attenuation limits for bypass sound routes through the head and through the torso were determined. The response of the head simulator in a sound field peaked in the 1 to 3 kHz frequency range, which is where the bone-conducted limit to hearing protection attenuation is at a minimum. The bypass sound levels measured on human subjects also increased in this frequency range. The performance of different hearing protection components including ear muffs, helmets, goggles, and face shields was measured on the head simulator and on human subjects, and the results are reported. General guidance concerning the design of devices for providing protection against bypass sound transmission is given. Finally, the need for additional protection of the torso is discussed. Dietz, A.J.; May, B.S.; Knaus, D.A.; Greeley, H.P. (2005) Hearing Protection for Bone-Conducted Sound. In New Directions for Improving Audio Effectiveness (pp. 14-1 – 14-18). Meeting Proceedings RTO-MP-HFM-123, Paper 14. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.aps. Hearing Protection for Bone-Conducted Sound 14 2 RTO-MP-HFM-123 1.0 INTRODUCTION In situations where personnel are required to work in extreme noise environments, the protection provided by hearing protection devices (HPDs) is often insufficient. Even if the sound transmitted to the ear canal was completely attenuated, the sound reaching the cochlea via bone conduction transmission paths that bypass the HPD can still be loud enough to cause hearing damage. Improved understanding of these bypass sound transmission paths will lead to improved hearing protection, allowing safe exposure to high sound levels. 1.1 Extreme Noise Environments Military jet aircraft operating at full afterburner can produce sound levels in the vicinity of 150 dBA. Crews working in close proximity to these aircraft need hearing protection that allows them to complete a work shift without exceeding the allowable exposure limit. With a hearing conservation limit of 85 dBA for eight hours with a 3 dB exchange rate and assuming sixty, thirty-second takeoff or landing events in a work shift, the allowable noise exposure during each event is 97 dBA. For events approaching sound levels of 150 dBA, hearing protection providing attenuation in excess of 50 dB is required. 1.2 Hearing Protection Limits Bone conduction transmission paths limit the maximum attenuation that can be achieved with double hearing protection (earplugs and earmuffs). For example, in the 2 kHz octave band, Real Ear Attenuation at Threshold (REAT) measurements by Berger et al. (2003) gave a mean attenuation of 37 dB for deeply inserted foam earplugs and 36 dB for earmuffs. However, the attenuation measured with both earmuffs and earplugs in place was only 41 dB, far less than would be expected from the combined component attenuation values. The attenuation was limited by transmission paths that bypassed the hearing protection devices. When a third level of protection was introduced in the form of a helmet, the total attenuation increased to 49 dB. 1.3 Bone Conduction Transmission Paths The various transmission paths that bypass the attenuation provided by double hearing protection devices are collectively known as bone conduction transmission paths. Four primary bone conduction transmission paths have been identified in the numerous research studies on this phenomenon (e.g., Tonndorf et al. 1966; Sohmer et al. 2000). These are: (1) inertial movement of the ossicular bones relative to the skull at low frequencies, (2) distortion of the temporal bone and cochlear shell at high frequencies, (3) osseotympanic transmission of sound radiated from the walls of an occluded ear canal, and (4) sound conduction via fluid pathways connecting the cochlea to the brain cerebrospinal fluid. The resultant sound level at the cochlea is a frequency-dependent vector sum of the contributions from each of these transmission mechanisms. A graphical representation of the various transmission paths is shown in Figure 1. The ability of hearing protection devices to attenuate the sound level at the cochlea will depend on their effectiveness against each of these transmission paths. Earplugs are effective against air-conducted sound transmission and against boneconducted sound transmitted to the ear canal. Earmuffs are effective against air-conducted sound and provide limited shielding of the head. A helmet further shields the head, attenuating both airand bone-conducted sound. Finally, even if the head is completely shielded, a body conduction path remains. In order to design effective hearing protection for extreme noise environments, the relative limits between the air, head, and body conduction transmission paths must be known. Hearing Protection for Bone-Conducted Sound RTO-MP-HFM-123 14 3 Helmet Earmuffs Earplugs Ear Canal