Listen Up! From the NHCA Experts…

© Jackie Niam

Bone Conduction: Pushing Protection to the Limit

By: Ted Argo, Principal Scientist and NHCA Expert

Sound arriving at the cochlea via the bone conduction (BC) pathway is not attenuated by hearing protection developed for the air conduction (AC) pathway, leading to an unexpected risk of hearing injury. (photo courtesy NHCA)

Did you know there is a limit to how much noise hearing protection devices (HPDs) can block? Sound reaches our ears both by air conduction (AC), where noise enters the ear canal, vibrates the ear drum and travels through the middle ear to reach the cochlea; and by bone conduction (BC), where sounds bypass the ear canal and travel directly through the skull to the cochlea.

For most industrial noise environments, HPDs effectively block the AC pathway. Properly worn earplugs reduce noise exposure by about 25dB, and adding earmuffs can increase protection by another 5dB. This is enough to reduce 115dB noise levels to a safe 85dB for an eight-hour workday.

However, in more extreme noise environments—such as noise from jet engines or explosives—sound can exceed 140dB and bypass HPDs through the BC pathway. BC sets a hard limit on attenuation, as even the most advanced HPDs cannot block more than about 50dB—the attenuation provided by the skull itself. For example, a properly fitted HPD might attenuate 60dB of sound, but only 50dB of protection will be provided due to the BC pathway. This means safe exposure durations are significantly reduced in these environments.

Understanding that sound exposure includes a BC component is critical to protecting people in extreme noise environments. Noise-monitoring tools, such as noise dosimeters, are only designed to measure the result of exposure for the AC pathway and do not account for health risks associated with the BC pathway. Given these limitations and armed with the understanding that adding more protection may not help, consider instead engineering or administrative controls to reduce or eliminate these exposures and ensure a safer working environment. IHW

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