Hearing Through Your Bones

A recent report from the Department of Veterans Affairs found that 58,000 of the 1.3 million soldiers who have served in Iraq and Afghanistan are on disability for hearing loss. And in 2006, the V.A. reportedly spent $539 million on payments to veterans with hearing-related ailments. This number is expected to rise in the coming years. When we think of hearing, we usually imagine our ear canal. However, sound also travels through the bones in our skulls, and at high noise levels can be just as damaging. To understand the nature of bone-conducted hearing, Margaret Wismer of the Bioacoustics Research Lab at the University of Illinois Urbana-Champaign studies this phenomenon on behalf of the Air Force, using computer simulations generated at the Texas Advanced Computing Center.

“The Air Force is interested in reducing hearing loss, by building better hearing protection devices for people that work in very noisy environments, which means under airplanes,” Wismer said. “The Air Force wants to know all the pathways by which sound can reach the ear and cause damage to hearing, because even if you block the air pathways, noise can still reach the eardrum through these bones.” 

The fact that sound travels through the skull has been recognized for ages. Beethoven, for example, found a way to hear music through his jaw after he became deaf, by biting a rod attached to his piano. Bone-conducted hearing also explains why we sound strange to ourselves on a recording: we’re used to hearing our voice through bone-conducted sound-waves; when it comes exclusively through our ear canal, our voice seems distorted. Despite extensive study, however, the phenomenon is not well understood.

Wismer used a CT scan of a skull to create a virtual model with the complex geometries of a real head. Her software simulates the pressure signal of compressed and rarified air interacting with and traveling as an acoustic wave through the bones of the virtual skull, and eventually reaching the eardrum. By repeating the simulations at different frequencies and at different input points, she obtains a highly-detailed picture of what’s actually happening with bone-conducted hearing.  Her high-resolution and three-dimensional simulations require the parallel processing power of Ranger, the nation’s most powerful academic computing system. Using Ranger, Wismer is simulating bone-conducted hearing to better understand the phenomenon and ultimately to help design more effective hearing protection devices.

Wismer is currently studying the output of her simulations on Ranger, looking at intensity plots, watching 2D and 3D movies of how sound-waves travel through bone, and exploring how occlusion effects —such as when individuals wear protective earplugs — can actually make bone-conducted sound more damaging. All of these insights feed into the Air Force’s and industry’s ultimate goal of creating better hearing protection devices, with less leakage and less bone-conducted sound.

Preventing hearing loss isn’t the only reason to study sound being transmitted through the skull. The armed forces and commercial companies have already begun to tap the bone pathway for alternative communication devices — so-called ‘bone phones.’ These have the advantage of keeping the ears open while transmitting cleaner sound waves. Such a device will rely on enhanced knowledge of how sound travels through the head — the type of insights that will come from Wismer’s software, and the application of high-performance computing systems.