Proper hearing in an operational environment is vital to mission success. The loss of this key sense can not only impact unit readiness, but also result in negative consequences for the individual.

"Sound localization is a critical component of situational awareness, or to put it in layman's terms, knowing what is going on around you," said Robert Williams, an engineer with the Defense Health Agency's Hearing Center of Excellence, at Aberdeen Proving Grounds, Maryland.

Recently, military researchers conducted a series of studies to learn more about how hearing can impact situational awareness. They found that service members who could not locate the source of a sound not only had diminished performance, but experienced a higher number of fatalities in the test scenarios.

Service members who took part in the research used programmable headsets during paintball and field exercises to simulate varying levels of hearing loss and how it can affect sound localization. These studies were conducted by Walter Reed National Military Medical Center in Bethesda, Maryland; and Naval Surface Warfare Center Dahlgren Division in Virginia.

"Sounds can be used to draw our attention to certain things, which can then be verified by our eyes," said Williams. "Your eyes can see in front of you, but they don't work as well in the dark, or when obscured by smoke, fog, dust, or other obstacles. But you can hear in the dark, or through fog, and the ability to hear can alert us to hazards or other things that we need to be aware of."

According to Dr. Felix Barker, DHA Vision Center of Excellence, director of rehabilitation and reintegration, vision and hearing are our two most important remote sensing capabilities, and essential to the warfighter.

"When they don't work optimally in a threat situation, this can make the service member effectively "blind or deaf" to the threat," said Barker. "Both senses are also highly interactive. Hearing produces initial awareness of the threat, even when it is coming from behind you. Hearing then helps define the target location so that the threat can be acquired visually, engaged and destroyed."

For those with hearing loss, Williams explained that the ability to localize sounds may also be diminished. "This could occur because they may not be able to hear a sound at all, or because our brains use certain frequencies of sound to determine their location. If we lose the ability to hear these frequencies, we may still be able to hear a sound, but not be able to localize it."

Another factor influencing sound localization is certain types of hearing protection devices, which can cause localization errors and affect situational awareness.

"Unfortunately, hearing protection often diminishes the ability to localize," explained Dr. Theresa Schulz, HCE prevention branch chief. "We lose the information provided by our pinna-- the part of the ear that holds up our glasses. Some hearing protector manufacturers make efforts to minimize the disruption to localization, but none are as good as the open ear. While localization is important, so is protecting the more basic ability to detect, identify and recognize sounds."

"There are a number of studies conducted by the Department of Defense and academia to create localization assessment methods for hearing protection, which have enhanced the DOD's knowledge of how localization affects operational performance,' added Schulz.

Certain military career fields are particularly dependent on a keen ability to localize sound, not only for mission effectiveness, but for survival, according to Schulz. "Ground troops and force protection personnel must be able to localize sounds when on a perimeter watch, for instance, to hear someone approach," said Schulz. "The ability to hear a sound and determine the direction can also help service members find targets. They must be able to detect and localize sounds as well as understand verbal commands whether face-to-face or over a radio."

Studies conducted by the National Institutes of Health show the brain uses many sensory cues to determine the location of a sound. When hearing and vision work together, a person can often determine where sound is coming from faster.

"Both threats and opportunities can be identified using our ears and eyes," explained Schulz. "Hearing a sound is often the first clue to where a sound is coming from, and then we look in a limited area to identify more specifically the source of the sound. Our brains use the difference in loudness between our two ears and the difference in timing of the sound's arrival between our two ears to determine where the sound came from."

By the same token, vision and hearing do not work independently, explained Dr. Mike Pattison, VCE's program manager of readiness and operations optometry. "The two senses work together to create our perception of what we experience in the real world. For most sighted people, in situations where the visual input is unclear such as at dusk or in a dark building, hearing takes on a more prominent role," said Pattison.

Schulz noted that while sound localization is possible with some hearing or vision loss, if both hearing and vision senses are impaired, this ability can become greatly jeopardized.

"Vision and hearing can augment and compensate each other, but if there is a dual sensory loss, much of this critical redundancy is lost. It's so important to always protect your vision and hearing senses by wearing the right protective equipment for your mission, and getting regular hearing and vision checkups to stay mission ready, and to survive and thrive."