WRIGHT-PATTERSON AIR FORCE BASE, OHIO — It is mistakenly thought that vision is the most important of the five senses. In reality, hearing is the one that dominates and clues humans in to their surroundings.
On the battlefield, troops’ auditory perception takes a beating. The Veterans Affairs Department compensates servicemen more than $1 billion annually for hearing loss-related problems. But even as the Defense Department provides measures to prevent hearing damage, troops complain that using such devices limits their ability to fight effectively because their environmental acoustic cues are muffled or distorted. For those reasons, some troops choose to forgo wearing earplugs at the cost of long-term aural disabilities.
In an effort to improve — and protect — troops’ hearing ability on the battlefield, scientists at the Air Force Research Laboratory are studying how the human auditory system locates and processes sounds. They are translating their findings into technologies that not only will provide troops with 3-D audio displays but also will preserve their acoustic health.
When a noise occurs, the sound waves arrive at one ear before the other. Humans are sensitive to that interaural time delay of 10 to 800 microseconds. The brain processes the staggered signals to help pinpoint the location of the noise, said Richard McKinley, principal engineer at the lab’s battlefield acoustics branch.
Inside an anechoic chamber, 277 Bose loudspeakers are embedded in a geodesic sphere that is suspended in midair by netting. In its center rests a chair where subjects sit with microphones inside their ear canals and listen to chirps. Scientists measure how the sounds change when they reach the ears.
Based upon those studies, researchers have figured out how to reproduce spatially distanced sounds in a stereo headset using high-speed digital signal processors. Normally when sound is piped through headphones, its directional quality is lost. The resulting surround-sound effect is great for listening to a favorite CD, but not so practical for military pilots who have to monitor multiple radio channels. Though they can increase the volume of certain frequencies to help distinguish the conversations, the voices still overlap each other.
By spatially separating the radio input so that one voice seems to emanate from someone standing 30 degrees to the left, and the other voice 30 degrees to the right, for example, an operator can naturally differentiate the channels and home in on one conversation, just as he does at a cocktail party, said McKinley.
For the Joint Strike Fighter program, McKinley leads the vibro-acoustics team that is developing 3-D audio capability for pilots. Inside their helmets, they will hear directional alerts from threats, such as incoming missiles, and from locations of friendly and enemy forces on the ground. Engineers also are working on active noise reduction earplugs for maintenance crews. The goal is to protect people’s hearing, but make them feel present in the environment. “Give them those spatial auditory cues that they need to identify targets, to avoid becoming a target and to communicate effectively with the people they’re working with,” he said.
The problem with current headphone technology in the cockpit is compounded for troops in command-and-control centers who are plugged into large communication networks. If they miss a transmission, they have to ask the operator to repeat himself. A simple miscommunication could translate into lives lost.
A team at the lab is developing a software interface tool intended to make the process more intuitive and less stressful. The multi-modal communication program integrates the spatial 3-D audio capability with voice recognition, automatic speech transcription and chat tools on a single computer display.
Engineers have designed the tool so that operators can aurally separate each radio channel into one of nine directional locations. An open-source speech recognition system developed by Carnegie Mellon University transcribes the conversations in real time.
“Not only are we transcribing, but we’re capturing the actual audio message, just like TiVo or DVR,” said Victor Finomore Jr., engineer research psychologist. Operators can replay the audio transmissions. If there are errors in the transcription, users can type corrections into the text, which is synchronized with the audio and time-stamped.
“One of the reasons we’re trying to make this so versatile is because they really don’t have anything like this in theater,” said Dianne Popik, research audiologist. “They really have no way of capturing and reviewing it at all. We’re expecting that this will change how they work.”
Internal experiments with users have shown that even when overloaded with 12 radio channels, they could respond to between 80 and 90 percent of the incoming signals as opposed to 30 percent on current communication technologies.
“We need to have it reach escape velocity and get it out there,” said Popik. “I’m hoping within the next year we’ll have this in several simulation facilities.”