A New Breakthrough Could Help the Elderly Echolocate Like Bats
You too can see using echolocation, but you'll need two young, healthy ears.
Image: Dave Thomas/Flickr
A new study into human echolocation has found that while humans can use sound to see the world around them, it's an ability that only works best with two functional, youthful ears. It's a breakthrough researchers say could help designers of hearing aids maximize spatial awareness for people with hearing problems—particularly those with visual impairment, too.
Most famously used by bats—and more recently, Marvel's Daredevil—echolocation is the ability to use reflected sound waves to locate objects in physical space. The possibility of human echolocation has been studied for years, but results have often been difficult to interpret due to confounding variables.
Lead researcher and University of Southampton audiologist Dr. Dan Rowan said that his team was "particularly interested in finding what aspects of sound people needed to hear" to do echolocation. Like prior studies, they asked subjects to locate a stationary object either to the left or the right of their heads—but unlike prior studies, this was the only input subjects received.
"We just got rid of all variables other than the sound cues themselves," Dr. Rowan said. That turned out to be easier said than done.
For that, the team had to head to Southampton's Institute of Sound and Vibration Research (ISVR) anechoic chamber—a room so bumpy that no sound ever reflects back to its source. The walls of this room do to your ears what the belly of a stealth bomber does to a radar tower. The team placed the board in various locations, but recorded the soundscape of the space without the confounding echoes that would otherwise be created by other objects or surfaces in the room.
This allowed Dr. Rowan and his team to create what's known as a Virtual Acoustic Space—similar to how music studios can emulate how a guitar or piano might sound in, say, an echoey church. The team used this auditory model to recreate the exact soundscape of the room while they played their recordings of the object in its various experimental locations.
By using a VAS, the team was able to "characterize the general echoey-ness of a room that has no echoes itself," Rowan explained. Using recordings, rather than physical boards, the team could also avoid having subjects unconsciously feel the circulation of air in the room, which might help them locate an object without echoes.
Participants heard the modulated recordings through specialized headphones that let the team cut out just some aspects of the sound at will, or all sound to just one ear. With this ability, they could simulate single-sided deafness, or the partial hearing loss associated with natural aging.
They found not only that subjects could only reliably locate objects when they had good hearing in both ears, but that high frequency sounds were by far the most important portion of the overall soundscape.
"Statistically, it's only a matter of time before most people have some hearing impairment," Dr. Rowan pointed out. "Blind people are effectively blind and hearing-impaired people in waiting." That's especially true for high-frequency hearing, which usually fades first as people age.
While it's becoming increasingly common for some public health services to offer just a single hearing aid to someone with two impaired ears, Dr. Rowan said his research shows that audiologists need to ask themselves what a person needs their hearing for. One person with vision problems might be able to get by with a single hearing aid, depending on their lifestyle, while another might need the extra spatial awareness offered by the echolocation of two functioning ears.
In the future, this sort of understanding might be used to help design hearing aids that actually enhance our spatial awareness—visually impaired or not. They could produce sounds themselves, much like how a bat does, but would need fundamental studies like this one to determine how to translate these artificial echoes into a form more immediately sensible to the human auditory system.
"Our approach is to use this research to design hearing devices that do the sorts of stuff that are difficult for [humans] to do," Dr. Rowan said. "We're not like bats… we evolved for speech, not clicks and echoes."