As I spoke with Micah Frerck at this week's Biomedical Engineering Society meeting, I had to focus to block out the cacophony around us and tune in to his voice. But for people who are hard-of-hearing or deaf, these kinds of environments can be torturous—a hearing prosthesis would amplify all the sounds equally, making it nearly impossible to carry on a conversation. Fortunately, Frerck, a bioengineering student at the University of Utah, is building a device that might be able to help.
Part of the problem is that hearing aids and cochlear implants (electrode arrays that translate sound into nerve stimulations) are bad at determining where a sound came from. For people with healthy hearing, the brain localizes the source of a sound by comparing the time the sound wave arrives at each ear. So if a wave hits my right ear first and then my left ear, my brain knows the sound came from the right. If the wave hits both my ears at about the same time, I know the sound came from in front of me.
To help people with hearing prostheses to localize sound, some scientists and engineers have proposed to mimic human ears by planting two microphones into a device. That's challenging because the device needs to be small, but it's easier to tell where sound came from if the microphones are farther apart. "To fit two microphones in one ear, you make the distance between them smaller and you make the difference in time of arrival smaller," Frerck says. "It becomes almost impossible to measure because you're into nanosecond time-of-arrival differences, and it requires a lot of digital systems processing."
Frerck wanted to accomplish sound localization without resorting to digital processing. He looked to nature for inspiration—more specifically, to mosquitoes. "Here we have animals that are really, really small, and they're known to produce sounds and to listen to sounds. And how they do it is with their antennae."
As a sound wave hits the mosquito antenna, its hairs undulate back and forth at the base. That movement is translated into electrical activity that the brain interprets as sound.
Frerck has created a similar sort of antenna from polysilicate, which looks like a tiny tree. The millimeter-tall device is flat and stands upright on a surface. When a sound wave jostles the antenna, "fingers" at the base of the tree distort and pull apart. As they do so, they create a capacitance, or stored charge, which can be translated into an electrical signal.
The flat shape of the antenna is what allows it to sense direction. Since its side is extremely thin (approximately 2.25 micrometers across), sound waves coming from that direction will have a hard time wiggling the antenna. In contrast, a wave that hits the flat area will have a large effect and a large electrical output.
Frerck says the device potentially could be incorporated into hearing aids and cochlear implants. The effect, he says, would be like using a shotgun microphone instead of an omnidirectional mic to pick up sound. By turning her head, the user could aim the microphone at a companion so that ambient sound won't drown out the conversation.
That's probably still five to 10 years away. For now, Frerck is testing the antenna's sensitivity, frequency response, and directional response. And before the device can be mass produced, he'll also have to find a way to automate the process of standing up the antenna—in the current prototype, Frerck had to do it manually, under a microscope.