Hear–and now

Building a better hearing aid with 3-D imaging


Imaging Imaging

3-D images of an ear canal are used to design personalized hearing aids.


Just 20% of those who could benefit from wearing a hearing aid actually do, according to the National Institute on Deafness and other Communication Disorders. MechE Professor Douglas Hart wants to better that number.

The problem, Hart says, is fit. Getting useful sound amplification depends on a tight fit between hearing aid and ear canal. Unfortunately, the current method of modeling patients’ ears is messy and not always accurate, leading to a device that often fits and functions poorly.

With 36 million Americans suffering from hearing loss, addressing the issue of fit has become an imperative. “A lot of people are likely walking around with hearing aids that don’t fit,” Hart says, “because they don’t know what they’re supposed to feel like.” His solution: a new way of scanning the ear canal with 3-D imaging technology—a process that is faster, easier, and more accurate than the plaster-mold technique now in use.

Patients who require a hearing aid usually have to spend an hour with an audiologist, who fills the patient’s ear canal with a gooey silicone substance. After about 15 minutes, the gel hardens into a mold that is removed from the ear and shipped to a hearing-aid manufacturer, who scans the mold and builds a customfit hearing aid using a 3-D printer.

The problem is that it can be difficult to achieve a tight seal between the hearing aid and the patient’s ear canal with this method. But a tight seal is necessary to prevent feedback between the microphone and receiver, which can produce squealing sounds annoying to the wearer and anyone nearby.

The “Holy Grail” of hearing aid technology

Doug Hart

Professor Douglas P. Hart

Getting a precise 3-D scan of the ear canal is the “Holy Grail of the hearing-aid industry,” says Scott Witt, head of research and development for hearing-aid manufacturer Phonak. “Taking these impressions is still the messiest, least exact part of the process.”

With Hart’s innovation, a stretchy, balloon-like membrane is inserted into the ear canal and inflated to take the shape of the canal. The membrane is filled with a fluorescent dye that can be imaged with a tiny fiber-optic camera inside the balloon. Scanning the canal takes only a few seconds. In fact, the entire fitting process takes only a minute or two.

Because the camera captures 3-D images so quickly, it can measure how much the surface of the ear canal deforms when the pressure changes, or how the canal shape changes when the wearer chews or talks.

The new technology is similar to a recently commercialized 3-D scanning system that Hart developed for dentistry, designed to replace the silicone molds traditionally used to make impressions for dental crowns and bridges. Impressed by his work in this area, hearing-aid manufacturers approached him to see what he could do to improve their fitting process.

Hart’s researchers have built a prototype scanner to demonstrate the proof of concept, and are now working on a handheld version of the device. Once it’s ready, they plan to do a study comparing the fit of hearing aids built with the new scanner to that of traditional hearing aids.

Witt believes the MIT scanner has more potential than any other proposed imaging system he has seen in the past several years. The new technology could be seamlessly integrated into existing manufacturing practices, he says. “We could do it right now. The rest of our manufacturing process is set up to receive digital scans,” he says.

The Deshpande Center for Technological Innovation funded the development of the new technology, which Hart described in a 2004 article in the journal Applied Optics. He patented the system in January. While his priority has been to bring the technology to hearingaid manufacturers, he believes the innovation will also be useful in building fitted earphones for MP3 players or custom-fit earplugs for military personnel and those who work in noisy environments.meche logo