One of the most common occurrences of aging, ranging from mild annoyance to depressing isolation, is hearing loss. The latest numbers from The Centers for Disease Control and Prevention's National Center for Health Statistics (compiled between October 1994 and March 1996) show that a third of Americans 70 years and older are hearing impaired (see figure). And it's not an old wives' tale that men do not hear as well as women—they are 1.5 times as likely to suffer hearing loss.
While some long term hearing loss may be attributed to environmental factors such as workplace conditions, often the hearing mechanism of the middle ear breaks down due to aging. In the inner ear, the hair cells or nerves that pick up sounds can become damaged or loose sensitivity. While these maladies are irreparable, electronics can help.
Sound design. Conventional, outer ear mounted hearing aids today come in three types.
Oldest, and very cheap ($100 class), is the simple analog amplifier.
Programmable devices have emerged, but they still use analog processing.
All-digital models are the latest development and they include digital control and signal processing. Cost: $1,000 range.
John Caizzi, RF (radio frequency) engineer for Phonic Ear (Petaluma, CA), says, "The digital devices can do so much precise processing with less power." Lower power drain means longer battery life, and he adds, "Users are very sensitive to having to change batteries frequently."
An audiologist can adjust the programmable hearing aids using a dedicated programmer or PC-based software, plugged via cable into a port on the hearing aid. Vince Kapral, manager of electrical design at Symphonix Devices (San Jose, CA), says, "A first approximation is made based on the user's audiogram. Then the program is modified based on the user's response during controlled tests in the audiologist's office." The audiologist makes a final adjustment after the user has experience in day-to-day activities.
Parts. In hardware developments, Phonic Ear's Caizzi highlights parts that are enabling smaller and more comfortable (i.e. lighter) hearing aid products. Such components include ultraminiature tantalum capacitors from AVX (Charleston, SC), chosen by Phonic Ear designers for their Sprite personal assistive listening device. Phonic engineers liked them because they offered the highest bulk energy density of any 0603 size surface mount capacitor—contributing to a small circuit board size.
The Sprite fits behind the ear and, to augment hearing aid functions, contains a 1V narrowband FM receiver that can pick up transmissions from a microphone worn by a lecturer in a classroom or other structured environment, such as entertainment venues and houses of worship. Previous devices used a separately worn receiver wired to a hearing aid. Via a mode switch at the base of the device, a Sprite user selects from twin channels for clearest reception, or, depending on need, can choose FM-only operation or turn the device off.
Another key component, Caizzi says, is a custom IC for audio processing developed by a Phonic Ear sister company in Denmark. The chip can mitigate both soft and loud sounds. The Sprite's small behind-the-ear size includes a helically wound resonant monopole antenna. Helical winding, unlike a straight wire antenna that may need orientation to the signal, ensures signal pickup regardless of signal polarization.
Future developments Caizzi sees include user-held directional microphones for Sprite devices, directly linked phone accessories, and a Sprite-like receiver that can be snapped onto a conventional hearing aid.
Get inside your head. Last year saw a major breakthrough in hearing technology, when the FDA approved use of the first (and so far only) middle-ear implantable device, the Vibrant® Soundbridge™ from Symphonix Devices (see DN 3/2/1998, pg. 124). Unlike hearing aids, possible feedback into the device from sound reflecting off the eardrum is eliminated.
The Sprite behind-the-ear assistive hearing device contains a miniaturized FM receiver.
Symphonix engineer Vince Kapral says that "all components are critical for device operation." These include the signal processors and transmitter circuitry, down to the high-density batteries. "But in Soundbridge, the breakthrough is the transducer in the middle ear, the Floating Mass Transducer™ or FMT," he emphasizes, and notes the company holds a patent on the 0.09-inch long, 0.07-inch diameter device. "This is basically a high magnetic density samarium cobalt rare-earth magnet, with springs on each end, inside a biocompatible titanium can. Wound around the can is a wire, which carries electrical signals that vibrate the magnet."
Kapral notes the design challenges in developing the Soundbridge. "The transducer had to be small in size and in weight, so it didn't weigh down the bones and allow enough movement," he says. "The doctor slides a titanium clip on the housing over the bone, then bends it for retention. This is done as outpatient surgery, with the signal receiving electronics (the Vibrating Ossicular Prosthesis™) implanted under the skin behind the ear." All that is visible externally is the Audio Processor™ housing the small microphone, battery, and audio power electronics—parts that may need more frequent replacing. "The audio is modulated on an RF carrier to a pickup coil in the implant," Kapral adds.
The Soundbridge's Floating Mass Transducer (FMT), in a cylindrical titanium housing attached to the central of three middle ear bones, is stimulated by a surrounding wire coil to produce sound for the user. The upper left shows the external Audio Processor (AP), containing the battery, microphone, and signal processing electronics. Programmable DSP technology allows response to be tailored to individual patient requirements. The AP is mounted over the implanted Vibrating Ossicular Prosthesis, which delivers the audio signal to the FMT to vibrate the middle ear bones.
Soundbridge meets the EN 60601-1-2 standard for medical electrical equipment. There are no day-to-day restrictions on a user because of RFI considerations from cell phones or any electrical devices. Kapral adds, "Design techniques, such as careful component selection and attention to inter- connects, minimize the effects of electromagnetic interference."
The Soundbridge also uses five of the AVX ultraminiature tantalum capacitors. Also noteworthy are Siemens programmable digital signal processors (DSPs) for adjustments to the gain and frequency response based on the individual's specific hearing loss. Siemens also markets Soundbridge for Symphonix in Europe.
Advancements in middle ear implant technology continue. Kapral feels we'll probably see, or rather hear of, in the next five to ten years totally implantable devices, higher-output devices, signal processing to treat tinnitus (head noises), and smaller implants for children.
Such advances in electronic design will ensure fewer of us need to suffer isolation from loss of hearing as we age.
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