I can hear you now

Hearing loss often accumulates gradually, so many older people experience no large day-to-day differences in what they hear. Eventually, though, they realize—or get told—they have a problem. As one who suffers from deafness in one ear and significant loss of hearing in the other, I often rely on others to "amplify" lines in movies and plays. So, I had a personal interest in learning more about the manufacture and test of hearing aids. Recently, I visited Siemens Hearing Instruments in Piscataway, NJ, and met with Gene W. Mulvihill, Jr., manager of in-the-ear electro-acoustic design, and Todd Putvinski, director of R&D, to learn more about the processes.

The design of a typical hearing aid starts in the same place the device will find its ultimate use—the human ear. Testing in an audiologist's office involves plotting a customer's threshold hearing levels against frequency for each ear. The plots often look like those for low-pass or band-pass filters. The results of these tests determine what type of hearing aid the customer may need to help overcome his or her sensory loss.

Customers may choose options such as programmable frequency responses for settings such as movies, normal conversation, meetings, and so on. New models also change their directional patterns to home in on conversation and reduce ambient noise. Given the variety of options a customer can choose, the company can offer over a million hearing-aid configurations.

Figure 1. A desktop anechoic chamber accompanies the Fonix 6500-CX tester to facilitate standard tests on hearing aids. This type of tester works equally well for production testing or for diagnostics or adjustments in an audiologists's office.

Mulvihill added that even the standards themselves present challenges (Ref. 1). "The standards mainly cover behind-the-ear devices, but you can't make the same assumptions for custom-manufactured in-the-ear hearing aids. We don't mass produce hearing aids—we customize each hearing aid to match a person's hearing loss and cosmetic preference." The wide variety of hearing aids Siemens manufacturers makes it difficult to streamline testing. "We have to make testing efficient so we meet demand without creating a huge backlog of products waiting to go through testing," said Mulvihill.

Mulvihill and his colleagues always look for ways to improve the testing process. "Often, the standards lag the technology, and that's the case with the latest directional hearing aids. When Siemens leads in a technology, we get to help write the standards. We have people on an ANSI committee looking into a standard for three-dimensional testing," added Putvinski.

Even without any formal standards, Siemens continues to work on designs for better 3-D hearing aids and on the tests needed to ensure their quality. "We hear in a 3-D world, not in a flat plane, so without a standard in place, we have to learn what does and doesn't matter when it comes to testing directional hearing aids," said Putvinski.

Production testing at Siemens mostly uses test equipment from Frye Electronics (Tigard, OR). The company manufactures a series of analyzers that can test hearing aids in an audiologist's office or on a production line. By employing the same test equipment audiologists use, Siemens ensures the audiologists can replicate test results.

In this test setup, a hearing aid is placed on a 3-D rotating fixture in front of an open speaker. Siemens hopes to develop this setup, which is controlled by LabView, into a production-line unit for performing directional tests on hearing aids like the Triano device.

Mulvihill noted that these types of test instruments undergo a daily verification to ensure they work properly. But for the most part, the instruments calibrate themselves, and operators run a self-calibration several times a day. These calibrations verify the frequency response of the microphone and the chamber. "We use an external reference to verify the sensitivity of the microphone, and at least once a week we use a standard 1-kHz tone to set the tester's output level." Every year, each piece of equipment goes through a formal calibration.

To control the testing process, Siemens developed its own software, called FTDS—final test data system. The project started years ago and used MS-DOS and programs written with Borland C. The software has evolved and now uses a Microsoft Visual Basic interface. Although the software continues to work well, Mulvihill continues to upgrade it.

At first, the software simply reported test results, but now it can automatically obtain test limits through the company's network. Because each hearing aid carries its own serial number, employees connect to the company's order system and then pull in test parameters for a specific hearing aid. The network connection also lets the company save test results.

Engineers at Siemens had thought about building test systems and writing software with test-software packages. "But we've shied away from data-acquisition cards in a PC," said Mulvihill. "The biggest challenge isn't acquiring the data, it's making sure we test to the standard, verify the tests, and keep the tests current. We're always willing to look at alternatives for future developments." One test system in development already uses National Instruments' LabView software.

In addition to production-line testing, the Siemens engineers also perform tests on designs for new hearing aids and ensure that their testing processes meet standards. For this sort of testing, "KEMAR," an industry-standard dummy who "lives" in an anechoic chamber, plays the starring role. (KEMAR's name comes from Knowles Electronics Mannequin for Acoustical Research.) The use of a standard dummy lets anyone with similar equipment reproduce test results. But the standards for KEMAR—yes, there are government dummy standards—relate more to R&D than to production testing.

KEMAR's masters, Oleg Saltykov, senior electro-acoustic design engineer, and André Koch, a consultant who works as an electro-acoustic design engineer, place the dummy on a turntable so they can position it for directional testing. A 360º rotation lets them test for directional effects and for shielding, an effect caused by the head blocking sound from a given direction. In addition to using KEMAR to test whether hearing-aid designs perform the way the designers expect, the engineers can also use the dummy to validate the tests they run on the production line.

Not all R&D testing needs to take place in the large anechoic chamber. Some tests can use smaller desktop chambers that more closely duplicate those used on the production line with the Frye Electronics equipment. In addition to the Frye testers, the people who use the R&D lab have access to audio test instruments from Rohde & Schwarz and components and accessories from Bruel & Kjaer.

The lab also includes probers and testers that perform electrical verification of the hybrid circuits used in hearing aids. Hybrid circuits undergoing R&D testing may get bonded out to 18-pin headers for insertion into custom-built test equipment. This equipment duplicates the test equipment at the hybrid manufacturer. Because Siemens qualifies many of its vendors at their sites, the company does not need to perform an incoming test or inspection of every hybrid circuit.

Mulvihill and Putvinski see changes ahead for hearing-aid testing. "Test engineers love to use a product to test itself. If you think about it," said Mulvihill, "a hearing aid can make sounds and it can listen to sounds. Most tests relate to what kind of sound a hearing aid can produce and how good it sounds."

"Because we have a digital signal-processing (DSP) chip inside a hearing aid," added Putvinski, "the circuitry can do a lot of new things. Here's an example: A hearing aid contains a pair of microphones and we need to match their sensitivities and phase to get the hearing aid to work properly. The DSP chip can create test tones, feed them from the hearing aid's speaker to its microphones, monitor the signals produced by the microphones, and make adjustments on the fly."

The engineers at Siemens have plans to quickly test hearing aids that have a directional response. Now, their prototype tester accepts a hearing aid and rotates it on a swiveled base. The tester provides a "level of goodness" or go/no-go test that lets production people catch any mistakes in the production process. As an extension of the basic test, an operator can put the hearing aid into its directional mode, align it using a laser, and rotate the hearing aid. Software based on LabView performs a quick test to check frequency response and directionality of the hearing aid.

This type of directional test normally requires a large test chamber and can take up to 20 minutes. The engineers use the prototype tester to determine how to scale it down to fit in the production area and still give good results. They don't expect to get the same results they would in a chamber, but so far, results correlate well with those from the chamber tests.

Although the prototype relies on manual rotation, in the future position control and device programming could occur automatically. Future use of automation depends on how accurately and consistently human operators can perform the tests. And as always, taking various approaches to testing involves tradeoffs. Normal hearing-aid testing requires small closed chambers, but Siemens is investigating test-station designs that allow for testing in an open environment. The open system would require more expensive equipment that would filter out ambient noise and pass only audio test signals. Because the company may need as many as 60 stations, it must approach new testing strategies with an eye toward its budget.