Design Articles

When Low-Power Design Gets Personal

I took delivery last week on a pair of sub-miniature, 16-channel wireless hearing aids. These little puppies are awesome.

By John Donovan, Low-Power Design

Phonak hearing aid

I lost my hearing in Hong Kong in 1996. Well, everything much over 1 kHz, that is. By all rights I should have lost it during rock concerts back in the 60s, but I guess the crowds made it hard to get too close to the speakers. Getting too close to pile drivers turned out to be a big mistake.

In Hong Kong I lived for a few years on Lantau Island and took the ferry to work every day to my office in Central. They were upgrading the Central ferry piers, which included spending several weeks driving huge steel I-beams directly through asphalt down to bedrock right next to where I got off the ferry. The piercing sound of a pile driver banging a steel I-beam into bedrock could be heard all around the harbor. When you’re walking next to it for two blocks it’s extremely painful and can do permanent damage to your auditory nerves, which according to the doctor is what happened to me. I noted with pained amusement that this all happened right outside the offices of the Occupational Deafness Compensation Board.

Many years ago I worked as a stereo technician and could easily hear notes above 15 kHz. Suddenly my hearing was down 20 dB (1000x) at 5 kHz vs. 1 kHz—the chart resembles an expert-level ski slope. Since speech intelligibility depends heavily on higher frequencies, this was a serious problem. I could easily converse with one or two other people in a quiet environment, but as soon as the noise level would rise—or even if the television was on in the background—I’d lose the thread. Holding a conversation in a noisy restaurant or bar was completely out of the question.

I got fitted for a couple of the hot, new (1996) “completely in (ear) canal” (CIC) hearing aids, which looked and felt like chewed peanuts. They used 6-channel DSPs to cover the range from 500 Hz to 5 kHz. No programming was involved, just a one-time frequency compensation made by the audiologist. AGC was primitive, and they were of only limited help in noisy environments. As soon as I stepped out in the street after getting them fitted, I was greeted by two jackhammers, which caused them to completely shut down. I popped open the battery compartments and they made great earplugs. This technology was not ready for Hong Kong.

Low-Power Wireless State of the Art

ear with hearing aid

That was then, this is now. I took delivery last week on a pair of sub-miniature, 16-channel wireless hearing aids. These little puppies are awesome.

The Phonak Audéo SMART hearing aids sit behind your ears, with an almost invisible wire connecting to a tiny transducer that fits in your ear canal. Unlike my old ‘chewed peanut’ CIC devices, these allow unamplified sound to enter around them, so you can hear low-frequencies directly, with—in my case anyway—only the highs boosted. These come with 8/16/32 DSP channels and a number of programs that adjust automatically for different acoustic environments, including the aforementioned noisy restaurants and bars, where I’m happy to report they work superbly.

They’re also wireless. After initially placing them in my ears, the audiologist tuned each device up individually from his computer across the room. No wires, no “Stick your head in this acoustic box.” At the click of a mouse he showed me different programs for different listening environments. Cool!

Each device has two microphones, one pointing forward and one behind you. They communicate with each other to focus on a 45 degree cone in front of you; any loud sounds outside of that cone are attenuated; they can even notch out a single point source 45 degrees behind you to the left. You can tap either earpiece to select different programs to suit a wide range of acoustic environments, ranging from listening to a flute solo to sitting in the front row at a Metallica concert. I’ve found that the automatic setting can handle everything to my satisfaction, though Metallica might be ill advised.

The Audéo’s wireless technology has a transfer rate of 300 kBits/s using continuous phase frequency shift keying. The transmission frequency is 10.6 MHz with a bandwidth of 300 kHz. This frequency was chosen to be able to support the transfer of complex broadband data with virtually no interference.

The magnetic field intensity needed for hearing instrument wireless communication purposes is low intensity as they are placed on the head in close proximity to each other. The measured field strengths for Audéo hearing aids is 3 mV/m at 1 m, which equates to 0.18 picoWatts. The magnetic field strength is < -62 dB μA/m at 10 m. The Specific Absorption Rate (SAR) value of the Audéo hearing aids is under 0.001 W/kg, more than three orders of magnitude less than what the FCC allows for cell phones. Don’t expect them to warm your brain up first thing in the morning—that’s what coffee is for.

If you’re an iPod addict, you can buy an optional Bluetooth device to connect your hearing aids to your iPod or to replace the headset on your computer-based VoIP phone. The thin palm-sized Bluetooth gadget also lets you redirect your hearing pattern in any direction, including to the right or left in the car if your spouse is driving.

The only limitation I’ve found is that pure sine waves—such as the “Put on your seatbelt!” signal from my car—are distinctly choppy. This doesn’t seem to be an AGC problem but more likely the result of slow data conversion. You can only run DSPs so fast if you want the tiny zinc-air hearing aid batteries to last 7-10 days. I don’t hear any choppiness or distortion when listening to music, but then I can’t do a meaningful comparison since the portions of the spectrum that the Audéos boost I can’t hear very well without them. I’m sorely tempted to do a teardown, but I doubt that the warranty would cover it.

I’ve been writing for years about low-power wireless and experimenting with new technologies as they came along. These little gadgets are the most impressive use of low-power wireless that I’ve seen to date. They’ve brought home to me in a personal way just how much technology can contribute to your personal well being.

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