Hearing The Unhearable

My wife was watching a crime drama, and one of the plot twists involved a witness’ hearing aid malfunctioning so that he could hear electromagnetic waves around him. It’s not so implausible, if you think about it. Many hearing aids have a t-coil, which is essentially an inductor that’s designed to couple with the speaker in a telephone. If that went haywire, maybe you could hear all the changing magnetic fields around you, and if you could escape the constant hum of the mains power line, it might even be interesting.

So of course, she turns to me and says “we need to make one!” It shouldn’t be hard at all — a big inductor and an amplifier should do the trick. In fact, it’ll probably be easy enough that it’ll make a good introduction-to-electronics project for my son. But there are also enough unknowns here that it’ll be interesting. How big a coil? How close? How sensitive? What about that mains frequency bit? Ferrite core or not?

None of this is rocket science, for sure, but it will probably be full of kludges, discoveries, and straight-up exploration. In short, the perfect weekend project. And in the end, it’ll expose something that’s normally invisible, and that’s where the fun lies.

This must be the same urge that drove Faraday and Marconi, Volta and Maxwell. There’s something amazing about directly sensing, seeing, hearing, and understanding some of the stuff that’s outside of our limited hearing and eyesight, and yet is all around us. I can write down the equations that describe it — I learned them in school after all — but there’s no substitute for poking around in your own home. Who knows, maybe in a few more weekends we’ll build ourselves an all-band receiver.

What’s your favorite super power?

51 thoughts on “Hearing The Unhearable

  1. I cut a piece of circuit board the shape of a hearing aid, stripped off the copper, and wound some fine waire around it. Seemed to work fine.

    Nowadays, some have a direct conection for an external microphone. Bluetooth is big, thiugh I don’t know if it’s yet internal. Some have a small external box, with a bluetooth receiver (and an external mic jack) which is inductively coupled to the hearing aid at 13.56MHz.

    1. My dad picked up one from costco that comes with a round medallion necklace for bluetooth. It’s a pain. Tying a lemon to a necklace would not only exaggerate the metaphor, but cost much less and probably provide better pairing.

  2. What frequency range do you want to recenve, though? most interesting E-M emissions are outside of 20Hz-20kHz audible frequency range. One could downconvert, but listening to 20kHz band at a time is not very efficient—it’d be actually a cool idea to compress the spectrum so yould listen to entire 20MHz from 88 MHz to 108MHz as a single .02-20kHz sound stream.
    So, the challenge is a tunable, compressible DC to daylight E-M receiver :)

    1. If you don’t care much about fidelity, you can do what some bat detectors do — threshold and use a counter to divide the frequency down. 20 MHz into 5 kHz (let’s make it pleasant) is divide by 4000 = 12 bits. Piece of cake!

      DC to daylight gets a little trickier, but if you added another 12-bit counter, you’d have it for sure.

      But as anyone who’s been near a GSM phone as it receives an SMS knows, there’s a lot of intermodulation.

  3. You normally want a sharp high pass filter for T coil to cut out power line frequencies. Typical voice application is 300 to 3K Hz. High end is automatically limited by parasitic capacitance of the coil. Too much inductance and high end will suffer,
    Too small and not much signal so it gets noisy. Knowles makes the actual inductors used in hearing aids. Typical around 350 mH. They’re tiny and use #53 wire or around in there. Around a half thousandth of an inch diameter. Not something I’d want to wind. An unshielded ferrite core inductor 50 mH to 500 mH will work. Not a transformer or toroid. Just a plain coil wound on a ferrite bobbin.

  4. “Tapir” from Elektor.
    About 10 years ago I built it using through hole components. Works great. From a digital wrist watch with 13 years battery life time you can hear each second !

  5. My favorite super-power (that I possess; so choices are limited) is that I can see black light pretty well. This was because of cataract surgery, and the pseudophakic lens has different optical properties than biological ones and admits more longwave UV. This drove me crazy at first because only one eye was done, creating an annoying shimmering as one eye saw the usual dark purple and the other saw a bright blue-white. When the second eye was done this annoyance was alleviated, but discos are not quite as special anymore.

    As to what you can practically do with this super-power, I did read that in WWII aphakic folks were used to receive covert communications via longwave UV, though this seems apocryphal. (google “aphakic ww2 communications” for relevant hits)

    1. Do flowers look different?

      Lots of common plants show up with wild patterns in UV photographs, because bees see UV and plants need bees (or maybe the causation goes the other way). Just wondering if you also see them differently.

  6. I used to work for a video production company with a machine room and a studio full of analog video gear – switchers and the like. There were certain parts of the building I avoided because the RF noise would generate audio frequency feedback in my hearing aids. I once diagnosed a failing RAID array by “listening” to the drives.

  7. The basic idea that there are things we can not perceive, but can understand with science is one of the powerful things with kids in learning about science and the universe. I have used the magnetic pickup example with kids; they roam around finding new weird stuff. A small module that shifts frequencies is best; then you can ‘hear’ walwart power supplies, and even sounds made by bats and insects. I love it when kids consider the things they can not see because they are too small (cells) too big (the sun/universe) too fast (electromagnetic waves) too slow (plate tectonics) etc. etc. But you can MAKE instruments so you can ‘see’ and ‘hear’ them. I have 16 grandchildren so I’ve got to do this a lot :-)

      1. Actually that’s a good example for kids: “Stuff even the smartest people could not understand 200, years ago”. What instruments do you need to actually see what the sun is doing, on it’s surface and below? What do you need to know and what do you need to calculate to find out how massive the sun actually is? How do astronomers figure out how big stars are when they are gadzillions of miles away??

  8. It’s funny, when learning in school about “…Faraday and Marconi, Volta and Maxwell…” and all of the other historical greats, we never discussed or discovered whether they *had fun* performing their experiments. They were presented as these dry, two dimensional elders, conservative in their persons, writ large in history but vanishingly small in everyday humanity.

    I like the idea of any of them giggling and animated over some unanticipated result, some surprise outcome they couldn’t wait to share with friends or family, the neighbor’s kids, the dog that slept on the piles of notes and papers that fell forgotten beneath the cluttered workbench – the idea that they would laugh at all.

  9. Tip to HaD.
    Do an article on the human hearing based “radars” that existed in conjunction with all flying forces before WW2 and real radar. The art above looks a lot like a head worn version of the giant ears navigational aid for boaters I have seen from back then. In Nam the VC laid down in bottom of bomb craters an listened for approaching aircraft and fire farther away than normal listening could garner.

  10. Any telephone tech has a “toner and wand”, which is a signal generator and inductive probe and amplifier. It’s used for tracing and identifying wires, but the wand picks up all sorts of other noise, exactly as you describe. Touchscreens sound particularly interesting, but the whole electronic world is awash in signals.

    Before making something new, consider picking up a 200EP probe, it might just do what you expect.

  11. Years ago, one of my neighbors in the houses at the Radio Australia transmitter site had his mother visiting.
    He noticed she was distressed, and when he asked her what was wrong, she said she was hearing voices, but in a foreign language. Well, it turns out the beam from one of the antenna arrays went straight through the houses and she was picking it up on her hearing aid.
    If I remember correctly, it was the Japanese program.
    In house 1, (there were 7), the flouro lights in the shed would come on powered by RF, and at our house, my 2M mobile ham radio was unusable in the driveway. At some times of the day, the phones had loud background program running.
    It may have been something to do with the 100KW AM transmitters driving a 14db gain curtain array!

    1. Thanks for that. I’ll have a look.

      Funny, in my mind the VLF radios are only for use out in the countryside, b/c in the cities there’s just too much man-made noise. And here I am _wanting_ to listen to the noise, so what’s better than a sferics radio? :)

  12. If you look up information about the Whirlwind computer, this something they used to prototype a lot of early user interface concepts, as it was a real-time system (not batch job). They also used it to prototype the SAGE system, used by the USAF for decades to track airborne targets, as well as a predecessor to modern air traffic control systems. The Wikipedia page doesn’t say much about it but there was an extensive write-up in “A History of Personal Workstations” which talked about light cannons, light pens, early touch-screen developments, etc.

    It used a horrific amount of power and created a lot of EM noise. A lot of people who wanted to work with it for “unofficial” projects (usually UI-related stuff) had to do so at night. At least one of the engineers could get in his car, drive toward the building where it was housed and tune his AM radio to a particular frequency. Listening to the EM noise from it, he could tell if it was working well or having issues. If the latter, he wasn’t going to get to use it that night so he might as well turn around and go back home. He could make that determination a couple blocks away from the building, so “loud” was the EM noise from it.

      1. That book included papers presented at a conference, as well as panel discussions about the various things. Originally borrowed it because I was looking for info about the Xerox PARC Alto. It had a good write-up on that, presented by someone from PARC, but there was so much other, interesting stuff in there.

        Rather ponderous tome but … totally worth it reading the whole thing.

  13. Sounds a bit like the Mythbusters radio teeth experiment, which they apparently deliberately nerfed for unknown reasons. After going on at length about two different metals in an acidic solution solution possibly making a point contact diode, their test with a real human jaw put the metals on teeth on *opposite sides* of the jaw, not in contact at all.

    Busted? Not at all since they didn’t actually test what they said they would.

  14. A lot of hearing aids have the feature of tuning in to an ‘Induction Loop’ connected to a buildings PA system. Usually these are located in shopping centres and entertainment venues but also in some churches. Often they are a single conductor laid flat under carpet but can also be run through conduit in a serpentine path in a concrete slab. In Australia, Silicon Chip magazine had a project on building and tuning one of these loops.

    1. I once ran a loop around my room, a single wire.

      But when I played records, things got noisy. It was obvious the cartridge was picking up from the loop, and causing feedback. No tone, but a hissing or mushiness that onky happened when using the turntable.

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