David Schneider and 240 Meters of Wire Create a Low-Cost T-Loop for Audio Accessibility

A standard stereo amplifier, 240 meters of two-conductor copper wire, and a few microphones make Quaker meetings more accessible.

The system relies on the telecoil already present in hearing aids. (📷: RawPixel.com)

David Schneider was looking to make a local assembly house a more accessible location for Quaker meetings whose attendees relied on hearing aids — turning to a surprisingly simple T-loop to broadcast audio to the telecoil built into most aids as standard.

"Commercial installers of this equipment charge thousands of dollars - a lot for something that’s probably not going to solve our particular problem. But for very little money, I figured, we could install a DIY audio induction loop and at least let people try it out," Schneider writes. "A little searching online uncovered some brief commentary posted by someone who installs such systems at music festivals. What he described was quite simple — creating a multi-turn loop using a multi-conductor wire (one for which the total resistance is between 4 and 8 ohms), and attaching it to a 200-watt audio amplifier, just as you would an 8-ohm speaker.

"Before diving into this, I enlisted an older friend (a retired electrical engineer) who wears a T-coil hearing aid to perform an experiment. I constructed an induction coil from a six-turn square loop of magnet wire that was about a half meter on a side (I taped the wire to a flattened cardboard box), using wire of the right diameter to make the loop resistance 8 ohms. I then attached it to the speaker terminals of an ordinary stereo receiver, one that was collecting dust in the back of my garage. Using that arrangement, I was able to convey audio to the T-coil in his hearing aid with his head about a meter and a half away (on axis) from my coil."

Schneider's project then moved to some calculations, based on the experimental results — resulting in a bill of materials including a 240-meter length of 20-gauge two-conductor wire, placed in a six-turn loop located in the building's attic. "Using two conductors would allow me to create two parallel loops," he explains, "powering each through one channel of my stereo receiver.

"This cobbled-together induction loop appears to convey signals just fine to hearing-aid wearers below. At least that’s what my retired electrical engineer friend reported when I set the stereo receiver attached to the loop to output a program from our local public-radio station. The only dead spots he found as he walked around the room were in the far corners."

Schneider's project write-up was published in the February 2020 print issue of IEEE Spectrum and republished on the magazine's website; the commentary piece that inspired the project, meanwhile, can be found on the Electrical Engineering StackExchange.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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