I regularly attend an unprogrammed Quaker meeting, where a lot of the folks are older and have hearing impairments. This style of worship poses a special challenge for these people because, unlike the convention at many other religious gatherings, there is no one person who addresses the congregation. At an unprogrammed Quaker meeting, anyone may speak from anywhere in the room.
Some years ago, the congregation installed microphones in the room, which pick up voices and amplify them. These signals are then transmitted by FM radio over special frequencies near the radio broadcast band. The thing is, a person who needs help hearing others must use a small FM receiver and earphones, which is a little awkward, in part because it makes you stand out.
One way to mitigate this problem is to take advantage of the fact that some of these people wear hearing aids equipped with a telecoil (or T-coil). These hearing aids can be switched to a mode whereby they pick up audio-frequency signals electronically instead of using their built-in microphones. This system for passing the signal wirelessly doesn’t involve radio transmissions—it just uses magnetic induction. Suitable audio induction loops for energizing T-coils are found in all sorts of places, including museums and theaters; even some taxicabs are equipped with them. You may see their presence advertised with a blue-and-white sign that looks like an ear with a “T” next to it.
Many of the hearing-aid wearers who attend our Quaker meeting live in a nearby retirement community where induction loops have been installed to help hearing-impaired residents enjoy lectures and films. These people have had good experiences in such settings, and so they have been hoping that an audio induction loop would help them to hear what’s being said at our Quaker meeting, too.
I worried, though, that their disappointment with our current system reflected the difficulty of picking up voices cleanly in a reverberant room using microphones positioned at the periphery. And 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.
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 multiturn loop using a multiconductor 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. This test was tougher than needed to assess requirements, because the magnetic field at my friend’s head was mostly horizontal, and my understanding is that T-coils are positioned to pick up vertical fields.
Based on that test (and the Biot-Savart Law), I figured out that I should be able to create a magnetic field of the same strength about 3 meters away from a long wire, which is exactly what you would need if said long wire were part of a loop installed in the attic of the building where the meeting is held.
Next I used a wire-resistance table to determine that 240 meters of 20-gauge wire should be around 8 ohms. That would be long enough for me to create a six-turn loop in the attic. So I bought a roll of 1,000 feet (304 meters) of two-conductor 20-gauge wire for about US $125 on Amazon. Using two conductors would allow me to create two parallel loops, powering each through one channel of my stereo receiver. Because each channel of the receiver can output as much as 80 watts, I figured I’d be close to the 200-watt figure I saw quoted online. Preliminary tests with six turns of this wire laid out on the floor of the building showed the signal-pickup range to be about 5 meters.
Encouraged by those initial tests, another friend and I recently placed a similar wire loop in the attic of the building—which was a lot harder than laying it on the floor because we had to snake the wire around an obstacle course of roof trusses and HVAC ducting. We ended up with a loop that’s got a rather complicated geometry and doesn’t lay flat on the joists in many places.
Nevertheless, 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.
That said, I don’t expect the addition of this informally constructed induction loop—or even a professionally installed one—could ever make up for the weak link in our system: the mounting of the microphones on the walls of the room, far from the people speaking. Unfortunately, there’s not much of an alternative, short of giving everyone TED-talk-style microphones to wear, which wouldn’t really mesh with the Quaker penchant for simplicity.
This article appears in the February 2020 print issue as “A DIY Audio Induction Loop.”
Source: IEEE-Spectrum – Fulltext