Imagine being asked to provide sound reinforcement for a meeting that occurs in a large room, where anyone can be the speaker, and in a situation where microphones would hinder the flow of the meeting. Throw in a couple of attendees who have hearing disabilities, and you’ve got quite a challenge to make sure everyone gets heard.
Such a situation faced [David Schneider] at his Quaker meetinghouse, which he ended up solving with this home-brew audio induction loop system. The worship style of conservative sects of the Religious Society of Friends, as the Quakers are formally known, is “silent worship”, where congregants sit together in silence until someone feels moved to share something. Anyone can speak at any time from anywhere in the room, leading to the audio problem.
Rooms mics and a low power FM transmitter didn’t work because those using radio as aids to hearing the service felt awkward, so [David] decided to take advantage of a feature in the hearing aids worn by some members: telecoils. These are inductive receivers built into some hearing aids to send sound directly to them using magnetic fields generated by a loop in the listening area. [David]’s loop ended up being 240 meters of 20-gauge copper wire in the attic above the meeting room. The impedance ended up close to 8 ohms, perfect for feeding directly from the speaker terminals of an old stereo amplifier. Pumping 160 Watts into the coil allows the hearing-aid wearers below hear the service now.
There’s still work to be done on the input side to improve audio quality, but [David]’s solution is elegant in that it helps those who need it most using technology they already have. And perhaps those who need but don’t yet have hearing aids can roll their own.
It works to use one loop of CAT5 ethernet cable, and tie the 8 wires in it so that they make 8 turns.
And don’t forget the load resistor. 4 or 8 ohms in series.
” [David]’s loop ended up being 240 meters of 20-gauge copper wire in the attic above the meeting room. The impedance ended up close to 8 ohms”
I don’t know what the impedance of the ethernet-wire loop is but wouldn’t you want to subtract that from the value of your load resistor? I would imagine it is going to be different with the geometry of every room and also with different wire. I doubt it’s ever nill though if David measured 8 ohms with his own solution. Things seem to have worked out quite conveniently if his loop was 8, nice!
Also useful for Star Trek style behavior control if those hearing impaired Quakers get uppity.
Just kidding, of course this shouldn’t be done.
I was thinking the same thing. You would think that they just put an icicle in Kirk’s ear in some episodes.
Escape is quite impossible —
as demonstrated by your collars of obedience.
Cool hack
What drives the input of the audio amp if room mic’s don’t cut it?
BTW, that cat 5 cable trick, I used that to make a seat heater for my old Jeep Wrangler. I made a big spiral out of it and held it together with gorilla tape. The seat already had a split in it so it was easy to jam in the split and put some foam on top of and I have a friend who does leatherwork, he made me a new bottom seat cover out of the back from an old leather couch. It worked quite well. I ran it to a cig lighter plug and it was nice on cold days.
From what I understand, the room mics drive the amp, but the sound quality is lacking. I think it’s because the only place they can mount them is on the walls, and everyone is facing away from the mic closest behind them. The might do better with ceiling-mount mics designed for audio conferencing rooms.
I remember using the wire loop trick decades ago. The city hall here for one. This is a very (at least was) common addition to public buildings. Did a lot of them in my time, and I,m old.
Good ideas live forever.
This is an old technique was often used in schools in language labs.
It’s a hack!
Not a hack – it’s missing the official blinking LED.
Nicely thought out Friend, Been using Loops and Phonac Induction ear pieces for Actor and Extras on movie sets since the 50’s for musicals, when dialog required a quiet set. I even used them up to 10 years ago on a couple sets
Now hack it to grab RFID data from everyone who has their ID card from work with them. It won’t work on the newer encrypted cards, but it should be fine against the more common (at least where I live) 125 kHz cards that just send their unique ID in the clear repeatedly whenever powered.
Where’s the Arduino?
David, you might find hanging a few mics in the centre of the room more beneficial. It would place the microphones closer to the sound source and have the speaker facing (approximately) the mics. The shorter that distance, the better. The Audex M1255’s (or similar) provide excellent pickup. In terms of number and placement, be sure to look up the 3 to 1 rule for mic placement.
The 6 turn loop results in a very high electrical impedance that significantly dampens higher frequency output from the loop. Higher frequencies (2500-6000 Hz) are essential for intelligibility. The high frequency loss is further exacerbated by the amplifier. As a voltage drive amplifier, it will not adequately produce the higher frequencies. You need to use a current drive amplifier with no more than two turns of wire and calibrate the system to IEC60118-4 standards. Do that and you will have ecstatic listeners! Until you are able to do that, turn up the treble control to achieve some high frequency compensation.
Also, take a look at https://hearinglosshelp.com…
Bill
I was thinking that a 10-band graphic equalizer between the mics and the amp might help compensate for the lost high frequencies.
Doesn’t this just hum and buzz from all the AC wiring? Light dimmers being the worst.
Wiring that meets code rarely presents an EMI problem. Light dimmers are bad, but only to within a yard/meter or so …