Inductor/Capacitor wireless audio

a.mlw.walker · Nov 30, 2012

Hi Guys,
I bought a device that is used for T-Coils and induction hearing aids. I wanted to see how it worked, and whether I could a) replicate it, and b) improve its range.
Its a device with a 3.5mm headphone jack, that attaches to a 10uF cap, the negative side of the cap to a toroidal inductor, and then back to gnd. A thick loop of wire passes through the toroidal inductor and is supposed to hang around the users neck. Then the audio signal is induced to the hearing aid.
Ive been reading a bit about LC circuits, and I was wondering firstly, if I know the value of the capacitor, and assume that the heighest frequency the human ear can hear is 20Khz, sampled at 40(ish) KHz, can I work out the henries of the indcutor. I assume you would place the resonant frequency at the top end of the human voice (about 8Khz) and work the equation backwards.
So if resonant freq in hz = 1/(2*PI*sqrt(LC))
then L = ((1/(2*PI*f))^2)/C
If C = 0.00001 F
and freq = 8000 Hz
then L = 3.96*10^(-5) Henries.

If that is right(???) how could I then work out the range at which i would be able to pick up the signal?
thanks guys

duke37 · Nov 30, 2012

The loop is intended to be connected to an audio source and then produces a magnetic field which the hearing aid can detect. The loop is not resonant and the capacitor is probably intended to protect the device driving it.

The signal strength will be strongest inside the loop and will be very low outside the loop. In order to get a longer range you will need a larger loop so you can sit inside it. A larger loop will need to be driven by a power amplifier to pass sufficient current through the loop resistance. Proper loop amplifiers have feedback to provide a defined output current, this can ba aproximated by using a normal amplifier with a series resistance, the inductance of the loop will tend to reduce the current at high frequencies and give top cut.

A 10W amplifier should be sufficient to run a loop around a house. It should have automatic gain control to give a reasonably constant signal level. You can have a one turn loop with a high current or a mutiturn loop with higher voltage and lower current.

There are standards for the signal strength and I think Practical Wireless published an article 20 or 30 years ago on how to set up a loop. You could try searching for induction loops.

a.mlw.walker · Nov 30, 2012

Right. So there is very little way of sitting outside of the loop and picking up a good signal. If you are aligned with the center of the loop, does the signal get projected? I.e if you are a distance away from the loop but looking thugh the loop can it be picked up ok? - I.e can you "fire" a beam of signal across a room very directively?

duke37 · Nov 30, 2012

The 'lines of force' diverge as you move from the centre of the loop. If I remember right, a loop of side 10m will work reasonably well 2m from the loop plane. There is no way you can 'fire' a signal but you can alter the shape of the loop to put the signal where you need it.

duke37 · Nov 30, 2012

http://en.wikipedia.org/wiki/Audio_induction_loop

davenn · Dec 4, 2012

I have installed quite a few induction loop systems in school auditorium/assembly halls
A 10metre sqr loop is probably going to require at least a 50W amplifier to get a decent coverage. The loops I was installing were in the order of ~ 8m x 15m rectangle and we were using amplifiers of ~ 100 - 120W

Dave

Inductor/Capacitor wireless audio

a.mlw.walker

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