Magnetic Field Transmission

Question

I have heard from here that if you put 1 turn of wire around your house, you can hear music all around inside your house. It doesen't tell me very much on all of the components etc. to drive but I am assuming that you have to have an amplifier on the receiver and the transmitter. To simulate, before actually performing the experiment, I wrapped 1 turn of wire around a cardboard box. For the transmitter, I found a stereo system, which can produce a very loud sound. The only problem is because it acts as a short, I can only turn it up to a certain volume. For the receiver, I just hooked up a speaker to a coil, and I heard a faint sound. I know with the scale of a large house, I will need some time of amplifier. How would I set up the circuit for amplifying the low signal sound found on the coil. Would this be possible with a TL082CP op-amp and if so, how would I set up the circuit? Also does anybody know of any better way of transmitting the sound through it or is the stereo fine for the setup around my house?

Also, I came across a very strange thing when I found a head-set and put one ear-piece in the magnetic field. I heard the audio out of the other. I had nothing connected up to the jack and it seems impossible for this to happen with a stereo head-set. Why is this happening?

Answer 1

This does work. I heard back in the 70s that a museum in the US had loops in various rooms and issued a special headset to each visitor - each room they entered gave a different commentary. I suspect that they may have used modulation techniques BUT I know it works because back then I tried it. I could just about hear stuff in headphones and I built a small amp (single transistor) and it was cool.

I powered it from an audio amp with a 6 ohm current limit resistor to prevent short circuits and it seemed fine.

TL082 - there is something you can use on this site - I've seen a circuit for a tunnel transmitter that relied on capacitive connections to the receiver. It could also transmit. If I can find it i'll post a link. It used a TL082 but virtually any op-amp configured for a gain of about ten to 100 would do the job.

Link - here

The headset - due to the cable or some other coupling impedance in the headset both earpieces could become "lightly" connected and thus one acts as receiving loop and couples energy to the other.

Answer 2

It is called audio induction loop and is commonly used to aid people that have a suboptimal hearing.

Here is a nice example.

Answer 3

Quickie setup: a Radio Shack telephone pickup coil (the one with the suction cup,) plugged into a small Radio Shack amplified loudspeaker.

The transmit loop can be many turns in order to multiply the ampere-turns proportional to the magnetic field. Find some old 25-pair telephone cable? You can lay it around your room circumference, then wire all the conductors to form a single spiral. Or just add a resistor in series with a 1-turn coil to get it up to 8ohms.

Fancy setups use modulated high frequency, which can provide stereo, eliminates any powerline buzz, and an AGC flattens the loudness changes as you get near/far from the transmit conductor.