Remote modulated induction of a speaker.. possible?

Discussion in 'Wireless & RF Design' started by TSOMB, May 26, 2018.

  1. TSOMB

    Thread Starter New Member

    Jul 31, 2014
    Hi folks, got a possibly stupid question here...

    I would like to power a small speaker via inductive / modulated magnetic field from a distance of approx 10ft. My thought is to produce a magnetic field and influence the coil in the speaker (8ohm) with some sort of transmitting device.

    The idea is for a prank in nature, so there cannot be any wires connected, no physical link or circuit attached to the speaker.. This must be completely wireless, and via inductance or even RF if either is at all possible...

    Any suggestions or ideas would be helpful :)
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    You won't be able to directly audio modulate a small speaker coil from that distance easily using simple magnetic fields but you can transmit audio modulated near-field RF (WiTricity) to power the receiver circuit (with some sort of audio signal detector) or just use a small battery for a receiver/amplifier circuit.
  3. TSOMB

    Thread Starter New Member

    Jul 31, 2014
    I dont mind if the transmitting unit is large or even requires A/C mains power to operate, or if the modulated signal is clear or just static.. even a dead open carrier sound would be fine. My main concern is how to efficiently transfer a small current to the coil as you would between tx/rx antennas. At least thats the idea... I think :)

    Do you have any advice on how I would broadcast the signal more efficiently? I was looking at treating the receiving speaker coil as an
    antenna and trying to match it with the transmitters antenna.. or at least something like this. Am I on the right path at least?? lol

    I cant find any info related to this topic :/
    Last edited: May 26, 2018
  4. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    Won't happen with audio frequency magnetic fields to a speaker coil at that distance with anything you could build. I would expect low mG levels at that distance from even an efficient emitter the size of an oven.
  5. Tonyr1084

    Distinguished Member

    Sep 24, 2015
    Real life experience: One night someone on the CB radio called for help. A friend of theirs rolled their jeep and needed hands to help turn it up right. This was back in the 70's when "Good Buddies" and "Truckers" named "Rubber Ducky" were all the rage. Someone, I don't know who, nor do I know how, was transmitting with illegal power. Even though my CB radio was off, they were coming booming through my radio. Through my car speakers too. I unplugged my CB (from the cigarette lighter, it was setup to be portable) but the audio, as clear as a bell, came booming through over my speakers. I don't know how that happened, but it did happen.

    Well, we did turn the jeep up right and got on our way. But I've experienced similar issues like that. Running normal power I was able to transmit my order at a McDonalds drive through and everyone in the store heard my order loud and clear. Not only did I order at the speaker I also used my CB to transmit my order. What can I say - I was young and, well, an idiot at times. But it was a lot of fun too. So I think you can achieve your goal, but I also suspect the method by doing so may be deemed illegal by the FCC (Federal Communications Commission).
  6. Bordodynov

    Well-Known Member

    May 20, 2015
    The loudspeaker coil is shielded by a magnetic system. What you've heard is a detected signal received by the lead wires. Detection can occur on any nonlinearity, even on simple contacts. There is a system of directional, local sound without using a loudspeaker. This system is based on the use of ultra sound.
    nsaspook likes this.
  7. Danko

    Active Member

    Nov 22, 2017
    Assume transmitter located in center of cube for which each side is 20 feet (10 feet from transmitter to wall).
    Then area of cube surface is 345600
    Assume your antenna has 1 area and you want to receive 100mW power.
    If efficiency of transmitter is 100%, then you will need almost 35KW power for transmitter.
    Last edited: May 27, 2018
  8. BR-549

    AAC Fanatic!

    Sep 22, 2013
    Yes, it can be done. It takes a little luck and a high power AM transmitter. Adjust transmitter carrier to resonate with speaker leads. If audio device is tube powered....unit sometimes needs to be powered up. The tube PA will rectify rf and leave audio for speaker.

    If audio device is transistor powered......many times audio unit does not even need to be on and still get audio at speakers.

    I have experienced both examples many times. A few hundred watts of speaker lead rf resonance can easily do it.

    The IF frequency can be snuck in on too. If unit has one.
    The FCC does not consider it a prank. It's considered direct interference.