Connect with us

Capturing EM interference with a microphone

Discussion in 'General Electronics' started by jh, Nov 4, 2005.

Scroll to continue with content
  1. jh

    jh Guest

    Hi,

    I was hoping somebody here might be able to help me with a question.
    First, some background. A couple of months ago I was trying to record
    the sounds of the insides of my computer for an experimental sound
    project. I first tried it with a cheap, crappy lapel mic that came with
    a pocket voice recorder. It worked just fine.

    Then I borrowed a fairly nice, high quality microphone and tried it
    again. Sure enough, this microphone picked up a lot more sounds... in
    fact, it recorded all sorts of beeps, buzzes, and hums that weren't even
    there, apparently some sort of electromagnetic interference. I was
    amused to find that this high-quality microphone was much more prone to
    picking up this interference than the cheap one I tried earlier.

    The thing is, the interference sounds were much more interesting than
    the real sounds. Holding the microphone near the graphics card, it
    recorded different noises depending on what was being displayed on
    screen. The fans sounded like something out of a science fiction movie.
    My personal favorite sound came from the power cord while the computer
    was asleep: it made a bizarre sequence of changing pitches that repeated
    every couple of seconds.

    The only problem is, all of these great interference-caused phantom
    sounds were almost drowned out by the actual normal sound produced by
    the fans, hard drive, etc. in the computer. Needless to say, the
    microphone was quite adept at recording these sounds.

    So my question is this: is it possible to build a device, or modify a
    microphone, so that it picks up ONLY the electromagnetic interference,
    but no actual sound?


    Thanks,
    Josh

    p.s.: I hope people don't mind that I'm not including my real email
    address. It's probably bad etiquette, but I'm kinda paranoid about spam.
     
  2. Ken Moffett

    Ken Moffett Guest

    Any way you can wrap the mic in a "sound proofing" material, so you damp
    out the acoustic pickup and leave the EMI sounds?
     
  3. Jasen Betts

    Jasen Betts Guest

    the reason for that is that the dynamic microphone has a voice coil (like in a
    loudspeaker) anf that coil picks up all the electromagneic interferance (EMI)
    inside your computer.
    You'll need to shield it against EMI. while not blocking too much sound
    that that won't be easy. basically you need to surround it with a conductive
    shell, electret mikes (like the "ctrappy lapel mike") are constructed that
    way.

    Bye.
    Jasen
     
  4. John Larkin

    John Larkin Guest


    Try something like this...

    |---------------
    ----------------+---///////-------+--------|
    | | |--------------
    K r |
    A | |
    | | |
    +-----------------+--------+

    where the antenna on the left is maybe 6-12" of wire, K-A is a small
    schottky diode, 1N5711 maybe, R is 100k, output is audio coax.

    Oh, ////// is a small inductor, 100 uH or something. This should pick
    up both baseband signals and detect RF. Drive a high-impedance audio
    input.

    John
     
  5. Rich Grise

    Rich Grise Guest

    .
    Sure! Just duct tape a piece of foam rubber over the part where the
    sound gets in. :)
    No, that's perfectly fine netiquitte, precisely because of spam, and not
    only that, but it's _bad_ netiquette to request private answers by email,
    (which you haven't done, so no worries :) ) because the purpose of USENET
    posts is to share with everybody. :)

    Cheers!
    Rich
     
  6. Harold Ryan

    Harold Ryan Guest

    Hi Josh:
    The high frequency noise is from the cmos technology switching between 1 and
    10 nanoseconds or 100MHz to 1 GHz. This noise passes thru the microphone
    cable and into the amplifier circuitry. At these high frequencies, the
    signal is rectified and what you hear is the peak detection of the noise.
    The only other noise is the switching power supply that creates a 1 MHz
    pulse every 20 to 40 useconds. Therefore, the best way to hear electronic
    noise with the existing amplifier is to just use a piece of wire about 2"
    long that connects the shied or ground wire to the signal wire. You don't
    even need the microphone. This is basically a UHF antenna.
    Enjoy
    Harold
     
  7. Guest

    An easy way, use an in-ear earphone, or other earphone. They've got a
    coil of wire inside to pick up EMI, and a jack plug on the end. Tho
    they're comparitively lo-impedance but, it's an experiment innit?

    Like others have said, put a bit of plasticine over the sound holes.
     
  8. An easy way, use an in-ear earphone, or other earphone. They've got a
    Using an earphone to capture EMI looks usefull to me, but I'm not sure I
    understood the principle. Please, what to do exactly with an earphone?
    Should I connect it to aditional power source (like batery) to drive the
    earphone, or no need? Should I, on the plug side of a cable, make something
    like antenna?

    Nikola
     
  9. Guest

    Just connect it to the mic socket on your tape recorder (or an
    amplifier or whatever), instead of a microphone.

    It's a bit lo-impedance to be ideal, but should work.
    No! Just plug an earphone into the mic socket. That's it!

    Altho, as an experiment, you could try the antenna. Just a piece of
    wire, connected to one of the contacts on the jack plug.

    I'd like to hear about your results.
     
  10. Ok, you'll know when I get result.

    Basicly, the point is to examine EMI in navigation devices on ships, becouse
    it can be the reason for some malfunctioning, sometimes. Using osciloscope
    or other measuring devices is hard (hard to cary it all the time when going
    to service). Once I saw a man wearing in-ear phones, probably driven with
    batery, and touching the electronic circuits with conductor (from
    earphones). He was using earphones instead of an osciloscope. So, what to do
    to use same principle to measure EMI?

    Nikola from Croatia
     
  11. GregS

    GregS Guest

    I thought I posted something, but here goes again.

    A telephone pickup coil is an inductive pickup.
    I think they still sell these.

    I listen to noise via a portable handheld amplifier, just
    an op-amp. I have a switch to either connect a coil or just a
    rod antenna. The gain is adjustable. Noises from magnetic vs
    electrostatic can sound very different. Magnetic also penetrate metal.
    Static magnetic fields can be heard when positioning the coil,
    but a Hall Effect sensor would be best at this.
    I still need to configure to measure other noises. I need
    to connect a photodiode to listen to light noise, 60 Hz etc. DC
    light should be free of noise. I also need to measure vibration. A
    vibration sensor will connect to bottom of handheld device, so
    when its sitting on a table, the noise can be heard. I also need
    to measure LF audio, as in ventillation noise and changes of
    less than 10 HZ. This is not just a usefull device I carry, its
    necessary!

    Therte is a product which is basically my electrostatic sensor
    It says inductive pickup, but I think its really just a rod.

    http://www.electrical-contractor.net/The_Store/EX/40180.htm



    greg
     
  12. Marc Noon

    Marc Noon Guest

    Here's a good link.

    http://www.logic1.com/rf-engineering/rfdetector/

    Also try and put a radio real close to your computer and see if it slows
    down any processes. Start at 100Mhz transmitters and work up.

    I guess the experiment would be like a counter for 10 second... see if it
    counts the same number after 10 seconds has elapsed. Then try to scale the
    distances.

    Marc
     
  13. Marc Noon

    Marc Noon Guest

    Here's a good link.

    http://www.logic1.com/rf-engineering/rfdetector/

    Also try and put a radio real close to your computer and see if it slows
    down any processes. Start at 100Mhz transmitters and work up.

    I guess the experiment would be like a counter for 10 second... see if it
    counts the same number after 10 seconds has elapsed. Then try to scale the
    distances.

    Marc
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-