200 Hz absorbent material

Discussion in 'Digital Audio' started by longofest, Dec 21, 2007.

  1. longofest Editor emeritus

    longofest

    Joined:
    Jul 10, 2003
    Location:
    Falls Church, VA
    #1
    So, looking for a needle in a haystack, and hoping there's an audio-engineer or RF engineer out there that has enough experience in this that can help (I've done some googling, but to no avail as of yet).

    I love my iPhone, but its GSM radio interference with audio devices irritates the hell out of me as a music and audio lover (avoiding saying "audiophile" here...).

    The interference is caused by power bursts that are at an approximate 200 Hz interval (one reference). I notice that when I put my hand around the antennae, most of the interference goes away, yet I only loose about 1 bar (granted, I'm in northern VA, so strong signal to begin with). I figure with 200 Hz in the audio spectrum, you should be able to use regular sound-proofing materials to weed out the 200 Hz interference from the iPhone. Of course, we'd want a material that is transparent to higher frequencies (the proper GSM frequencies).

    With that... anyone have any ideas? Foam is probably not a good solution, as I'd want this to be somewhat compact. How about different (specific) rubbers? What are the absorption qualities of them?
     
  2. balamw Moderator

    balamw

    Staff Member

    Joined:
    Aug 16, 2005
    Location:
    New England
    #2
    I'm an RF engineer (of sorts).

    You really can't do anything about this without encasing the iPhone in a Faraday cage, which would (of course) render it quite useless as a phone. As you expect, foam which can often work well for high frequencies would have to be feet thick to have an effect at these low frequencies. Since the 200 Hz is a beating -like byproduct of the pulsed 900 MHz signals, you are also not guaranteed that it isn't being generated by the presence of the high level 1-2 GHz signals in your audio device.

    All GSM devices do this to one degree or another, just keep your phone a bit farther from audio amplifiers, or shield them better. FWIW the signal drops off quite quickly so even moving the phone an inch or two away can often eliminate the interference. Also try reorienting the phone as the effect can be quite directional.

    EDIT: also FWIW modern cell phones (GSM and CDMA) use varying degrees of power control from the base station to the handset, so anything you do that induces any attenuation of the desires 900 or 1800 MHz signal (like putting your hand or foam around the antenna) will generally be offset by turning UP the signal in the phone to compensate for it, and will generally lead to shorter battery life.

    EDIT: Link to a previous discussion on this issue as it relates to GSM interference w/iPods. http://forums.macrumors.com/showthread.php?t=166491

    B
     
  3. QuarterSwede macrumors G3

    QuarterSwede

    Joined:
    Oct 1, 2005
    Location:
    Colorado Springs, CO
    #3
    What I can't believe is that the FCC (in the states) allowed GSM to be sold as is being that it interferes with all audio devices ... at least that I've had my phone near (all in house and car).
     
  4. Tarkovsky macrumors 6502

    Tarkovsky

    Joined:
    Jan 4, 2007
    Location:
    London/Norwich
    #4
    Interesting thread, I'm sure everyone on this forum has experienced that phenomenon. This really isn't my area of expertise but what about putting the amp in a Faraday cage?
    I think this link has some info on some to acoustically dampen at certain frequencies - http://www.ethanwiner.com/acoustics.html
     
  5. longofest thread starter Editor emeritus

    longofest

    Joined:
    Jul 10, 2003
    Location:
    Falls Church, VA
    #5
    Just to clarify... the 200 Hz is an EM wave, not an audio wave. I think I confused myself in my OP. So, 200 Hz audio absorbent material won't necessarily absorb 200 Hz EM waves.

    Appreciate the post balamw. However, like I said, using my hand can block the interference and not block too much of the other GSM signal. I gotta think there's other materials that can do similar. Distance isn't much of an option, as in a car scenario, it will cause interference anywhere in the car.
     
  6. balamw Moderator

    balamw

    Staff Member

    Joined:
    Aug 16, 2005
    Location:
    New England
    #6
    :eek: get better shielding for the amps for your car then. You probably have some ungrounded/unshielded wires (think antennas) and too much gain, so the signal can couple to the input of the amp.

    Read the link you provided again, it's a pretty decent explanation. The problem is that the other electronic equipment (where you hear the interference) contains nonlinear devices (usually an amplifier).

    These devices will create the signals you hear in the device, not in the phone. Since the effect is nonlinear it doesn't take much to reduce the effect. Typically if you can reduce the input signals by a factor of two, the products will be reduced by a factor of eight.

    This is likely what you are doing with your hand. cutting down the power the phone can put out by a little bit. (Note also that your signal strength indicator is NOT a good measure of the real effect your are having on the link since it only measures the base station to phone link and not the other way.)

    B
     
  7. gotzero macrumors 68040

    Joined:
    Jan 6, 2007
    Location:
    Mid-Atlantic, US
    #7
    I have video-shielded studio monitors which go crazy if I am talking on the iphone in my seat a few feet away.

    I gave in and got a headset...
     
  8. WinterMute Moderator emeritus

    WinterMute

    Joined:
    Jan 19, 2003
    Location:
    London, England
    #8
    Balanced cabling in your audio rig will sort out most RF problems, but it's simply not usual in domestic hi-fi and computer kit and you have to make a real effort in pro-audio if you want to balance the whole thing.

    take a look at "virtual earthing", that may help, however, only electrically balanced cabling and shielded components are gonna be immune.
     

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