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Audible VLF

  1. Dec 10, 2014 #1
    USDOC Radio Frequency allocation table (http://www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf)
    lists part of the radio spectrum as "Audible." Radio waves are electromagnetic (e.g. light), while audible waves are pressure waves in a medium (e.g. air.) They travel at very different speeds, and are very different things. We see one and hear the other (depending on the frequency.) An audible RF signal makes absolutely no sense to me. I wouldn't think a government document would get basic science so blatantly wrong, so i think i may be missing something here. If I Google "Audible VLF", there are a lot of hits on the subject. They seem to talk about something like receiving radio frequencies from lightning strikes, and playing them as audio. Or hearing buzzing in electrical lines. Neither scenario i would interpret as being an audible radio wave.

    What do you suppose the allocation chart is trying to say? Is it just wrong? Or perhaps correct, but misleading? Or am i completely missing something?
     
  2. jcsd
  3. Dec 11, 2014 #2

    Bystander

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    It's just EM at frequencies from 10 -10kHz, plus or minus whatever definition you choose for audio frequencies. Different media obviously.
     
  4. Dec 11, 2014 #3
    Agreed. An oscillator producing an EM frequency in that range could be heard audibly if you connected to a speaker. The speaker just does the translating from EM to waves of pressure by design. VLF signals are pretty easy to translate to audio since they occupy those frequencies already. I've heard of some folks just taking a really long wire and plugging it into the mic port on a laptop. If you crank the gain up high enough, you essentially create a basic VLF receiver. Ideally, you'd want around 150 meter-long wire for VLF though, but you could probably pick up some interesting sounds with something shorter around dawn or dusk if you're outside away from heavy EMI. HF bands and below tend to be pretty noisy in crowded areas these days.
     
  5. Dec 11, 2014 #4

    davenn

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    Yup a lot of fun listening to various atmospherics ... Dawn Chorus, Whistlers etc

    Dave
     
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