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Geophone Audio recording advice

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  1. Mar 24, 2015 #1
    Hello,

    I am a music student and most of my work is concerned with field recording/ phonography. I am very keen to record the output of a geophone but I do not know how to connect/ convert the outputs for audio. I also know there is a similar thread to this but I don't think it's specifically answers the question I am interested in.

    I want to record infrasound and then move it in into the human register in post production. Artist/scientists like Felix Hess and Jez Riley French have worked with infrasound as source material before and the results are stunning.

    My problem is I don't know how to convert the geophone for audio use, nor do I know which geophones are most suitable for audio recording. I have emailed the manufacturers, but got no response.

    Any help hugely appreciated

    Will
     
  2. jcsd
  3. Mar 24, 2015 #2

    davenn

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    hi Will

    welcome to PF

    Geophones are sensitive to very low frequencies as they are designed to record earthquakes

    generally sensitive between 1Hz and 15 Hz depending on the model of geophone
    on this page of mine http://www.sydneystormcity.com/g_phones.htm
    and scroll right down to near the bottom you will see an example of a geophone
    with the text above it ..... "This is a view of the Geo-Space, 4.5Hz, 4000 Ohm coil, geophone seismometers"

    I use several geophones and some other sensors for earthquake recording
    There is a basic 3 stage Op-Amp amplifier circuit immediately above the geophone pic
    if you want to be able to hear the sound, you would have to use a frequency multiplier to bring up to the audio freq range

    Dave
     
  4. Mar 24, 2015 #3
    HI Dave,

    Thanks so much for getting back to me. I am a bit of a dunce with electronics, as I will now demonstrate...how does the 4 core cable connect to the outputs of the capsule? Is there a specific order positive/ negative etc. If I am using a single capsule will normal 2 core audio cable work? In terms of the pre amp, is this nessasary if I have a mid range pro-sumer audio recorder that comes with pre amps? I contacted an artist who works with geophones—Jez Riley French https://engravedglass.bandcamp.com/track/leigh-woods-geophone

    and he said this to me...

    The tricky thing is that to get the best results you have to open them up and re-attenuate. Geophones aren't built to output audio - they're measurement devices really so most 'off the shelf' ones will need amplifier circuits and shielding etc.

    Did you have to 're-attenuate'? If at all possible I would like to find the simplest solution, with the least soldering, possible—perhaps there are DIY amplifier kits I can buy? The other problem is that they would need to be rugged enough to take out into the field.

    Thanks you so much for your help! There's no reason for you to answer my silly questions but I am very grateful that you have!

    Best

    Will
     
  5. Mar 24, 2015 #4

    davenn

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    geophones, like the one on my site, as you can see have 2 connection pins.
    They are not polarity sensitive

    that makes no sense at all ??

    and opening one up will almost likely destroy it.
    Their internals consist of a finely balanced magnet within a coil(s)



    Dave
     
  6. Mar 25, 2015 #5
    OK, so i'll disregard his advice then. However, can I just check whether I need the pre-amp if I have a mid range audio recorder? And do the geophones need +48v phantom power? Sorry to keep coming back to you, I just really want to start implementing them into my recordings. Thank you so much for your help.
     
  7. Mar 25, 2015 #6

    davenn

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    depends on what the frequency response is like at 0 - say 20Hz ?
    if it's a normal audio preamp circuit, my educated guess would be that it's very insensitive
    it's pre-amp is more likely to cover the hearable audio range ~ 300Hz to ~ 10 kHz


    geophones don't need any power supply, they generate a very small voltage by the magnet moving within the coil ( similar to a microphone ... but where the magnet is fixed and the coil moves)

    Don't be sorry .... you are asking good questions and I am happy to help as much as I can :smile:

    cheers
    Dave
     
  8. Apr 2, 2015 #7
    Hi Dave,

    If I have a middle of the road Tascam portable recorder, we're probably safe to say that it's not optimised for infrasound. Iwas wondering where you got the preamp from on your webpage? Is it a kit? Or did you buy it?

    Thanks again for your help

    Will
     
  9. Apr 2, 2015 #8

    jim hardy

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    Will - Are you planning to tape record these signals with a magnetic tape deck ?

    Are you handy with electronics ?

    To record sub-audio infrasound signals is not something audio tape equipment will do well if at all.
    But it's a problem that was solved in 1950's by the guided missile guys out at White Sands, "Inter Range Instrument Group" or IRIG.
    They use the signal of interest to modulate the frequency of a carrier in the audio range which can be easily recorded on tape.
    A voltage to frequency converter is the gizmo that does that, and don't be scared by the term.
    To play back they play back the frequency modulated audio carrier into a frequency to voltage converter , thereby reproducing the original signal including its DC component.

    It's exactly like FM radio but at a few khz instead of 88 mhz.

    IRIG standardized their carrier frequencies, see
    http://www.spiraltechinc.com/otis/IRIG_files/Table6-7.htm
    excerpted from here
    http://www.spiraltechinc.com/otis/IRIG_files/IRIG_Chap6D.htm#Chap6D 6-10

    In the power plant we used a 32 channel tape recorder to capture in plant 60 hz voltages and currents, as well as a few DC signals for purposes of investigating upsets.
    Our tape machine ran at 1&7/8 inch per second, just like a home tape deck, but the tape was an inch wide.
    It used IRiG standard wideband FM group 1, per that table 6-7 link. Carrier was 6.75khz , which lets you capture and reproduce signals from DC to 1250 hz.

    Observe that if you record at 1&7/8 and play back at 3&3/4, you double the carrier frequency , fortunately to the next standard one.. And you also double the frequency of the original signals you've recorded, which is what Will wants to do - translate infrasound up to audio range.

    VCO and V to F circuits are in the Signetics PLL Applications book , here.
    https://archive.org/details/bitsavers_signeticsdcsPLLApplications_5800304

    A handy electronics hobbyist could build them from LM565 and LM567.
    I did it in 1973, but it's not a project for a beginner. I had to interface a peculiar signal to the tape machine so made my own VCO and wired it right to the recording head.
    Beauty of FM is you record at high level so don't need bias signal.


    Our tape equipment was eventually replaced by computers when they got small enough to fit in the room.

    So, frequency modulation of an audio carrier might be an approach for Will .
    http://www.sonoma.edu/users/m/marivani/datasheets/misc/LM565CN.pdf

    Hey - Les Paul added a third head to his Ampex.... this is lots easier.

    Dave - do you guys use FM in seismological recording?
    Will - does anybody use FM in recording music?
    If yes to either , there might be equipment around. I haven't seen a IRIG FM tape recorder since late 1980's.

    Just a thought.

    old jim
     
  10. Apr 3, 2015 #9

    davenn

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    Hi Jim

    no we don't, we are just dealing in a few Hz. The 4.5Hz geophones used in seismology, like my ones, and without any bandwidth extenders,
    operate from around 1 Hz to around 10Hz with pretty sharp dropoff either side of that

    the signal from the 'phone is amplified anything up to ~ 500 times and fed to the A to D converter

    Dave
     
  11. Apr 3, 2015 #10
    Hi Jim/ Dave,

    I'm not very handy with electronics to honest, I've made a few things but not to that level. I will be recording onto a Tascam digital recorder http://www.tascam.eu/en/dr-70d.html. I'm not too worried about hearing the infrasound 'live', I would pitch up the content until it's within human audibility and playback through a Sub woofer I have.

    If this is the case, do I still need the converter? Or can I just run an extreme filter and use equalisation to flatten the signal? It seems like a preamp to get good low end frequency register is the best solution? But i'm not sure which preamps are designed for this purpose?

    Will
     
  12. Apr 3, 2015 #11

    jim hardy

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    Will's Tascam recorder brochure says

    so there's a mission mismatch, it'll not do well in those octaves<20 hz .

    Sounds like Dave digitizes his signals with Analog to Digital Converter(ADC) instead of trying to treat them like audio.

    ADC turns the signal into a long list of numbers, which the computer stores.
    Digital to Analog Converter(DAC) makes that list back into a waveform.
    So it's in principle not any different from conversion to frequency and back.
    One can play the list back at any speed.

    Here's a paper by some guys using frequency modulation so their signals can be captured with a PC sound card
    http://metalab.uniten.edu.my/~farrukh/fuzzy/Low%20freq_papar.pdf\ [Broken]
    they used only 140hz carrier, by this interesting little IC that i'd never run across before
    http://www.ti.com/lit/ds/symlink/vfc32.pdf

    Honestly though , i'd recommend Dave's approach, a PC instead of a tape deck. There are ADC's and DAC's available for PC's.
     
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