Geophone Audio recording advice

[Will Parker]

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

 

[davenn]

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

 

[Will Parker]

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

 

[davenn]

...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?

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

Did you have to 're-attenuate'?

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

 

[Will Parker]

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.

 

[davenn]

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?

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

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.

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

cheers
Dave

 

[Will Parker]

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

 

[jim hardy]

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 let's 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

 

[davenn]

Dave - do you guys use FM in seismological recording?

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

 

[Will Parker]

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

 

[jim hardy]

operate from around 1 Hz to around 10Hz with pretty sharp dropoff either side of that

Will's Tascam recorder brochure says

Audio performance
Frequency response (LINE IN or MIC IN to LINE OUT, JEITA)
20 Hz – 20 kHz +0.5/−2 dB (Fs 44.1/48 kHz)
20 Hz – 20 kHz +0.5/−3 dB (Fs 96 kHz)

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\
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.