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Neky Lyk - 25 Mar 2006 20:08 GMT

i stumbled upon this little piggy while looking for reading material. i've
been reading about infrasound and it's use lately and i was quite intrigued
by this sensors design; it has no microphone. the name alone got my
attention: (O)ptical (F)iber (I)nfrasound (S)ensor.; OFIS.
here's the link explaining the design and tests:
http://klops.geophys.uni-stuttgart.de/~widmer/JASAfinal.pdf

i know it's probably old news to some of you but i want to know your opinion
about it, maybe some potential problems, approx. cost etc.
and i just noticed there's a public lecture about Comprehensive
Nuclear-Test-Ban Treaty on my university :)
i think i will attend it, there's gonna be a discussion about detection of
nuclear tests, perhaps they'll talk about using new infrasound developments
for detection purposes.

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Angelo Campanella - 26 Mar 2006 19:21 GMT

> i stumbled upon this little piggy while looking for reading material. i've
> been reading about infrasound and it's use lately and i was quite intrigued
> by this sensors design; it has no microphone.

Not quite so. Conventional microphones do not work or do not work well
for infrasound measuremant, due to the "audio" roll off at frequencies
below 20 Hz. From about 1000 seconds per cycle (.oo1Hz) to 20 Hz is
where all the action is in infrasound. At such low frequencies, the
wavelength being so long, the microphone does not have to stand alone
outside, but rather can be in a well or a pit, or a remote cabin for
that matter. The "microphone" in this case can be something as simple as
a tin can with mylar streched over its mouth. A light beam reflected off
that film, aimed at a knife edge, photo sensor beyond... well you get
the picture.

> The name alone got my
> attention: (O)ptical (F)iber (I)nfrasound (S)ensor.; OFIS.
> here's the link explaining the design and tests:
> http://klops.geophys.uni-stuttgart.de/~widmer/JASAfinal.pdf

I'm sure thsoe folks have some neat home-made (lab-made) infrasound
sensords of this sort.

> i know it's probably old news to some of you but i want to know your opinion
> about it, maybe some potential problems, approx. cost etc.
> and i just noticed there's a public lecture about Comprehensive
> Nuclear-Test-Ban Treaty on my university :)

A nuclear blast sends a huge pressure pulse into the atmosphere that can
be sensed hundreds of miles away. An array of such infrasound sensors
(several on a line hundreds of feet long aimed in specific directions)
can sense said pressure pulse with some directivity. It's all just basic
acoustical physics.

> i think i will attend it, there's gonna be a discussion about detection of
> nuclear tests, perhaps they'll talk about using new infrasound developments
> for detection purposes.

The developments are not so new. This work has been going on for some
years, but attending such meeings is a good way to catch up, if you are
interested.

Angelo Campanella

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Neky Lyk - 27 Mar 2006 22:53 GMT

> > i stumbled upon this little piggy while looking for reading material. i've
> > been reading about infrasound and it's use lately and i was quite intrigued
[quoted text clipped - 10 lines]
> that film, aimed at a knife edge, photo sensor beyond... well you get
> the picture.

i meant no standard microphone. Bruel&Kjaer type 4193 has a "standard"
capsule if i'm not mistaken. the fact that gravel and materials like that
pose small or no barrier for infrasound is why they put it in a ditch.

> > The name alone got my
> > attention: (O)ptical (F)iber (I)nfrasound (S)ensor.; OFIS.
[quoted text clipped - 3 lines]
> I'm sure thsoe folks have some neat home-made (lab-made) infrasound
> sensords of this sort.

it's under research from what i read. as far as i know the army is financing
it.

> > i know it's probably old news to some of you but i want to know your opinion
> > about it, maybe some potential problems, approx. cost etc.
[quoted text clipped - 6 lines]
> can sense said pressure pulse with some directivity. It's all just basic
> acoustical physics.

i like to think i know basic acoustical physics, otherwise i wouldn't be
interested in infrasound :)
anyway, i think the main problem is the frequency/directional characteristic
thing. on lower frequencies, e.g.1Hz, you would need a 340 meter tube(an
array of them!) to have a usable directional charateristic(for location
purposes). and you need your freq. band to be as wide as it can get(spectral
source recognition) but the sensors size isn't exactly unlimited, so thats a
problem. now when you multiply all that meters of fiber-optic with their
price per meter, adding a quality interferometer and other expenses you get
quite a sum. that's another problem. obviously someone is willing to finance
that. i'm glad, researchers get to work, to inovate... just not for
everyone's pocket :)

> > i think i will attend it, there's gonna be a discussion about detection of
> > nuclear tests, perhaps they'll talk about using new infrasound developments
[quoted text clipped - 5 lines]
>
> Angelo Campanella

i sure wanna catch up :) talking of which: can you recommend some e-books or
literature on infrasound? it's really rare/hard to get in my country...

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Angelo Campanella - 26 Mar 2006 19:26 GMT

> attention: (O)ptical (F)iber (I)nfrasound (S)ensor.; OFIS.
> here's the link explaining the design and tests:
> http://klops.geophys.uni-stuttgart.de/~widmer/JASAfinal.pdf

I recall reading that paper... It works by changing the transmissivity
(altering internal refelction parmeters) of the very long optical fiber
via stretching or compression when the external atmospheric pressure
changes. A clever way of get mechanical amplification, relatively
noise-free...

Angelo Campanella

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Neky Lyk - 27 Mar 2006 22:55 GMT

> > attention: (O)ptical (F)iber (I)nfrasound (S)ensor.; OFIS.
> > here's the link explaining the design and tests:
[quoted text clipped - 7 lines]
>
> Angelo Campanella

i agree, very clever. thats why i'm impressed. just not sure about the
thermal influence on the fiber/tube.

Reply to this Message

Neky Lyk - 28 Mar 2006 02:42 GMT

"Angelo Campanella" <a.campanella@att.net> wrote in messag
news:A1BVf.30948$bn3.8699@bgtnsc04-news.ops.worldnet.att.net..

> Neky Lyk wrote
>
[quoted text clipped - 3 lines]
>
> I recall reading that paper... It works by changing th

transmissivit

> (altering internal refelction parmeters) of the very long optica

fibe

> via stretching or compression when the external atmospheric pressur
> changes. A clever way of get mechanical amplification, relativel
> noise-free..
>
> Angelo Campanell

i agree, very clever. thats why i'm impressed. just not sure about th
thermal influence on the fiber/tube

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Natural Science Forum / Physics / Acoustics / March 2006

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