Ambient sound, infrasound, magnetic fields, whistlers?

[cdev]

Every once in a while I wonder if it would be fun to put a transducer of some kind (the technical term for the professional ones is I think geophone) in a sealed container underground and monitor it for small earth movements. (Similarly for magnetic fields)

To see if it picked up earthquakes anywhere?

People do this for magnetic fields, the earth actually produces a lot of sounds in the low frequency realm.

I also frequently hear sounds that are just at the lower range of hearing (20-30 Hz-ish) and wonder what they are. Sounds that low in frequency are very hard (almost impossible) to locate with binaural hearing alone.

Also, because I kind of live in or perhaps on the fringes of a vast sound producing area the sounds that can be heard and perhaps monitored in the audible range are rich and complex.

One can definitely track the rise and fall of sound produced by traffic on the major highways. This time of year it begins long before dawn. Weekends the level of traffic is much lower.

Just as it is with RF, the higher up one lives (on a hill) the more and farther away are the sounds you hear.

[mark03]

Well which of the three do you want to pursue first?   (infrasound, magnetic fields, or 0-10 kHz RF where whistlers are found)  Don't forget ambient electric field; that's interesting too!

There are some great DIY infrasound resources over at techlib.com. 

The folks who do this professionally, like the test-ban treaty monitors, use fancy wind-noise reducers often consisting of large/complex PVC pipe structures, but apparently you can see lots of interesting activity in the near-infrasound, just below audible, without going to extreme lengths.  The transducer of choice has traditionally been a capacitance manometer (google MKS baratron).  I think eventually this paradigm may change as MEMS devices continue to improve.  Obnoxiously, mfgs don't provide a noise level in rms Pa/sqrt(Hz), but reading between the lines, I think we are somewhere below 1 Pa/sqrt(Hz) for a single sensor.  You could set up an array of 100s of these covering a square km or so, and by combining the signals get the wind filtering without the bother of a mechanical filter, and maybe ok noise performance. 

Just as it is with RF, the higher up one lives (on a hill) the more and farther away are the sounds you hear.

This is probably because in a normal thermally stratified atmosphere (not a temperature inversion), sound waves bend upward.  During an inversion, such as often forms in the predawn hours, it is sometimes possible to hear sounds from much further away.  One of my project ideas, regrettably still in the daydream phase, is to "transmit" bandlimited pseudonoise through a loudspeaker and see how far away it can be detected with a small microphone array and synchronized correlator.  There is enough ambient noise basically anywhere to dither the received signals, so ADC resolution and microphone self-noise are not hard limits; you can just correlate longer.  Atmospheric stability (coherence time) must be the limiting factor, but I do not know what are typical values.  In favorable conditions I can hear train horns from miles away, so similar range ought to be possible.

[cdev]

Thank you for this info!

 I suppose the difficult part of localizing where sounds come from if you have multiple microphones (or similar) is correlating two similar signals to extract the time difference of any given sound, so you can make a good guess as to what "angle" the sound is coming from.

Really, there are a hell of a lot of different kinds of challenges which go into doing something like tracking sounds, or signals, and attempting to locate its origins and make some guesses about that.

It makes us realize just how much work has gone into setting up large systems - where maybe one person has a setup to extract a given signal and then is correlating with other receivers. They all have to coordinate so that whatever they extract can be used together.

Kind of like what is being done for arms control, (not just nuclear arms, cities also have set up similar systems to locate gunfire by sound) And of course, also lightning with the blitzortung.org, wwll and I think some other 'sferic' signals lightning tracking nets.

Seeing videos of the nighttime Earth from space, at times parts of the Earth are just crackling with a huge amount of storm activity.

A lot of people would probably love to have nets they could plug into for learning more about electronics oriented things of this kind.

[cdev]

Actually I need to build a capacitance manometer.

I want to have ongoing monitoring of my heat recovery ventilator's state of balance (or imbalance). 

Its top two speed settings seem to be okay but the lowest one seems to be off a bit.

There are some great DIY infrasound resources over at http://techlib.com. 

Yes, thats a really great web site for simple but fun project ideas.

Seems like they tend to use very inexpensive common parts.

A bunch of projects from techlib would be a recipe for a lot of inexpensive fun.

I would love to build a bunch of the different projects there.

Not very complicated, but lots of bang for the buck, so to speak. Fun stuff with a common theme of seeing the unseen it seems.

For example, radiation detectors.. static electricity detection, field strength meters. Great ideas for science teachers.

Great ideas for parents with kids, too.

Everybody could benefit from seeing the unseen in new and different ways.

[coppercone2]

check out www.vlf.it

[StillTrying]

Reminds me of these infra sound recording experiments.
https://www.eevblog.com/forum/projects/infrasound-sensor-what-makes-a-narrowband-2-7-hz-signal/25/

I just used the scopes csv files as an 8 bit recording device, and played them back as wavs at x8 to x200 speeds.