Author Topic: Infrasound sensor - what makes a narrowband 2.7 Hz signal?  (Read 7402 times)

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Offline JBealeTopic starter

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Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« on: March 27, 2018, 03:10:07 pm »
To measure low-frequency sound I made a crude differential pressure sensor using an electret mic sealed into a tube with separate 1/4" hose barbs connecting to each side of it (photo below).  I'm using a simple x100 amp going to a Teensy 3.2 for the ADC, then into a PC and plotting the spectrum with Audacity.  This system records infrasound, or pressure variations down to around 1 Hz.  I have it on my bench indoors so the largest signal is always 17 Hz + broadband noise whenever the heater fan (HVAC system) comes on, and big vertical spikes when a door swings open or closed.  During the night when it is quieter I also see different near-17 Hz signals from other fans in several neighboring houses.

Usually there is very little visible at lower frequencies except broadband wind noise, but after a week of recording, this morning I saw a very narrowband signal wandering from 2.6 to 2.7 Hz. I assume it must be from some machine, but I wonder what. The spectrum plot is from a 10x resample so the real signal is 1/10 the displayed frequency and 10x the indicated time (looks like 27 Hz and 12 minutes but the actual 2.7 Hz signal was mostly present over about 120 minutes; around 4 am to 6 am local time). The vertical bands show times that the heater fan was running, and you see the background noise (likely from wind) increasing later in the morning.
« Last Edit: March 27, 2018, 03:31:47 pm by JBeale »
 
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Offline tpowell1830

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #1 on: March 27, 2018, 04:26:52 pm »
You do not have a flag to show where you are from, however, if you are in the US, this could be a bull alligator. The alligator mating call is very low frequency and very intense.
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Offline principia

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #2 on: March 27, 2018, 04:45:34 pm »
Are you near any wind turbines/farms?
 

Offline KD4PBS

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #3 on: March 27, 2018, 05:06:27 pm »
Aliens.
Or Mr & Mrs NextDoorNeighbor going at it quite happily for a long time, and you're picking up the results. 
Does your neighbor's wife always walk around with a huge smile?
 
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Offline JBealeTopic starter

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #4 on: March 27, 2018, 06:41:10 pm »
This was in San Jose CA. There is the Altamont Pass wind farm but that's over 30 miles away and I would expect such a signal to be more continuous, not a 2-hour stretch only once in a week, at 4 am.

But now that I think about it, there's the Ames Research Center which has the world's largest wind tunnel in Mountain View, about 10 miles away. I think I read it can generate low-frequency sound.
https://en.wikipedia.org/wiki/Unitary_Plan_Wind_Tunnel_(Mountain_View,_California)
 

Offline HighVoltage

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #5 on: March 27, 2018, 06:45:22 pm »
There is actually a really nice Android app, called "Infrasound"
You may use that to confirm your findings.
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Offline Tom45

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #6 on: March 27, 2018, 07:01:38 pm »
But now that I think about it, there's the Ames Research Center which has the world's largest wind tunnel in Mountain View, about 10 miles away. I think I read it can generate low-frequency sound.
https://en.wikipedia.org/wiki/Unitary_Plan_Wind_Tunnel_(Mountain_View,_California)

The wind tunnel seems a likely candidate.

Back in the 60s when I was in college a prof said that Boeing could only run their wind tunnel  in the middle of the night because the grid didn't have the capacity to supply both the wind tunnel and Seattle at the same time.
 

Offline richnormand

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #7 on: March 27, 2018, 07:08:17 pm »

But now that I think about it, there's the Ames Research Center which has the world's largest wind tunnel in Mountain View, about 10 miles away. I think I read it can generate low-frequency sound.


Have you tried to contact them and ask?
I once sent an email to the HARP facility and the associated university in Alaska and they kindly responded with their planned schedule and frequencies.  :)


Many times we ran our own wind tunnel at set times (off peak in the middle of the night) at the request of the power company with a substantial cost savings.
Infrasounds can indeed travel very large distances.

Of possible interest:
http://www.techlib.com/area_50/infrasound.htm
« Last Edit: March 27, 2018, 07:19:51 pm by richnormand »
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Offline JBealeTopic starter

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #8 on: March 27, 2018, 07:28:06 pm »
I was looking for an online schedule of operation for the wind tunnel and didn't see anything, I'll see if I can figure out how to contact somone there.

Thanks HV for letting me know about the Android infrasound app and it is very cool! Although it doesn't seem to record much below 5 Hz even with some strong signals; I'm not surprised as otherwise the phone wouldn't work outside with any wind.
« Last Edit: March 27, 2018, 07:30:16 pm by JBeale »
 

Offline Benta

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #9 on: March 27, 2018, 07:32:49 pm »
Police helicopter hovering in the area?
 

Offline jmelson

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #10 on: March 27, 2018, 09:26:42 pm »
Any (freight) railroads nearby?  I recall some locomotives can generate some noise about that frequency, either from the Diesel engines or the wheels hitting the joints.  I think I've heard some locomotive make a whump-whump-whump pretty close to 3 Hz when at a particular throttle setting.

Jon
 

Online edpalmer42

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #11 on: March 27, 2018, 09:33:38 pm »
Another way to think of 2.6 to 2.7 Hz is 160 rpm.  When I google for "160 rpm" I find a ridiculous number of hits, but nothing jumps out at me.

Ed
 

Offline JBealeTopic starter

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #12 on: March 27, 2018, 09:34:09 pm »
The police helicopter is not a bad guess as those probably do come by weekly. However they make a very distinct FM-sinewave around 20 Hz +/- some Hz from doppler as they orbit around. You can see an example at http://www.techlib.com/area_50/infrasound.htm

If there's a helicopter out there with a 2.6 Hz rotor signal, it's a pretty fearsome beast.  But actually I recall an experimental "gossamer condor" style man-powered helicopter trial that may have had such a rotor speed- so fragile it would only work in a dead calm, and it certainly wouldn't be running steady for over an hour.

Hmm, freight train sounds plausible if it was a shorter duration. There are some such around here, but I doubt over an hour long. On the other hand I'm not up to listen at 5 am, and if they ever did run a train that long I guess that's when it would be.
« Last Edit: March 27, 2018, 09:41:00 pm by JBeale »
 

Offline stj

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #13 on: March 29, 2018, 01:41:47 am »
2.6Hz could be from a big V8 idling.
anybody in your street got a muscle car??
 

Offline CopperCone

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #14 on: March 29, 2018, 01:49:49 am »
your neighbor has a bull alligator pit
 

Offline JBealeTopic starter

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #15 on: March 29, 2018, 05:24:07 am »
2.7 Hz seems awful low to me for any car engine. Maybe a steam engine? I've seen a lot of car engine noises in other recordings but starting much higher in frequency, and they always show harmonic overtones as well. I was zoomed into the spectrogram so in the image I posted you don't see above it, but apart from a slight drift, this frequency is a pure note with no harmonics within the passband up to 50 Hz. It is like a super-bass flute- which may describe the NASA-Ames wind tunnel mentioned above.
 

Offline richnormand

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #16 on: March 29, 2018, 08:18:22 pm »
It is like a super-bass flute- which may describe the NASA-Ames wind tunnel mentioned above.

Interesting thought. One could compute the length of a pipe organ to give this frequency and compare it to the NASA photo in Wikipedia or Google for scale.
Could be 1/4 or 1/2 wave also. Depending on air pressure inside the speed of sound in the wind tunnel could be altered a bit too.
Still, you should be within the right magnitude.

Using 343m/s at TPS as the speed of sound, 2.7 Hz gives me 127m wavelength. (1/2= 63.5 and 1/4=31.7m ). Any pipes, tunnels, elevator shaft or chimneys of that size around that could get exited by wind in the right direction and the right speed?

Would also be nice if you had something like a Geophone (seismometer, it can be tuned to go lower than specs for 2. to 4.5 Hz with some mods) and see if there is a corresponding signal from the ground. I am thinking of the famous Windsor (Canada) hum here, possibly coming from a plant on the US side.
« Last Edit: March 29, 2018, 09:24:51 pm by richnormand »
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Offline JBealeTopic starter

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #17 on: March 29, 2018, 11:17:18 pm »
That's a good point. As it happens I am also running the "Raspberry Shake" home seismometer, based on a 4.5 Hz geophone but it is used in an actively-driven circuit which extends the usable lower range below 1 Hz.  I see all sorts of broadband seismic noise from traffic etc. in that general frequency range, and various known rotating machinery signatures (clothes washer, dishwasher, etc during the day) but nothing at all like this narrowband pure-tone signal.
 

Offline richnormand

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #18 on: April 07, 2018, 09:08:49 pm »
Any progress at identifying the source?
Has it showed up again?

Inquiring minds want to know :scared:

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Offline Neomys Sapiens

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #19 on: April 07, 2018, 10:52:47 pm »
Would be interesting to look at the electromagnetical 'baseband' <1kHz and also above for any corresponding activity.
 

Offline StillTrying

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #20 on: April 08, 2018, 02:16:43 pm »
2.7Hz.
Are you within 2 or 3 miles of big ships.
.  That took much longer than I thought it would.
 

Offline JBealeTopic starter

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #21 on: April 09, 2018, 02:15:04 pm »
I've only been logging data on and off since then as I've been playing with the circuit, but so far haven't seen it again. No big ships in the immediate area. Electromagnetic interference is a good point, this is a high-gain amp and not well shielded.  If there was much of that going on though, I'd expect to see it much more during the day when more things are on.  Eventually I'll get the circuit packaged up and running 24x7 and maybe I'll know more at that point.

EDIT: Now that I think about it, there were 2 laptops running in that room, both with a spinning hard drive. Either one might have decided 4am was a good time to start some disk optimization program that might take hours, and have a 2.7 Hz seek pattern.  At some point I'll put the sensor in a shed detached from the house with no moving parts around, and see what happens then.
« Last Edit: April 09, 2018, 03:47:23 pm by JBeale »
 

Offline JBealeTopic starter

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #22 on: April 09, 2018, 05:51:38 pm »
In case of interest, I changed to an actual pressure sensor instead of an electret mic. The amplifier circuit is this: http://www.bealecorner.com/pcb/is1/  basically from a strain-gauge example on a Linear Tech datasheet. Not shown is a second-stage x20 amp before going into the ADC on a Teensy 3.2.

I saw a huge, persisting ~1 Hz signal yesterday when I was playing with it, and figured I must have triggered some oscillation. Then I walked down the hall and found out one of the kids' friends had come over and picked that moment to start bouncing up and down on a yoga ball at the other end of the house.  So I tracked that one down, anyway.  :)

EDIT: 12 bits of effective resolution wasn't so great, so I changed to one of the ADS1256 (TI 24-bit ADC) boards you can get premade from ebay.  That is giving plenty of range, at 100 Hz sample rate you can get almost the full 24 bits.
« Last Edit: April 20, 2018, 05:12:44 pm by JBeale »
 

Offline Twoflower

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #23 on: April 09, 2018, 07:01:25 pm »
Not sure it it helps. Have you thought to make it audible? For example play a recorded specimen at 100x the original speed? Maybe you can pick some information out of it.
 
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Offline StillTrying

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Re: Infrasound sensor - what makes a narrowband 2.7 Hz signal?
« Reply #24 on: April 09, 2018, 07:02:54 pm »
I'm going to have a go to see what I can pick up 0.2Hz - 20Hz with an electret, it will make a change from all the times I've tried to stop them responding to those frequencies!

http://www.openmusiclabs.com/learning/sensors/electret-microphones/index.html
.  That took much longer than I thought it would.
 


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