Infrasound sensor - what makes a narrowband 2.7 Hz signal? - Page 1
Forum section: electronics / projects

author: JBeale
author_rank: Frequent Contributor
timestamp: March 27, 2018, 03:10:07 pm
content: 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.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: March 27, 2018, 04:26:52 pm
content: 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.

author: principia
author_rank: Newbie
timestamp: March 27, 2018, 04:45:34 pm
content: Are you near any wind turbines/farms?

author: KD4PBS
author_rank: Contributor
timestamp: March 27, 2018, 05:06:27 pm
content: 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?

author: JBeale
author_rank: Frequent Contributor
timestamp: March 27, 2018, 06:41:10 pm
content: 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)

author: HighVoltage
author_rank: Super Contributor
timestamp: March 27, 2018, 06:45:22 pm
content: There is actually a really nice Android app, called "Infrasound"You may use that to confirm your findings.

author: Tom45
author_rank: Frequent Contributor
timestamp: March 27, 2018, 07:01:38 pm
content: Quote from: JBeale on March 27, 2018, 06:41:10 pmThe 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.

author: richnormand
author_rank: Unknown
timestamp: March 27, 2018, 07:08:17 pm
content: Quote from: JBeale on March 27, 2018, 06:41:10 pmHave 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

author: JBeale
author_rank: Frequent Contributor
timestamp: March 27, 2018, 07:28:06 pm
content: 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.

author: Benta
author_rank: Super Contributor
timestamp: March 27, 2018, 07:32:49 pm
content: Police helicopter hovering in the area?

author: jmelson
author_rank: Super Contributor
timestamp: March 27, 2018, 09:26:42 pm
content: 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

author: edpalmer42
author_rank: Super Contributor
timestamp: March 27, 2018, 09:33:38 pm
content: 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

author: JBeale
author_rank: Frequent Contributor
timestamp: March 27, 2018, 09:34:09 pm
content: 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 athttp://www.techlib.com/area_50/infrasound.htmIf 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.

author: stj
author_rank: Super Contributor
timestamp: March 29, 2018, 01:41:47 am
content: 2.6Hz could be from a big V8 idling.anybody in your street got a muscle car??

author: CopperCone
author_rank: Super Contributor
timestamp: March 29, 2018, 01:49:49 am
content: your neighbor has a bull alligator pit

author: JBeale
author_rank: Frequent Contributor
timestamp: March 29, 2018, 05:24:07 am
content: 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.

author: richnormand
author_rank: Unknown
timestamp: March 29, 2018, 08:18:22 pm
content: Quote from: JBeale on March 29, 2018, 05:24:07 amInteresting 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.

author: JBeale
author_rank: Frequent Contributor
timestamp: March 29, 2018, 11:17:18 pm
content: 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.

author: richnormand
author_rank: Unknown
timestamp: April 07, 2018, 09:08:49 pm
content: Any progress at identifying the source?Has it showed up again?Inquiring minds want to know

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 07, 2018, 10:52:47 pm
content: Would be interesting to look at the electromagnetical 'baseband' <1kHz and also above for any corresponding activity.

author: StillTrying
author_rank: Super Contributor
timestamp: April 08, 2018, 02:16:43 pm
content: 2.7Hz.Are you within 2 or 3 miles of big ships.

author: JBeale
author_rank: Frequent Contributor
timestamp: April 09, 2018, 02:15:04 pm
content: 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.

author: JBeale
author_rank: Frequent Contributor
timestamp: April 09, 2018, 05:51:38 pm
content: 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.

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 09, 2018, 07:01:25 pm
content: 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.

author: StillTrying
author_rank: Super Contributor
timestamp: April 09, 2018, 07:02:54 pm
content: 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

author: JBeale
author_rank: Frequent Contributor
timestamp: April 09, 2018, 08:48:40 pm
content: >http://www.openmusiclabs.com/learning/sensors/electret-microphones/index.htmlThanks for that link, it's a very good explanation of the electret capsule. I didn't realize they were that simple inside. The model I was using, I think it was a "Primo EM182" is about 10mm diameter and claims higher SNR than most other capsules; maybe due to a larger diaphragm and and/or better JFET (?)The experiment could be fun! I can almost guarantee you can detect a door opening from quite some distance away, more than you would expect just by listening with your ear anyway. That's on a day with calm weather. If it's windy outside (and/or if you have HVAC running) mostly you just pick up pressure fluctuations from that. Otherwise I'd expect this type of sensor would be available in burglar alarms, because when conditions are quiet you can really pick up motion from all over the house.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 10, 2018, 01:15:27 pm
content: A fun extension of your project would be to set up two or more of your sensors as a phased array. You could then get direction information which would help in identifying the source.

author: StillTrying
author_rank: Super Contributor
timestamp: April 10, 2018, 01:58:03 pm
content: Quote from: CatalinaWOW on April 10, 2018, 01:15:27 pmAt these VLFs you'd need quite a bit of space."Arrays are also used to estimate the direction of arrival of a wave and its propagation velocity. Typical sizes, i.e., apertures, are in the order of 100 to 1000 meters."https://www.knmi.nl/kennis-en-datacentrum/achtergrond/infrasound-and-seismology-in-the-low-frequency-array-lofar

author: JBeale
author_rank: Frequent Contributor
timestamp: April 10, 2018, 02:42:29 pm
content: I definitely have thought about multiple sensors for a direction-finding array. The spacing required is exactly the problem. Doable if you've got a big farm, etc. but for us suburban folks, it means you need to get some friends living some distance away also involved. Maybe I could interest someone at the kids' school for a science project.There is actually a kickstarter running now with exactly what you would need, a networked infrasound sensor. If interested check out "Raspberry Boom". It is from a group "OSOP" and I have their previous kickstarter item, a home seismograph which does locate earthquakes using a network of sensors. How feasible that is with infrasound, I do not know.

author: CopperCone
author_rank: Super Contributor
timestamp: April 10, 2018, 03:00:23 pm
content: m.youtube.com/watch?v=IWR3dc7kkAA

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 11, 2018, 03:47:30 am
content: Space requirement depends on accuracy required. Think of this in the time domain. Speed of sound is around 330 meters/sec. If your sensors are 10 meters apart (should be possible in many urban situations), the distance difference is approximately sin(theta)*10 where theta is the angle between the source of the sound and the perpendicular to your sensor baseline. Or 1.7 meters for a 10 degree offset. So if you can resolve 1.7/330 second (or 5 millisecond) time of arrival differences you can tell angle to roughly ten degrees. For nice sinusoids of the type you are hunting measuring the phase will probably be easier than measuring time of arrival, but the basic concept is the same.There are issues, but there are advantages to a short baseline. Such as no ambiguity due to full cycle phase shifts from large angular offsets. There are lots of other things to consider. Multi-path being probably the worst.

author: CopperCone
author_rank: Super Contributor
timestamp: April 11, 2018, 02:51:23 pm
content: Do you have any examples of such a low frequency phased array system?I am interested in building one.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 11, 2018, 04:47:49 pm
content: I don't though I am sure that the nation's who fear submarines have built them. The people playing these kind of games don't publish much.

author: texaspyro
author_rank: Super Contributor
timestamp: April 12, 2018, 03:27:34 am
content: google: infrasound smu

author: StillTrying
author_rank: Super Contributor
timestamp: April 21, 2018, 02:14:11 am
content: A couple of rough experiments...Even in doors in very quiet conditions the air always seems very noisy ~0.5 - 20Hz, so noisy that I wired up a bare electretYellow, to compare with the 0.2Hz<80Hz X 50 amp onePurple.There's audio, - TV news on theYellowone, you can see how large the LF noise is compared with the audio.The .wav is 2 X 1 sec. of 8-bit samples of the randomness, collected off the scopes .csvs and converted to a mono 8 bit .wav, playing at 8 X speed. Experiments continue...

author: David Hess
author_rank: Super Contributor
timestamp: April 21, 2018, 02:56:44 am
content: Usually when I see something like this, it is caused by either aliasing, mixing of two much higher frequency signals, or AM detection. EMI protection should be used at every susceptible analog input and output. The various cellular communication and wireless networking standards which use TDMA are particularly troublesome because the detected AM shows up as DC offset shifts.What was the sample rate and what kind of filtering was used?

author: JBeale
author_rank: Frequent Contributor
timestamp: April 23, 2018, 04:50:03 pm
content: Thanks for the scope plot; interesting to see. If the yellow trace is simply a bare electret going direct to the scope, with no low-pass filters, I think it really is air pressure because I would expect RFI to look more bursty. If you directly pickup RF from wifi, cell phones etc. you should see very short vertical spikes during the brief transmissions. If there is even the slightest stir of wind outside, it can cause low-frequency pressure changes indoors like this. In my area the winds die down after sunset and there is very little wind noise from midnight - 6 am.You could prove it by enclosing the electret in some kind of a sealed, airtight jar.

author: Kalvin
author_rank: Super Contributor
timestamp: April 23, 2018, 05:56:14 pm
content: If you want to make signal audible in real-time, you could try AM modulating for example 1000 Hz carrier with the signal. Alternatively, you could FM modulate the carrier. FM modulation can be achieved quite easily with a MCU by creating a sine lookup table (for example 1000 values) and make the signal-to-be -detected adjust the index step size. Of course you can use any other more sophisticated spectrum shifting algorithms.

author: StillTrying
author_rank: Super Contributor
timestamp: April 24, 2018, 12:49:21 am
content: "What was the sample rate and what kind of filtering was used?"I've just got an op amp on a bread board, it's definitely not very sophisticated!"You could prove it by enclosing the electret in some kind of a sealed, airtight jar."It took me 4 tries to get the electret completely air tight sealed into a small glass salt cellar with a ball of blutack in the neck,purpletrace.The secondpurpletrace is with a very small air leak in the blutack - the electret still at the bottom of the glass.After seeing the quiet trace in the sealed salt cellar, I think a lot of the mains hum is actually audio through the air, from the TV/PC/Lights."If you want to make signal audible in real-time"I'll try adding some sine to the wav files, but the sample rates are usually quite low 100 - 5000 Sa/s.

author: JBeale
author_rank: Frequent Contributor
timestamp: April 24, 2018, 10:48:33 pm
content: Nice work. Sure enough, from that experiment it seems pretty clear you are mostly picking up actual sound (or infrasound) with the bare electret. That was part of my interest in fact- what kinds of sounds can I pick up, which we don't normally notice?

author: StillTrying
author_rank: Super Contributor
timestamp: April 25, 2018, 05:10:05 pm
content: Longish post got 502'd"what kinds of sounds can I pick up, which we don't normally notice?"Any interest in trying to make the infra sound real-time hear-able?I tried just adding/mixing some 400-600 Hz at amplitudes above and below that of the infra sound, that did make some of the VLF changes hear-able.I tried fully modulating the amplitude of 400-600 Hz with the VLF, that was better at making the VLF changes hear-able. I can't figure how to simply do FM.These first 2 were very tiring to listen to for the VLF changes, even for just a minute or two.I tried just squaring up the VLF comparator-like, that seems to be the best so far in making the VLF changes audible.The squared up version sounds very much like a buzzing bad connection, I think this version is worth a few more lines of C, - a 50 sample running average to accurately follow the the mid-level, and at least some variation to the square's height/volume based on the actual current p-p height of the VLF. The running average is easy enough, I'm still thinking on thereached peakdetection. I'll post an audio samplewhenif I get anywhere.

author: CopperCone
author_rank: Super Contributor
timestamp: April 25, 2018, 06:59:16 pm
content: post the raw data im sure some people know exactly what to do

author: JBeale
author_rank: Frequent Contributor
timestamp: April 25, 2018, 10:19:34 pm
content: Interesting ideas with the FM modulation. All I had done before with audio was simply a direct speed-up, for example with playback at 100x speed I can identify some infrasound signals. Other than that I was just using spectrograms, which make really narrowband features easier to pick out, but for any more complex things I'd agree your ear is likely better than your eye.

author: Gregg
author_rank: Super Contributor
timestamp: April 26, 2018, 02:17:12 am
content: It could be paving equipment. Caltrans does a lot of work at night; they usually try to finish by 5 AM, but will go to 6 if it has to be done.

author: StillTrying
author_rank: Super Contributor
timestamp: April 26, 2018, 02:20:35 am
content: Here's a few seconds of the squared infra sound, it might sound awful but it certainly makes the silent LF very audible at X1 speed. I'd still like to get the squared amplitude to increase when a 'loud' infra sound comes along.Here's the lines that make some adjustment to the center 127/128 position of the input wave 15 samples ahead of the squaring comparator, ...there's probably a better way.Code:const char Hyst=3; // hysteresisfloat Center = 127.5;for (i = 1; i<NumSamples; i++){if ( ADCSample[i+15] > Center ) Center += 0.08;if ( ADCSample[i+15] < Center ) Center -= 0.08;if ( ADCSample > (Center+Hyst) ) ADCSample = 127 + 85; // set o/p highelse if ( ADCSample < (Center-Hyst)) ADCSample = 127 - 85; // set o/p lowelse ADCSample = ADCSample[i-1]; // no change}Edit: font=courier goes italic for some reason.

author: StillTrying
author_rank: Super Contributor
timestamp: May 05, 2018, 03:43:58 pm
content: Anyone still interested in infra sound experiments.Using the VLF to FM modulate a 600Hz sine wave certainly makes the VLF wobbles hear-able at X1 speed.

author: JBeale
author_rank: Frequent Contributor
timestamp: May 06, 2018, 06:54:40 am
content: FWIW, I've most recently been using the enclosed circuit (PDF), which goes from low-audio right down to DC (no series caps in the signal path). This shows the sustained increase in pressure in the room when the HVAC blower comes on, for example. Interestingly it does not go back to normal right away when the fan stops; maybe thermally driven air currents? I don't think it's purely a sensor temperature dependence, as the room temperature variation has a still longer time constant.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: May 11, 2018, 03:01:25 am
content: Quote from: Gregg on April 26, 2018, 02:17:12 amAnother possibility: a Kreepy Krauly or similar pool cleaner. The flow valve, in the main body housing, can oscillate at a few Hz, depending on water flow and pump pressure. If you stand near the pool (concrete), you can feel vibrations through your feet as the cleaner passes close by.Vibrations can carry through clay-based soils (and the pool's water of course). The concrete pool walls and floor might couple vibrations efficiently into the surrounding ground, especially if the Kreepy Krauly frequency is near a resonance frequency of the pool's structure.