Author Topic: Raspberry Boom (microbarometer monitor)  (Read 1851 times)

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Offline JBeale

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Raspberry Boom (microbarometer monitor)
« on: March 23, 2018, 04:30:38 pm »
This is not my project, I'm not associated except that I backed it because I think it's worthwhile.

A bit over a year ago I received my "Raspberry Shake" home seismometer from this OSOP group based in Panama.  For perspective over the past six years, I have backed 7 different Kickstarter projects, almost all embedded hardware devices of one kind or another.  Some of them ended up on the shelf after not proving very useful, or lacking good software support. The Raspberry Shake has been an exception, proving to be one of my best Kickstarter investments. It continues to work perfectly recording data 24/7 and has also received several online updates.  It feeds into a global server which shows live updates of earthquake events: https://raspberryshake.net/

This week I was glad to see that the OSOP group has announced a similar device they call "Raspberry BOOM". This time instead of a geophone it is using a microbarometer to measure low-frequency sound waves, down to 0.05 Hz (20 second period).  You will hopefully not be picking up any atmospheric nuclear tests, but volcanoes and meteors are possibilities.  At shorter ranges, it is possible although unproven that you could use this in a "shot spotter" network to localize fireworks and/or gunfire.

I've played around with a much simpler infrasound sensor (modified electret mic) good to 1 Hz or so, and it easily detects a door opening at the other side of the house (that is  inaudible to my ear), as well as people walking inside from the ~ 2 Hz (walking pace) sound waves emitted by a wood floor as it flexes under their weight like a drumhead.  It picks up signals from my heater fan (HVAC system) with great detail, including trends over time,  reminding me that I do need to replace that air filter. At any rate, there are a number of interesting possibilities from this type of sensor. 
Project: https://www.kickstarter.com/projects/1158786437/raspberry-boom-atmospheric-monitor-the-weather-and
« Last Edit: March 23, 2018, 05:03:48 pm by JBeale »
 

Offline JBeale

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Re: Raspberry Boom (microbarometer monitor)
« Reply #1 on: March 25, 2018, 05:49:29 am »
Addendum: after a bit more research, I've come to the conclusion that an instrument like the "Raspberry Boom" which is detecting only sound below 40 Hz, is good for sensing events that move large volumes of air:  meteors, volcanic eruptions, large-scale explosions, ocean surf, and of course wind.  It should pick up things that go "whoosh" or "boom" better than your ear does, and from longer distances. However it detects much less than your ear does of smaller things that go "crack" or "bang", for example small fireworks, handguns, popping balloons, etc; those events  show up mostly in the 200 Hz to 2 kHz range, and also higher frequencies when they are closer.
 

Offline Richard Crowley

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Re: Raspberry Boom (microbarometer monitor)
« Reply #2 on: March 25, 2018, 06:36:35 am »
Clever. Does it use a standard pressure transducer?  Or did they create (or modify) something to sense infrasonic?

Reminds me of other "crowd sourced" environmental sensor networks.  Like the radiation (radon?) monitor connected back to a central data collection point. Dave reviewed it in a video but I can't find it.
 

Offline JBeale

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Re: Raspberry Boom (microbarometer monitor)
« Reply #3 on: March 25, 2018, 03:26:32 pm »
The kickstarter page doesn't specify many technical details. When I asked privately, I was told the sensor is already in commercial production. The most specific information I've found is in a footnote to the specifications here:

https://manual.raspberryshake.org/_downloads/SpecificationsforBoom_SnB.pdf
The Raspberry Boom infrasound sensor was based on Jeffrey Johnson’s InfraBSU sensor and the work published in (1) Marcillo, O., Johnson, J.B., and Hart, D. (2012) Implementation, Characterization, and Evaluation of an Inexpensive Low-Power, Low-Noise Infrasound Sensor Based on a Micromachined Differential Pressure Transducer and a Mechanical Filter, Journal of Atmospheric and Oceanic Technology 29:1275-1284; and (2) Johnson, J.B. and Ripepe, M. (2011) Volcano Infrasound: A review, Journal of Volcanology and Geothermal Research 206:61-69.

Personally, I tried modifying a cheap electret mic capsule (simply by sealing it into the middle of a brass tube, and adding a hose connector with soaker hose to one end for a slow leak) and I got down to about 1 Hz, limited I believe by electronics (RC time constant of leakage of internal buffer FET discharging the electret capacitor).  The R-Boom mentions "estimated" 3-dB points of 0.05 Hz to 40 Hz, if accurate that is 20x lower in frequency than my simple hack.
« Last Edit: March 25, 2018, 03:36:01 pm by JBeale »
 

Offline rhb

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Re: Raspberry Boom (microbarometer monitor)
« Reply #4 on: March 25, 2018, 04:03:40 pm »
Ramsey Electronics sold a microbarometer capable of measuring a fraction of an inch change in elevation.  The kit used the NXP MPXA4115AC6U Integrated Silicon Pressure Sensor.  I never bought one, but I did research the pressure sensors.  This is not the most sensitive one.

Ramsey now just sells Faraday cages for law enforcement forensics and dropped the kit line :-(

Search on "ramsey electronics up24" and you can find the manual and lot's of discussion.

https://www.wxforum.net/index.php?topic=13756.0
 

Offline evb149

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Re: Raspberry Boom (microbarometer monitor)
« Reply #5 on: March 25, 2018, 04:26:25 pm »
Larger area is better for low frequencies, just the same (in the inverse direction) as a loudspeaker woofer.

So basically you'd want something that would act like a fairly hermetically sealed cone (leaks should equalize small differential pressures over N*100s time constants).  Then a means to sense the differential pressure on the cone between the read cavity of the housing and the atmosphere.

As in loudspeakers a horn could effectively concentrate the power over a larger area onto a smaller transducer.

And for directionality and/or very low frequencies it would be beneficial to have a distributed array / set of sensors so correlated differential data could be analyzed over different places along the wavefront.

Many electromechanical or other position sensing technologies could well be easily used, capacitive, inductive, optical and/or fiber optic, etc.

For better linearity and dynamic range one could make it a closed loop feedback system where the sensing 'cone' is kept static and the feedback needed to achieve that is monitored as the output signal. 

One could also make the overall system resonant to greatly increase the sensitivity near the resonant frequency/frequencies.

 

Offline JBeale

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Re: Raspberry Boom (microbarometer monitor)
« Reply #6 on: March 25, 2018, 04:34:23 pm »
Thanks for the tip about the NXP sensors. That one is absolute but for infrasound you want a differential one, which I see NXP also has, for example : https://www.mouser.com/ds/2/302/MPX2010-1127090.pdf

Attached is an example of a spectrogram I recorded indoors this morning with the electret-mic hack. No one was walking around or opening doors during this time; those things make a big vertical spikes on the spectrogram. The main signal is the HVAC blower fan which I assume due to a duct resonance makes a big 17 Hz sinewave in the pressure, plus a lot of broadband noise.  I was interested to see the signal around 0.5 Hz. It does not stop immediately after the blower cuts off but persists around 45 seconds, and then there is a 3 minute period when there is very little signal there, then it returns, and this pattern repeats each heater cycle.  So I guess this has to do with indoor thermal convection patterns, which I didn't expect to be able to measure.  There is also a faint feature around 18 Hz even when the blower is not on, possibly the duct resonance excited by external wind gusts.

The narrowband trace at 40 Hz on the first half of the plot is I think an alias (100 Hz sample rate so Fnyquist = 50 Hz) of the 60 Hz vibration of the refrigerator.

The plot is displayed using the Audacity spectrogram view feature, but that is limited to frequencies above 1.0 Hz so I rescaled my 100 Hz sample rate up 10x to 1 kHz so I could display down to 0.1 Hz actual frequencies.
« Last Edit: March 25, 2018, 04:38:48 pm by JBeale »
 

Offline evb149

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Offline rhb

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Re: Raspberry Boom (microbarometer monitor)
« Reply #8 on: March 25, 2018, 04:52:03 pm »
I've considered using that differential sensor with a pair of water filled tubes as a super precise machinists level.

You can build a unit with a 24 bit EBay ADC, sensor and the MCU of your choice.  The 24 bit ADCs are intended for use with strain gauge scales, so the sample rates are low, but for your interest it should be fine.  Seismic ADCs are better but also a lot more money.
 

Offline JBeale

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Re: Raspberry Boom (microbarometer monitor)
« Reply #9 on: March 25, 2018, 05:02:46 pm »
http://techlib.com/area_50/infrasound.htm
That's a very cool page!  I'm impressed by the effort he went to, building a very large diaphragm mic capsule with a mylar membrane. You can also buy smaller versions of that premade and ready to go for about $30, for example https://www.aliexpress.com/item/High-Quality-34-mm-Diameter-Microphone-Large-Diaphragm-Cartridge-Core-Capsule-For-Studio-Recording-Condenser-Mic/32806774299.html
 


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