Acoustic Observatory - Low-Cost

[Front Office]

Acoustic Observatory -- Low-Cost

I have built what I call an Atmospheric Acoustic Observatory. It could also be thought of as a high-resolution barometer. There's a write-up on it at http://www.comfortlight.com/baro.htm.

It is simple in principle, and it cost only about $50 for the main components including a red laser diode and a front-surface mirror that I cut into two tiny parts.

An observer can see pressure changes smaller than 1 ten-millionth of atmospheric pressure. It can "see" long-wave sounds that ordinarily can not be heard. Even with all the windows open in my house, it can sense when someone opens or closes a door, silently to the ear, elsewhere in the house.

One of my goals with this Observatory is to "see" long-wavelength sounds of thunder such as might come from approaching storms too distant to otherwise hear with normal hearing. It responds to the low frequencies of a human voice and passing car traffic, and it goes crazy when helicopters fly by within a mile or so of it.

I also want to see, and hope to see, "microbaroms" which might trail in the wakes of hurricanes that I hope will come up the east coast this fall. (I live near Washington, D.C.) Check out the Wikipedia article on microbaroms. Distant earthquakes, thousands of miles away, might also produce "atmospheric tsunamis" that might be detectable with this device.

A highly sensitive and low-cost seismometer could be built using the same basic principle of reflected laser light to amplify small motions. See the schematic diagram at http://www.comfortlight.com/SO.jpg.

It's been operating for a month now. The main mirror, or "first mirror" got dislodged several weeks ago when lightning hit about a thousand feet away. A military jet, several thousand feet over the house, also dislodged the mirror.

 

[Khashishi]

Can you get access to seismological data to compare with?

 

[davenn]

A highly sensitive and low-cost seismometer could be built using the same basic principle of reflected laser light to amplify small motions. See the schematic diagram at http://www.comfortlight.com/SO.jpg.

Yup that should show up quakes

of course torsion style seismometers using a mirror to reflect a beam of light have been around for a long time. You idea uses much the same principal. Google the Wood-Anderson torsion seismometer. I used to run one at home for a number of years before I got sick of the hassles and cost of using large sheets of photographic paper and went to a more conventional pen on paper drum, then in the mid '90's went digital.
The challenge you have is how to record the motion of the laser dot on the wall so that it can be used to produce meaningful data

as far as your barometer goes ... any earthquake motion it records is more likely to be movement of your observatory from the passing seismic waves rather than from any atmospheric disturbances traveling long distance.

Natural atmospheric disturbances you may well record are ..
1) distant storms - hurricanes ( as you said)
2) not so distant tornadoes
3) large meteors (bolides) --- the air shockwave from the 1908 Tunguska ( Russia) meteor explosion was recorded worldwide on barometers> The shockwave was reported to have circled the Earth a couple of times.
4) big volcanic eruptions --- The Krakatoa eruption in 1883 ( from memory) in Indonesia also produced a massive air pressure waves. The Low rumbling booms were audibly heard in Alice Springs (Central Australia) over 3000 km from the blast.

Look forward to hearing more from you as time passes and hear about things you manage to record :)
WELL DONE , keep up the experimenting !
cheers
Dave

 

[Bobbywhy]

Front Office, congratulations on building your sensitive infrasound detector! In addition to the atmospheric sources of infrasound already mentioned above by davenn, more sources in nature are waterfalls, earthquakes, and avalanches. Thunder generates a wide spectrum of sound energy, including at infrasonic frequencies. Some auroral processes emit infrasonic signals by the expansion of air during electrical discharges.

A number of animals use infrasound to communicate: elephants, (http://www.light-science.com/articles1003.html) giraffes, rhinoceroses, hippopotamuses, alligators, and whales all generate low frequency acoustic signals. The flightless cassowary birds, Rock Doves and pigeons emit infrasonic calls, and recently, the male peacock has been found to generate infrasonic energy to call for a mate by vibrating its feathered fan. See: http://www.birds.cornell.edu/brp/?lk=lpro/

Manmade explosions, both chemical and nuclear, can be detected. The defense establishment listens for low yield atmospheric nuclear weapons testing in the range of frequencies from 0.05 to 10 Hz.

You have described making qualitative measurements by observing the laser spot move around on the wall or ceiling. May I suggest that you consider making quantitative measurements? This would require detecting those movements and storing it for later retrieval and analysis. A digital-to-analog conversion would be used along with a computer for display of waveforms. Sound intensity level, frequency, and harmonics might be analyzed at your convenience. Software programs for seismic monitoring may work for this purpose. See LabVIEW at: http://www.ni.com/labview/

For some examples of this process, see the “Redwood City Public Seismic Network”. Their website describes new equipment for sale, useful free software downloads, and other PSN information. See: http://psn.quake.net/

As you may notice, some members here at Physics Forums are interested in your work and I, for one, would be pleased to read your experimental results.

Cheers, Bobbywhy

 

[davenn]

Hey front office and Bobbywhy

Bobbywhy has made a really good recommendation there. I am part of a worldwide PSN network that uses the software and hardware from the Redwood City Public Seismic Network site.
I know Larry personally and visited him on one of my USA trips. He's a great guy has an awesome knowledge of electronics design and software writing. He's listens to the users of the gear and constantly does upgrades of his system to take in features that have been suggested.
His system is the full package from multi channel low noise preamps, A-D converters 16bit and 24 bit units available. Datalogging software and a separate analysis program.

cheers
Dave

 

[Front Office]

Can you get access to seismological data to compare with?

It would be neat to record the output of this type of laser-beam-and-mirror system, and then compare its output to other records. But that's not possible with this real-time system.

 

[Front Office]

As you may notice, some members here at Physics Forums are interested in your work and I, for one, would be pleased to read your experimental results.

It's pretty much a moment-to-moment kind of instrument; i.e., you just watch the red spot and see if the bobbing up and down corresponds sounds you can hear, or, as it happens, to wind that is blowing. In that latter case, wind, the pressure variations inside my house look to be on the order of several Pascals. I estimate that a 1-mm motion of the red spot corresponds to about 3 millipascals of pressure change in the air, but when the wind is blowing, the spot goes up or down as much as 10 cm total amplitude. I've noticed that when I walk across the bedroom where it's located, walking as silently as I can, the red spot moves in a way that suggests that deflection of the wood floor is being picked up because of air pressure changes. However, I can't rule out that motion of the floor is not directly affecting the instrument through the floor; I have it placed near a wall, where the deformation of the floor should be much less than were it in the middle of the room. Also, late at night, when I lie in bed watching it and there's no perceptible air motion, the red spot occasionally moves in ways to which I cannot assign plausible explanations, sometimes quite slowly up and down. Who knows what I'm seeing, and how far it might be coming from. I'd like to devise, without having to put much effort into it, a way to record the output.

 

[Front Office]

Bobbywhy has made a really good recommendation there. I am part of a worldwide PSN network that uses the software and hardware from the Redwood City Public Seismic Network site.
I know Larry personally and visited him on one of my USA trips. He's a great guy has an awesome knowledge of electronics design and software writing. He's listens to the users of the gear and constantly does upgrades of his system to take in features that have been suggested.
His system is the full package from multi channel low noise preamps, A-D converters 16bit and 24 bit units available. Datalogging software and a separate analysis program.

I've been wanting for decades, to learn ways to digitize and record data from various experiments, but . . . well, you probably know how daunting certain things can seem upon first consideration, e.g., A-D conversion etc, and then how simple it all is once you learn it. In this instance, I seem perpetually stuck in the daunting first-consideration stage, though maybe I'm just lazy. It would be nice to have a neighbor who could teach me such things. I've been wanting to stick I wire up a tree or on the roof to look at voltage changes, especially as storms approaches; or just as a matter of course, like Ben Franklin did with his "http://fi.edu/franklin/bells.html," but with a way to both watch the voltage changes and record the data -- and also protect the instruments from lightning.

 

[cjl]

It's pretty much a moment-to-moment kind of instrument; i.e., you just watch the red spot and see if the bobbing up and down corresponds sounds you can hear, or, as it happens, to wind that is blowing. In that latter case, wind, the pressure variations inside my house look to be on the order of several Pascals. I estimate that a 1-mm motion of the red spot corresponds to about 3 millipascals of pressure change in the air, but when the wind is blowing, the spot goes up or down as much as 10 cm total amplitude.

How did you get this calibration value (1mm = 3mPa)? Also, how do you know that the pressure vs motion of the light is linear (if 1mm = 3mPa, then does 10mm = 30mPa?)? This definitely seems interesting, but I would be curious to hear how it was calibrated before assigning too much value to the quantitative results.

 

[mfb]

With wind, I would also expect significant effects from the kinetic energy of the moving mass. The mirror might oscillate in the wind, and gets an amplitude that exceeds the real pressure changes significantly.

How sensitive is your device to objects moving nearby?

 

[Bobbywhy]

OOPS! Please revise my post #4 above to change

from "digital-to-analog conversion would be used "

to "analog-to-digital..."

Thank you, Bobbywhy

 

[Front Office]

How did you get this calibration value (1mm = 3mPa)? Also, how do you know that the pressure vs motion of the light is linear (if 1mm = 3mPa, then does 10mm = 30mPa?)? This definitely seems interesting, but I would be curious to hear how it was calibrated before assigning too much value to the quantitative results.

Look at this schematic. The distance between the pushrod support and the fulcrum rod is about 2 mm. The laser beam travels about 4 meters from the mirror, so that the angular deflection of the mirror gets multiplied by a factor of about 2,000; when the diaphragm, in its center where the pushrod support sits, gets deflected one micron up or down the red spot moves up or down 2 mm. The 1-micron deflection of the diaphragm corresponds to a volume change in the ~20-liter water bottle of ~7E-7 liter, and I'm assuming volume change is inversely related to pressure change.

I don't 'know' that it's linear, I'm only assuming that. Clearly there are non-linear considerations here, mostly, I think, relating to the geometry of the setup.

In effect, it's not been calibrated, at least not in any rigorous way. These numbers are just estimates. Maybe I should make that clearer in my write-up.

 

[Front Office]

With wind, I would also expect significant effects from the kinetic energy of the moving mass. The mirror might oscillate in the wind, and gets an amplitude that exceeds the real pressure changes significantly.

How sensitive is your device to objects moving nearby?

When the wind blows (outside, of course) air pressure in the house changes, up or down, depending. There is no sensible wind motion in the house. But yes, if air were to blow right against the diaphragm, then I agree: there'd be an effect.

As for objects moving nearby, that's a question that's interesting to me, because, long ago I noticed that when cars pass by, near me, you can see or sense, by the motions of things like pieces of paper or cigarette smoke, and at distances of 10 feet of more laterally away from the track of the car, that a pressure wave is moving in front of and to the sides of the car. I'd like to figure out a way to visualize that low-pressure wave, because it might help my thinking about how a car pushes aside the air as it travels. People talk about "air friction," but that seems to me to give an erroneous view of the "drag" that impedes vehicular motion. It seems to me that the drag force acting on a car, or bullet, airplane, whatever, relates to the force (and energy) that gets expended in pushing air out of the moving object's path.

 

[mfb]

It seems to me that the drag force acting on a car, or bullet, airplane, whatever, relates to the force (and energy) that gets expended in pushing air out of the moving object's path.

That is the main source of air drag for many objects, indeed.

 

[cjl]

When the wind blows (outside, of course) air pressure in the house changes, up or down, depending. There is no sensible wind motion in the house. But yes, if air were to blow right against the diaphragm, then I agree: there'd be an effect.

As for objects moving nearby, that's a question that's interesting to me, because, long ago I noticed that when cars pass by, near me, you can see or sense, by the motions of things like pieces of paper or cigarette smoke, and at distances of 10 feet of more laterally away from the track of the car, that a pressure wave is moving in front of and to the sides of the car. I'd like to figure out a way to visualize that low-pressure wave, because it might help my thinking about how a car pushes aside the air as it travels. People talk about "air friction," but that seems to me to give an erroneous view of the "drag" that impedes vehicular motion. It seems to me that the drag force acting on a car, or bullet, airplane, whatever, relates to the force (and energy) that gets expended in pushing air out of the moving object's path.

Air resistance is an interesting subject, and the balance between form drag and skin friction (viscous) drag is highly dependent on the shape of the object. For cars, I don't know which one will be the dominant term, but it wouldn't surprise me to learn that form drag is a significant contributor.