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Subsonic sensor?


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#1 Old Guy

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Posted 13 April 2010 - 09:44 AM

For the technical and the technically curious (You know who you are ;-) take a look at Table 3 (Figure 3). It has a response down to 3.3Hz?!?

http://www.meas-spec...rms=ldt series*

The best part is that Parallax has them for <$2.

http://www.parallax....89/Default.aspx

Please, give me your "outside of the box" thoughts.
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#2 Robot

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Posted 13 April 2010 - 11:13 AM

Very interesting......................

I'm trying to figure out what length of "tip deflection" is needed to "see" the 3.3Hz.

"One experiment is worth a hundred calculations". The cost of this experiment would be reasonable :Spaz:

Adding the mass creates a pendulum, this might be exploited by something like a "lever arm" to a speaker?

I think "sensor" is the keyword, I initially have little faith in any audio modulation capabilities.

I was impressed previously with your links to "other" low Hz modulation capable devices.

Thanks for the link!
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#3 AnythingButNormal

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Posted 13 April 2010 - 03:18 PM

Very interesting......................

I'm trying to figure out what length of "tip deflection" is needed to "see" the 3.3Hz.

"One experiment is worth a hundred calculations". The cost of this experiment would be reasonable :Spaz:

Adding the mass creates a pendulum, this might be exploited by something like a "lever arm" to a speaker?

I think "sensor" is the keyword, I initially have little faith in any audio modulation capabilities.

I was impressed previously with your links to "other" low Hz modulation capable devices.

Thanks for the link!


Ahh, this is fantastic!!! I've had the idea now to create a strip of lights capable of sensing movement (much like those ghost pods) but lacked an appropriate sensor to operate the strip. This is EXACTLY the sort of thing I was looking for, and the sensitivity is EXCELLENT. I imagine this would be a MUCH cheaper way to create a sort of geophone as well. I'm curious to hear what plans the rest of you had in store for such a sweet little electronic gadget.
Investigator/Technical Manager: South Coast Paranormal Society, Augusta, GAwww.southcoastparanormalsociety.com

#4 AnythingButNormal

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Posted 13 April 2010 - 03:35 PM

HEHEHEEE, this is fantastic!!! I've had the idea now to create a strip of lights capable of sensing movement (much like those ghost pods) but lacked an appropriate sensor to operate the strip. This is EXACTLY the sort of thing I was looking for, and the sensitivity is EXCELLENT. I imagine this would be a MUCH cheaper way to create a sort of geophone as well. I'm curious to hear what plans the rest of you had in store for such a sweet little electronic gadget.


Plus these things generate voltage? Hooked in series with a zener diode and a tiny amount of weight added to help get the 2mm of deflection would yield an open circuit that only generates current above 5v of activity. When used as a regulated voltage gate for a mosfet, the potential uses of this thing are astounding! I see a lot of creative potential in this tiny cheap sensor. Especially to detect footsteps, knocks, opening of doors, or other vibrations in floors, windows, or walls where no one is present.

I am curious though, could this sensor somehow be enclosed to detect wind drafts or variations in windspeed? My natural instinct says it could if placed upright in a standalone enclosure with a wind tunnel directed at the strip. My brain is going nuts thinking of all the things I could do with this. Thanks much for pointing it out!!
Investigator/Technical Manager: South Coast Paranormal Society, Augusta, GAwww.southcoastparanormalsociety.com

#5 Old Guy

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Posted 13 April 2010 - 04:00 PM

I've got several questions in to an applications engineer. I'll let the forum know when they make contact...

"Adding the mass creates a pendulum, this might be exploited by something like a "lever arm" to a speaker?"

Or a membrane?

"Plus these things generate voltage?"

They do. Enough to kill the input on a Basic Stamp. Don't ask. :-P

Edited by Old Guy, 13 April 2010 - 04:04 PM.

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#6 CaveRat2

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Posted 14 April 2010 - 07:20 AM

Interesting.

I have requested design samples for evaluation and will let everyone know what comes of it. I do see potential though.

#7 Robot

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Posted 14 April 2010 - 08:08 AM

I've got several questions in to an applications engineer. I'll let the forum know when they make contact...

"Adding the mass creates a pendulum, this might be exploited by something like a "lever arm" to a speaker?"

Or a membrane?

"Plus these things generate voltage?"

They do. Enough to kill the input on a Basic Stamp. Don't ask. :-P


Owwww, stamps aint cheap!
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#8 AnythingButNormal

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Posted 14 April 2010 - 11:58 PM

"Plus these things generate voltage?"

They do. Enough to kill the input on a Basic Stamp. Don't ask. :-P

Owwww, stamps aint cheap!


No kidding right? When I saw that they output up to 70 volts, I thought, "Hmm, resistors are good. They are my friend." Can anyone say 100 Mega ohm?

So I gotta ask, are you planning to hook this up to another stamp, and if so how are you going to regulate the voltage? I was considering using some resistors, and a few series capacitors to help regulate the current so I could pulse drive a few LEDs off the sensor trigger. I'll admit that I don't have nearly the electronics experience that many of the others in this forum do.
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#9 Old Guy

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Posted 15 April 2010 - 02:36 PM

Well... I heard back from MSI tech support. Bad news for me:

"Unfortunately, using piezo film to detect sub-sonic acoustic energy isn't a very good way to go. In general, low frequency acoustic energy has a fairly low power content, due to the low instantaneous velocity of the wave fronts. To get reasonable sensitivity at these low frequencies, a large surface area transducer is needed. It can be done with piezo film, but a very large piece is required - a few sq feet. It's cost prohibitive. Try looking at another approach - dynamic electromagnetic transducers. Another term to describe these is...woofers. A large low frequency speaker will be your best bet. The surface area is big, their design is already optimized for low frequency operation, and the output is low impedance which is ideal for interfacing to electronics."

But I believe (Like Robot implied) that mechanical amplification (a lever) could still do the trick.

And I've had some secondary thoughts about my preamp in the isobaric woofer setup, too.
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#10 Old Guy

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Posted 15 April 2010 - 02:49 PM

So I gotta ask, are you planning to hook this up to another stamp, and if so how are you going to regulate the voltage?


Sure. But it needs a little something more than a resistor. Check the "Quick Start Circuit" in the Piezo Film Information pdf here: Parallax

The 5.1V Zener acts as a voltage clamp for anything over 5.1 volts. I probably used something stupid like a 1N914 or a 4001 or a what-the-heck-is-this? It's was a while ago. And hey - it's still got 15 good IO pins! :ghost:
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#11 AnythingButNormal

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Posted 15 April 2010 - 06:06 PM

"Try looking at another approach - dynamic electromagnetic transducers. Another term to describe these is...woofers. A large low frequency speaker will be your best bet. The surface area is big, their design is already optimized for low frequency operation, and the output is low impedance which is ideal for interfacing to electronics."

But I believe (Like Robot implied) that mechanical amplification (a lever) could still do the trick.

And I've had some secondary thoughts about my preamp in the isobaric woofer setup, too.


I'm no expert on anything sonic or electronic, but I do have a good grasp of physical science, material engineering, and optics. I disagree with the guy who gave you the answer. If you need a large surface area to pick up the waves, couldn't you make a "subwoofer" out of a large sheet of Mylar under tension and attach a peg or pole to the sensor. Then position the peg in the center of the Mylar such that you are already near the optimal deflection for your 3.3hz response? It would basically make an inverted subwoofer design using the switch/sensor as its modulator. Plus, Mylar is cheap and readily available at party stores. Larger piezoelectric materials generate more current, but my understanding is that it won't matter what level of current you get. If possible, I'd use glass or quartz to make your mechanical peg for the following speakers since they transmit vibration readily. A Pyrex stirring rod could be just the thing.

I'm not sure what your intended use of this device is, but I was thinking of building a subsonic sensor to detect frequencies emanated from floors or walls. Placing the whole manufactured subwoofer inside a recessed enclosure and setting the enclosure on the floor should allow me to pick up the vibrations of the floorboards, which transfer to the air, then mechanically to the piezoelectric film sensor. The biggest concern from a structural standpoint is anything that may provide damping for the waveforms since 3.3hz doesn't have squat for energy; however, amplifying your signal after the sensor should get you back up to detectable levels of audio. The sensor itself doesn't have much mass, nor does the Mylar, so it seems to me that this problem isn't much of a concern except at the interface between the Mylar and the enclosure. I'd wrap it around a wire frame, and heat seal the flaps over the frame.

I suspect that this enclosure would also pick up just as well standing on it's side. The other alternative I can think of is using a waveform guide tube from a modern subwoofer as your amplifier. If you could find an old or broken Logitech z5500 subwoofer that would be ideal, as this speaker design has some of the best low frequency response I've ever heard/felt from a surround sound system. Then work in reverse, point the waveform tube at the area to be listened to and attach the sensor to a mylar sheet positioned over the end of the tube where the subwoofer would be :ghost: .

I hope any or some of this is helpful OldGuy. Since I'm not good at math, I tend to work from a physical science perspective by understanding the principles and applying them to new or different techniques. I do have a question for you though, what amperage is this sensor putting off through it's range of deflection? It seems that material is lacking from the data sheet.
Investigator/Technical Manager: South Coast Paranormal Society, Augusta, GAwww.southcoastparanormalsociety.com

#12 AnythingButNormal

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Posted 15 April 2010 - 06:13 PM

I realized after this long winded diatribe, that I was planning a much simpler approach for my geophone design. I was going to attach a peg to the sensor, then have the peg rest on the floor or wall where the device is attached. With some tension on the peg before starting, I could zero the device with a potentiometer, and watch a meter peg spike when subsonic frequencies are detected. This could of course be attached to switch on alarms or lights. I planned to test my design on the highway overpass near my house since I hear passing tractor trailers cause the roadways to emit subsonic frequencies.
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#13 CaveRat2

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Posted 15 April 2010 - 06:23 PM

I would agree here. Using a piezo element entirely as a stand alone sensor would not provide the low frequency capability. However using one in a situation where it is directly coupled to the floor or ground would allow the entire area to serve as a sensor. It would operate much like the geophone concept and should provide sufficient sensitivity.

My plans to test the element once I get it will involve coupling it to an analog amplifier and simply measuring the output voltage for now. As stated Basic Stamps are not cheap, and until I get a sensor developed that provides a known range of signals I don't really see a need to go that route. Once the sensor is refined then I would consider that because it would simplify calibration and provide a known output. Plus it could also be utilized in different modes based on needs.

But for now I am only concerned with getting the concept operational. Calibration comes later.

#14 boatlesspirate

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Posted 17 April 2010 - 10:31 AM

I've always had this idea to either use water with a laser projected through it, or use a large use a mylar sheet with a mirror attached and the laser bouncing off of it. Then, the subsonic sound would cause a disturbance in the laser. And if projected of a distance of at least 10 feet, you would be able to visually see the sound cause movement of the laser, no?
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#15 Robot

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Posted 19 April 2010 - 08:10 AM

I've always had this idea to either use water with a laser projected through it, or use a large use a mylar sheet with a mirror attached and the laser bouncing off of it. Then, the subsonic sound would cause a disturbance in the laser. And if projected of a distance of at least 10 feet, you would be able to visually see the sound cause movement of the laser, no?



This is known as a "laser microphone" if you google this you will find they have been used for spying on people, often bouncing a IR laser beam off a window to a remote reciever. The laser beam is modulated by the vibration of the window or other acoustic pressure wave sensitive surface.
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