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infrasonic detector

I am not sure if it is sensitive enough for your use (a full atmosphere of pressure change produces only 2% of the excitation voltage as an output voltage), but you would use it somewhat like a microphone. Its advantage for low frequencies is that it compares the air pressure to an internal sealed vacuum, so its response really goes down to 0 Hz.

It contains a 4 resistor strain gauge bridge that needs an excitation voltage source and produces a pair of outputs, each at approximately half of the excitation voltage. When pressure increases, one output changes voltage in the positive direction and one changes in the negative direction.

You need a first gain stage that amplifies the difference between these two signals. You could just use one of the outputs, but amplifying their difference removes most of the errors caused by voltage ripple on the excitation input, and doubles the signal strength.

Go to

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and read: AN-20 page 3 (difference amplifier) AN-31 page 14-16 (instrumentation amplifiers)

Reply to
John Popelish
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I want to detect only low frequencies, as from a diesel.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson

I read in sci.electronics.design that Jeff wrote (in ) about 'infrasonic detector', on Thu, 22 Sep 2005:

Loudspeakers work quite well as microphones but you need a

**second-order** low-pass filter from very low frequencies to get a flat response. Peter Baxandall wrote an AES pre-print on the subject a long time ago, but it's probably still available. 
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

I read in sci.electronics.design that Jim Thompson wrote (in ) about 'infrasonic detector', on Thu, 22 Sep 2005:

You can buy them from the AES web site.

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You may find it difficult if you don't have the pre-print number.

Loudspeakers as High-Quality Microphones It is established, using the reciprocity principle, that if a voltage-driven loudspeaker has a flat axial frequency-response, then such a response will also be obtained when it is used as a microphone, provided the associated amplifier has zero input impedance and a V out/I in response rising with frequency at 20dB/decade throughout the audio spectrum. The design of a suitable amplifier raises problems with regard to signal-to-noise ratio and the ability to handle the very large low-frequency input levels that are liable to be produced. Two different approaches are described, both leading to practical designs. In conclusion, consideration is given to the use of the technique for subjective loudspeaker quality assessment, and for the measurement of frequency response and absolute sensitivity. Preprint Number: 1593 Convention: 65 (January 1980) Author: Baxandall, Peter J. E-lib Location: (CD aes9) /pp7680/pp8002/1688.pdf downloadable electronic version PDF (1.0 MB) $5.00 AES Member | $20.00 Non-Member hard copy $5.00 AES Member | $20.00 Non-Member

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

I read in sci.electronics.design that Jim Thompson wrote (in ) about 'infrasonic detector', on Thu, 22 Sep 2005:

Don't blame me if it sounds funny, then. (;-)

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

so... what do you suggest ? Do I build this DIY sensor with my laptop microphone ?

Reply to
merco

oh yes... but it could be an idea... strange but perhaps good ... i'm going to do it with an AD822...

Reply to
merco

As long as nobody knows, what you want to detect and what kind of mircophone you want to use(even if you tell me the type of your laptop I don't know the mic)... Nobody knows...

Marte

Reply to
Marte Schwarz

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