say you been wanting one of those kewl infrasonic sensor / microphone
units for monitoring those government missile launches, EMP firings
sonic booms or nuke tests  ........ just to mention a few.

you been wanting one but the $800 for the  differential transducer (
alone, not to mention all the rest ) a " slight " hinderance ??    

well here is a way to get a " fantastically " sensitive  infrasonic
microphone going for UNDER $100 and if you aren't all that handy
with a soldering iron .... for under $200 ready to go.

under $100 ........

go to this dealer's ebay auctions and buy one of these mamac
pressure sensors ....
http://cgi.ebay.com/MAMAC-TREND-PR-272-6-1-A-1-2-E-Low-Pressure-Transducer_W0QQitemZ320015158620QQihZ011QQcategoryZ53301QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

these sensors are far too insensitive BUT the  box contains ALL the LP
filter, power supply , and amplifier electronics you need . ready to
go.

remove the sensor in this box and in it's place substitute this
standard 4 wire ( bridge type )  Honeywell  DUXL01D 
( available from Digikey for $53.00 plus shipping ) 

http://sensing.honeywell.com/index.cfm?ci_id=140301&la_id=1&pr_id=81288
http://www.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch

add the adjustable syringe w/ leak ,  parts available here ......
http://www.smallparts.com/products/descriptions/stc-stcy.cfm#reducer
( trimmed capillary tube from old car dash  gauges works just fine for
the leak )    ....... I use  standard 50cc plastic syringes set to
between 10 and 30 cc depending upon the reference volume freq response
I want )  


and " presto " you have the exact equivalent of a $1500 infrasonics
sensor / microphone  almost for  pocket change.

to complete this setup into a working infrasonics monitoring station.
add the wall wart to the above sensor setup ........ jack the output
of the sensor / microphone into a $25 Dataq  a/d  kit
http://www.dataq.com/products/startkit/di194rs.htm ....... 

and that's the whole ball of wax ...... as good ( or even better
really because the government stuff is pretty old ) as the
government's monitoring equipment and a whole lot less expensive.


under $200 ...............
see " project status on .......
http://www.knmi.nl/~evers/infrasound/web-article/how.html


place the sensor under  your house or out in the barn etc.  
and you have your very own REAL infrasonic monitoring system.
frequency response adjustable from .05 hz to 10 hz ( government
monitoring range ) or   .003hz - .03 hz  (  for civilian
natural emissions  )  .... 
pc chart recording resolution and time limited only by the size of
your hard drive and settings in the dataq software that comes in the
kit.

or ......... for  $200 you can buy a ready made setup...... ready to
go ..........

see .......... ( the  home infrasonics bibles ) .......

the belljar texts .........
http://www.belljar.net/microbar.htm

and the  more affordable  and desirable alternative.....
http://www.infiltec.com/Infrasound@home/

I have built several of the above setups and from field testing by
several knowledgable groups ....... these puppies work GREAT !!!  
FAR better than trying to equalize and modify ( unsuitable )
back-electret microphone carts .....  and WAY better than those 
$600 Baratron differential  sensors .  

these perform VERY well portable ( like with ALL infrasonic
mikes you have to find ways to manage the wind noise / interference 
tho it's pretty easily taken care  of  )  and  wonderful in a home
setup with the plastic pipe wind screens 
( seen on the bottom of the page on this site )
http://www.knmi.nl/~evers/infrasound/web-article/how.html

sonic booms and mine blasting  are  EASILY  recorded at distances of
several hundred miles and up  with this setup.  ( plastic pipe wind
screen  used portable )   

you would be very surprised at the number of infrasonic " events  "
that can happen all around you and one would never know without the 
proper equipment to  detect it. the government is responsible for a
large number of these events in western Nevada , along with some
mining operations  ( but there are others WELL worth checking out as
well ) .

think  true infrasonics are beyond your ability ( especially
financially )  to own them ...... look this material over and think
again .

you can do it easily  yourself and get fantastic results ......