In a message dated 08/04/2006, greensky@.............. writes:
This may be of interest. It's an article dealing with the electronics of
infrasound detection using simple (cheap) electret condenser microphones.
http://earth.unh.edu/johnson/MICROPHONES/ISLA14.pdf
http://earth.unh.edu/johnson/MICROPHONES/J4.pdf
Hi Arie,
Also have a look at _http://earth.unh.edu/johnson/MICROPHONES/ISLA4.pdf_
(http://earth.unh.edu/johnson/MICROPHONES/ISLA4.pdf) I think that this
probably gives more realistic measurements.
While on holidays I plan to construct this device. It
will be my third sensor dealing with infrasound.
Good luck!
The problems with these microphones is that they don't really go to low
enough frequencies and they do drift with temperature.
The circuits described could benefit from a three or four pole low pass
cut-off filter.
You can get greatly extended LF range and reduced noise using a 2"
electret piezo speaker as the sensor. They have a capacity of ~65 nF, so with a 47
M Ohm input impedance, you can get a Tc of 3 sec, corresponding to a period
of 19 sec, 0.05Hz.
The straight sided 1L "food" flasks can be modified as low temperature
drift capacities. I use 100 M Ohm to get to 40 sec period with an OPA604 opamp.
It is an advantage to fill the capacity with the lowest density
polyurethane foam. This ensures a near isothermal pressure response as opposed to an
adiabatic / slump response and gives about 40% more output. The pressure /
temperature relaxation time constant of this system seems to be about 0.1 sec.
The thermal drift can be reduced if necessary by adding candle wax to
the inside of the flask to increase the 'thermal mass'. The thermal time
constant of an empty flask for ambient temperature changes may be about 1/2 hr.
You can also use a screw top glass jar with a restriction tube as a
simple series low pass RC input filter to limit the pulse / noise response.
You need to use a ~constant disk temperature to avoid LARGE voltage
drifts.
As with all sensitive LF microphones, you do need to limit the wind
noise, preferably with a loop of porous irrigation hose, maybe 10 to 30 m
diameter, with solid cross connecting tubes to the sensor in the centre.
Panasonic Electret Condenser Microphone WM-034DM
The Sellers web address:
Hi-Tech Enterprises, Inc. http://www.videoequipment.com
(Note they are a discontinued line)
You can download the data on these WM-034 microphones from:
_http://www.panasonic.com/industrial/components/audio/aud_mic.htm_
(http://www.panasonic.com/industrial/components/audio/aud_mic.htm)
If you buy the CURRENT Panasoic WM61A, you will get a higher
sensitivity. I suspect that the VLF response is likely to be similar. You can also use
these for intruder detection. You can't open a window or a door without
getting quite a large signal.
There is a comprehensive web page giving details of several types of
infrasonic sensor at
_http://earth.unh.edu/johnson/MICROPHONES/microphone_list.html_ (http://earth.unh.edu/johnson/MICROPHONES/microphone_list.html)
This is worth serious attention for anyone interested in infrasound.
Regards,
Chris Chapman
In a message dated 08/04/2006, greensky@.............. writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>This may=20
be of interest. It's an article dealing with the electronics of infrasound=
=20
detection using simple (cheap) electret condenser microphones.
&n=
bsp;=20
http://earth.unh.edu/johnson/MICROPHONES/ISLA14.pdf
=20
http://earth.unh.edu/johnson/MICROPHONES/J4.pdf
Hi Arie,
Also have a look at http://earth.unh=
..edu/johnson/MICROPHONES/ISLA4.pdf I=20
think that this probably gives more realistic measurements.
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>While on=20
holidays I plan to construct this device. It
will be my third sensor=20
dealing with infrasound.
Good luck!
The problems with these microphones is tha=
t=20
they don't really go to low enough frequencies and they do drift with=20
temperature.
The circuits described could benefit from a thr=
ee=20
or four pole low pass cut-off filter.
You can get greatly extended LF range and reduc=
ed=20
noise using a 2" electret piezo speaker as the sensor. They have a capa=
city=20
of ~65 nF, so with a 47 M Ohm input impedance, you can get a Tc of 3 se=
c,=20
corresponding to a period of 19 sec, 0.05Hz.
The straight sided 1L "food" flasks can be=
=20
modified as low temperature drift capacities. I use 100 M Ohm to g=
et=20
to 40 sec period with an OPA604 opamp.
It is an advantage to fill the capacity with th=
e=20
lowest density polyurethane foam. This ensures a near isothermal=20
pressure response as opposed to an adiabatic / slump response and gives=
=20
about 40% more output. The pressure / temperature relaxation time constant o=
f=20
this system seems to be about 0.1 sec.
The thermal drift can be reduced if necessary b=
y=20
adding candle wax to the inside of the flask to increase the 'thermal mass'.=
The=20
thermal time constant of an empty flask for ambient=20
temperature changes may be about 1/2 hr.
You can also use a screw top glass jar with a=20
restriction tube as a simple series low pass RC input filter to limit the pu=
lse=20
/ noise response.
You need to use a ~constant=20
disk temperature to avoid LARGE voltage drifts.
As with all sensitive LF microphones, you do ne=
ed=20
to limit the wind noise, preferably with a loop of porous irrigation hose, m=
aybe=20
10 to 30 m diameter, with solid cross connecting tubes to the sens=
or=20
in the centre.
Panasonic Electret Condenser Microphone WM-034DM
Th=
e=20
Sellers web address:
Hi-Tech Enterprises, Inc.=20
http://www.videoequipment.com
(Note they are a discontinued line)
=
You=20
can download the data on these WM-034 microphones from:
ht=
tp://www.panasonic.com/industrial/components/audio/aud_mic.htm
If you buy the CURRENT Panasoic WM61A, you will=
get=20
a higher sensitivity. I suspect that the VLF response is likely to be=
=20
similar. You can also use these for intruder detection. You can't open a win=
dow=20
or a door without getting quite a large signal.
There is a comprehensive web page giving detail=
s of=20
several types of infrasonic sensor at http:=
//earth.unh.edu/johnson/MICROPHONES/microphone_list.html
This is worth serious attention for anyone=20
interested in infrasound.
Regards,
Chris Chapman
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