Hi!
I agree with James. The preamp and amplifier/digitizer=C2=A0circuitry of wh=
atever you
you are using to record the infrasound is the first factor to consider when=
attempting to record these sounds. If the recording device doesn't go low e=
nough
in frequency without=C2=A0rolloff to get what you want, then you are wastin=
g your
time trying to find the proper transducer or microphone.
The transducer should then be a pressure transducer. An omnidirectional mic=
is a
pressure transducer, but depending on how low you wish to go below 20 Hz. a=
ctual
industrial pressure transducers or microflown transducers=C2=A0may be what =
you want
to use. But you haven't quantified what actual range of frequencies you wan=
t to
record, nor have you told us the purpose of your work. It is difficult to r=
ecord
sounds outside of the "normal" 20Hz - 20 KHz range that most mics and recor=
ders
can handle; anything outside of that range often requires special equipment=
,
gear that can only be specified when the exact requirements are known.
Many of the folks that are intimately familiar with infrasound recording ar=
e
either studying volcanoes, seismic events (including remote nuclear explosi=
on
detection) and elephants. There is plenty to learn from them: just Google
"infrasound microphone" and you'll get a lot of useful info!
One more thing about this field. The NOAA ESRL site has a paper about infra=
sound
and near-infrasound which is very interesting and would be a good introduct=
ory
paper on infrasound:
http://www.esrl.noaa.gov/psd/programs/infrasound/infrasonic.html
It clarifies the definitions for what we are talking about: "...periods bet=
ween
.05 to 1 second (20 to 1 Hz) are logically called near-infrasound (an analo=
g to
near-infrared). Periods between 1 and about 100 seconds (1 to .01 Hz) defin=
e the
range of infrasound (well below the human hearing), and signals with period=
s
greater than 100 seconds can be designated acoustic/gravity waves..".
So infrasound=C2=A0is really the world below 1 Hz to ~0.01 Hz. Is this the =
area of
your interest?=C2=A0This prompts one to look for special equipment such as =
a B&K 4147
mic and a PC acquisition (instrument recording) system that has good, flat=
response down to DC. But that is only one possibility of many.
Good luck and good hunting!
-Greg
=C2=A0
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