Very Interesting, Paul! Can we hear some of these recordings?
I followed the previous posted link to the ars.usda.gov site and found a mo=
ther
load of good info, i.e., links to papers by Richard Mankin.
He's done quite a bit of work recording different insects for study and the=
tools vary depending on what insect/environment they were studying. As it t=
urns
out, they used a lowly AT803B lavaliere mic=C2=A0[self-noise =3D ~ 29dBa; n=
ot very
amazing!]=C2=A0to study fruit flies, albeit with some DSP post processing. =
Other
recording devices used were PVDF film, omni mics wired in a differential ci=
rcuit
to cancel out ambient noise [classic noise-canceling microphone technique],=
infrared photocells, accelerometers with charge amplifiers, hydrophones=C2=
=A0and
geophones.
Geophones are interesting transducers since they are quite sensitive to
vibration. They pick up the vibrations of whatever they are resting upon. T=
he
operating principle is=C2=A0voltage generation=C2=A0by a magnet suspended=
=C2=A0on springs
within a coil of wire. Vibration [up & down relative to the axis of the uni=
t]
makes the magnet move=C2=A0and it induces a voltage into the coil. Because =
of the
mass=C2=A0of the magnet, the frequency starts to roll off steadily=C2=A0abo=
ve several
hundred=C2=A0Hertz, but you can get some nice sounds from them.=C2=A0I have=
been using
geophones for recording ambient vibration from machines, structures=C2=A0an=
d also
trees, but they have a certain inherent noise level at high gains that can =
be
rolled off with appropriate high pass filters [use your ears!]. The geophon=
es
that Mankin likes are the=C2=A0GS-32CT units=C2=A0and here is the link:
http://www.geospacelp.com/index.php?id=3D29
You can get them mounted in a ruggedized case with a nice long spike that c=
an be
inserted into the ground for picking up underground sounds. I suspect that =
there
are some MEMS accelerometers that might be usable as well, but I have no
experience yet with such devices. Also check out the Sanken microphone site=
--
they have some interesting products such as the COS-22. They used to make a=
special super sensitive microphone for insect recording that appears to be=
something like a wide geophone with a probe coming out of it. See:
http://www.atscomms.com/Sales/Products/Mics/Sanken/Contact_Mic/contact_mic.=
html
I suppose that if you could find one now it would cost a small fortune
Looks like experimentation is still the name of the game, just get some
transducers and try out some ideas. Check our Mike Rooke's excellent blog s=
ite
for some interesting applications of mics and other transducers. Ambient no=
ise
level will always be an issue [try some of those noise-canceling techniques=
] and
bear in mind that the type of preamp needed to get a good quality signal ma=
y be
very different from the one inside your field recorder. You may need a much=
higher preamp input impedance for certain transducers in order to achieve
optimum results.
Happy hunting!
-Greg
=C2=A0
|