>> > Any thoughts or comments would be appreciated.
Just some random thoughts.
The actual sound wave produced has it's frequency related to the turbine
speed, and will be infrasound, below human hearing at the speed these
things turn. If the turbine is making at least 6 rpm then a MKH-110-1
mic would pick it up as infrasound. If it gets up to 60 rpm than a
MKH-110 could pick it up. The low frequency spec for those mics is 0.1Hz
for the MKH-110-1 and 1Hz for the MKH-110.
It's possible the frequency could be rpm times number of blades too,
which would be a little higher, though still infrasound. Depends on
where you are located relative to the blades. The critical speed for the
two mics would be 2 & 20 rpm for a three bladed turbine.
But that's only the first part of your problem. None of the recorders
people use here are rated to record that low. They will probably have a
limiting filter in them to cut out such low frequencies which can be
powerful and easily overload them. So they won't record the infrasound
directly. And even if they did, our playback systems cannot reproduce it
at the proper power levels.
What you hear is the effect of that pressure wave on the other sounds in
the area, mixed with the sound of air flowing over the blades (the sound
of air moving over a airfoil) and echos of environmental sounds off the
blades. Some mic and recorder combinations may do a respectable job on
this as it's variations in volume of higher frequency sounds. The sound
if recorded right in a dimensional soundfield will sound mostly like
variations in distance to the blades. You'll hear each blade pass.
The pressure wave's effect on other soundwaves may be kind of like a
doppler effect, cyclically varying the frequency and volume.
A widefield stereo setup pointed up at the blades would be what I'd try,
the MKH-110 mod SASS would be my choice as it would feed the recorder
all it could record. Even the MKH-20 mod SASS would do ok if you abandon
trying for the infrasound component. Try any stereo setup, preferably
one with very quiet and smooth self noise and good sensitivity. I'd
expect that directly under the blades is not going to be the loudest
point but a little ways in front or back of them would get better.
A quality pair of headphones will probably be the best for listening,
ones that have very good low frequency reproduction. This is not what
speakers are any good at.
Walt
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