🚀 go-pugleaf

I've just recently obtained a Larson-Davis 2900 Sound level and vibration 
analyzer (about $9,000), which analyses analog signals down to 0.8 Hz.
It will do FFT, but More importantly, it does 1/3 octave band analysis with
digital filtering. This will side-step all your instrument development woes, in 
my opinion.

I've just used mine to measure vibrations in the few Hz to few dozen Hz region.

Filter settling time is about one minute, but when it settles, you have a
measurement accurate to a tenth of a decibel (1% amplitude ) for each band.

Hopes of seeing neat narrowband signals are usually dulled by reality.

When looking at community noise which included cricket chirps, I thought the
FFT would show them as a few lines, but lo and behold, they smeared over many
lines.  I guess the crickets were  FM'ing a little (or 'chirping' as in sonar 
parlance!).

The beauty of the 1/3 octave bands is that down in the few Hz region, most
off-the-shelf FFT analyzers fall flat on their face.  Typically, 400-line FFT
analyzers working across a 1,000 Hz band has filters that are 2.5 Hz wide (ALL
of them).  Down in your shedding frequency area, that's a whopping big chunk of
your working spectrum to average across.

On the other hand, the 1/3 octave filters are centered at the ISO frequencies
of 0.8, 1.0, 1.2, 1.6, 2, 2.5, 3.1, 4, 5, 6.3, 8, 10 Hz, etc.. giving handy
resolution where you need it most.

The LD 2800 (1 channel version) is off the shelf.  Larson-Davis is located in
Provo, Utah (801)-375-0177, FAX=375-0182, Ask for Derek Anderson. Tell him I
recommended this to you.

These units have a high impedance input, so that the hot wire anemometer should
work OK.  If the ac signal therefrom is feeble, you may need a preamp..., but
try it straight-in first..

                                     Ang. Campanella.