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Relating FFT magnitude to dBSPL
(11-23-2020, 03:40 PM)Jens Noritzsch Wrote: Dear Catherine,

you are perhaps aware that the acceleration spectrum does not help you that much with infrasound matters. I am also a bit unsure if smart device mics (the electronics part) are sensitive to infrasound at all, they would miss most of the range at least…

Hello Jens, thanks very much for your reply!  Your input helped me understand the FFT readings much better.  You are correct, it is not a source of what I want to know. Too bad for me. Undecided

Definitions can get unclear and contradictory with mechanical wave frequencies between about 1 Hz and about 50-60 Hz -- 'sound' is arbitrarily said to be what 'most' humans can 'hear'  (20-20000 Hz)but hearing isn't the only way humans perceive vibrations, and the nerves of the hearing center sends messages to many parts of the brain. I talk about frequencies only because I know nothing about the kinetic perspective and sciences. Sorry!

I've read that infrasound (2-20 Hz) acts like seismic ground waves so is appropriately detected via accelerometer in a smartphone, not the microphone. Also it causes resonance in building materials and human tissues such as vertebrae, lungs, eyes, viscera, teeth, sinuses, etc., which are mechanical waves traveling thu various coupled media (ground, structure materials, air human tissues).  Sound or not sound? Opinions disagree between fields.
If the infrasound levels are high enough to cause vibrations, these could be seen in the acceleration spectrum. “dbSPL” is an acoustics measure (*S*ound *P*ressure *L*evel) that makes no sense for kinematics. Acceleration is given in m/s² (velocity change per time interval)

Vibrations are tangible, and can be seen on surfaces of beverages (coffee, tea, water) by watching reflections.

The Fourier transform of the recorded acceleration gives you an impression of the relative intensities of frequencies, that's why it is given in arbitrary units (a.u.). If you put your smartphone on a washing machine rotating at 1200 rpm, you would see a sharp peak at 20 Hz, for instance.

This helps very much! I understand better now! Many Thanks!!

Catherine Julian

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RE: Relating FFT magnitude to dBSPL - by Catherine Julian - 11-23-2020, 06:00 PM

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