Wind farm infrasound—Are we measuring what is actually there or something else? (Part 2)

In predigital acoustics, low frequency analysis used analog narrow band filters and cathode ray oscilloscopes for special problems leading to the general use of peak values. Analog filters have time constants that can affect the derived rms values requiring caution where high crest factors are involved. Modern narrowband digital analysis based on a FFT of the time signal extracts the periodic function that occurs in the time domain that are then displayed as discrete peaks in the frequency domain. FFT analysis of turbines show discrete infrasound peaks at multiples of the blade pass frequency in addition to sidebands in the low frequency range spaced at multiples of the blade pass frequency. Are these signals actually there or are they a product of modern day analysis. Is the infrasound signature a clue to a different area of investigation? In Jacksonville, Part 1 presented the complexity of the investigations and showed how the raw Pascal data are lost when converting to SPL and then A-weighting. Part 2 presents the results of different filtering techniques for different wind farms.