In the olden days of acoustics (pre digital), 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 is based on a FFT of the time signal to extract 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? The paper will show the results of testing to compare old fashioned and modern day analysis.
There are challenges in obtaining accurate full spectrum recordings of the source signals or obtaining a linear analogue output (valid from DC) for existing full spectrum WAV files.
The use of unfiltered time signals in terms of pressure variations (double sided) shows a clear modulation in terms of the pressure level that is not so apparent when expressed in terms of the dB SPL (one-sided), and becomes non-existent when considering the Aweighted SPL.
In a “traditional” narrowband analysis of 10-minute sample the results indicate the presence of discrete tones associated with the blade pass frequency and harmonics of that frequency.
Adopting the UK approach  to examine individual 1/3 octave bands that stand out above the ambient Leq level (when A-weighted) show that there is a modulation of those frequencies occurring at the blade pass frequency with the time signal having a mixture of pulses related to the blade pass frequency and harmonics of that frequency.
The internal sound pressure levels are below the nominal threshold of hearing (obtained using 1/3 octave bands) for both the general audible frequency range and the infrasound range.
With additional amplification, the playback of the signals led to the identification of typical turbine audible components that produced maximum levels as a time varying signal which varies at the rate of the blade pass frequency.
The preliminary results of the investigation suggest that the amplitude modulation method adopted in the UK may very well have a corresponding relationship to the presence of discrete infrasound frequencies described by the author in the Cape Bridgewater study  as the Wind Turbine Signature (the “WTS”), and that an increase in the amplitude modulation by the UK method relates to an increase in the WTS.
The work to date has not identified the discrete infrasound signature is generated by the turbines in the normal sense of a sine wave. A question yet to be addressed is “Is the WTS the result of a FFT analysis of the modulation of the low frequency noise generated by the blades?” The preliminary results indicate another current line of our investigations into the thresholds of infrasound (hearing and perception – for both sine wave and pulsed waves) should be extended to include inaudible modulated low frequency noise.
Further work in examining other infrasound measurement results (different turbine types and large ventilation fans) will be carried out in light of the preliminary analysis described herein.