Cooper, S. Wind Farm Infrasound, Are We Measuring What Is Actually There….
170th Meeting of the Acoustical Society of America
Jacksonville, Florida 02-06 November 2015
Wind farm infrasound – Are we measuring what is actually there or something else?
Steven E. Cooper
In the olden days of acoustics (pre digital), low frequency analysis used analogue narrow band filters and cathode ray oscilloscopes for special problems leading to the general use of peak values. Analogue 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 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.
General acoustics deals with audible sounds, dB(A) levels, octave bands and sometimes 1/3 octave bands. Environmental acoustics incorporates these measurement parameters and looks at noise levels emerging above background levels.
Normally the acoustic assessment of a wind farm is based on the energy average of the A-weighted level over a sample period (e.g. 10 minutes). The A-weighting filter dramatically reduces the contribution of low frequency components when compared with the mid band and high frequency components.
Wind turbines generate noise detected in the normal range of hearing and also generate energy in the infrasound range of hearing (generally expressed as below 20 Hz). Wind turbines follow the laws of physics in relation to fans and create a specific signal at the blade pass frequency and harmonics of that frequency. However, due to the size of the turbines and different wind speeds that may occur over the vertical profile of the turbine the noise across the radiation plane in front (or behind) the swept path is not uniform.
Measured levels of infrasound in proximity to wind turbines are generally below what is assumed to be a threshold of hearing for such frequencies. Similarly, our testing of infrasound levels in houses impacted by turbines consistently find levels below the threshold of hearing.
Guidelines, Standards and permits used in Australia , ,  &  provide an allowance for “special audible characteristics” identified as low frequency noise, tonal noise and amplitude modulation. The nature of variation in noise levels as a result of the operation of turbines, that may also include the concept of tonality and amplitude modulation, are not addressed in the A-weighted Leq level.
Audible noise in the low frequency region (below 200Hz) is often reported by residents for both internal and external locations. In some cases, the low frequency sound is supplemented by a mid-band varying sound resulting in descriptions of both a swish and a thump.
In guidelines used for wind farm assessments in Australia  &  “amplitude modulation” is identified as the variation of the A-weighted level occurring at the blade pass frequency. However, extraneous ambient noise can often influence the A-weighted levels and mask the amplitude modulation.
A more detailed concept to distinguish between amplitude modulation and excessive amplitude modulation has been used in the UK  as a tool for identifying the audible characteristics of turbines that do not appear in the variation of the A-weighted level.
The author has previously raised the question as to whether the variation of the measured signal is “amplitude modulation” or “modulation of the amplitude that is occurring at an infrasound rate”.
The abstract for this paper covers a wide area of investigations which are not complete. As a result, this paper looks to the results of infrasound measurement with respect to amplitude modulation and uses a method recently suggested in the UK . Further work in relation to discrete frequency analysis (using pre digital analysis techniques) has not yet been completed.
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 A- weighted 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.
Source: Acoustical Society of America Volume 25 http://scitation.aip.org/content/asa/journal/poma/25/1/10.1121/2.0000177