Reproducing wind farm infrasound for subjective testing – Just how accurate is the reproduced signal? 

In response to investigation of residents’ complaints concerning the operation of wind turbines, independent acousticians have identified the presence of a discrete infrasound/low frequency signature associated with the operation of the turbine to be present when such turbines are operating.

The discrete signature of turbines when using narrowband analysis reveals peaks at the blade pass frequency (and harmonics of that frequency) to occur in the lower portion of the infrasound frequency band, generally below 10 Hz and a peak with sidebands around what may be the gearbox output shaft speed.

Attenuation of infrasound over distance occurs at a lower rate than that of normal sound, resulting in the discrete infrasound signature of turbines being recorded up to 7 km from wind farms, and in some situations even greater distances.

Infrasound measurements of the natural environment in rural areas free from the influence of wind turbines whilst revealing similar broadband levels of infrasound (for example using dBG or even 1/3 octaves) do not experience a discrete periodic pattern similar to that associated with rotating blades on wind turbines when assessed in narrow bands.

In seeking to assess the audible characteristics of wind turbine noise, being different to that of general traffic or environmental noise, laboratory studies have sought to use speakers to generate or to reproduce recorded signals for test subjects in a controlled environment. …

As the impact of the turbine’s inaudible infrasound on people has not been studied in controlled studies, of critical importance in the laboratory assessment of wind turbine “noise” is the question as to whether the source signals generated in the laboratory are full spectrum and reproduce the original signal (that includes by narrowband analysis infrasound). …

Tachibana [Yokoyama S, Kobayashi T, Sakamoto S & Tachibana H, “Subjective experiments on the auditory impression of the amplitude modulation sound contained in wind turbine noise”, International Meeting on Wind Turbine Noise, Glasgow 2015] used a set of reverberation chambers to evaluate full spectrum sound of wind turbines. However, the primary issue presented in the paper was looking at the A-weighted level with different low pass filtering and modulation. Reference [5] did not examine infrasound specifically but concluded that frequency components below 25 Hz are not audible which is to be expected for the levels that were tested. As a side issue to the investigation of the A-weighted levels and audibility of the modulation, the audible modulation effects were identified as associated with low frequency.

Walker [Walker B & Celano J, “Progress report on synthesis of wind turbine noise and infrasound”, 6th International Meeting on Wind Turbine Noise, Glasgow 2015] provided results of generating infrasound signals synthesised from narrow band Leq analysis to find no impact. No frequency response was provided to define the output of the synthesised infrasound signal generated by a speaker. There is an assumption the system equalisation curve resulted in a flat spectrum.

Walker [Hansen K, Walker B, Zajamsek B & Hansen C, “Perception and annoyance of low frequency noise versus infrasound in the context of wind turbine noise”, International Meeting on Wind Turbine Noise, Glasgow 2015] started with external wind farm noise samples from the Waterloo wind farm that were then synthesised from the narrow band frequency spectrum to provide the source signal.

Tonin [Tonin R & Brett J, “Response to simulated wind farm infrasound including effect of expectation”, International Meeting on Wind Turbine Noise, Glasgow 2015] used a synthesised infrasound signal applied to a pnuematic driver connected to modified hearing protectors.

Crichton [Crichton F, Dodd G, Schmid G, Gamble G & Petrie K, “Can expectations produce symptoms from infrasound associated with wind turbines?”, Health Psychology, 33(4), 360-364 (2014); Crichton F, Dodd G, Schmid G, Gamble G, Cundy T & Petrie K, “The power of positive and negative expectations to influence reported symptoms and mood during exposure to wind farm sound?”, Health Psychology, American Psychological Association 2013] used single infrasound tones inserted into broad band noise for an assessment of “wind turbine infrasound”. …

Issues of concern with the use of simulated “infrasound” are:

• Whether the synthesised signal (obtained from adding sine waves) reproduces the actual time signal that occurs in the field.
• “Infrasound” applied as single tones and then attributed as being the signal generated by wind farms.
• Testing of synthesised signal and claiming the results apply to wind farms.
• Accurately reproducing the Wave file signal by the use of speakers.

Steven Cooper, The Acoustic Group, Lilyfield, NSW, Australia

171st Meeting of the Acoustical Society of America, Salt Lake City, Utah, 23-27 May 2016. Noise: Paper 4aNS10

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