The general concept for describing the noise environment in proximity to a wind farm is expressed in terms of the A-weighted level that will vary dependent upon the wind strength. The compliance methodology in general use for wind farms relies upon a measurement that includes wind and an average line of fit through such data. Measurements at residential receivers when conducted using full spectrum recording/analysis revealed unique characteristics extending into the infrasound region that are normally inaudible that would appear to be present when disturbance is noted. There are no traditional dose-response investigations for the full spectrum of wind farm noise on which to describe the soundscape. A different approach to assessing wind farm noise emissions was used for the Cape Bridgewater wind farm study identifying different concepts for describing the soundscape of the wind farm.
In considering the development of soundscape for wind farms the use of the A-weighted level has limited value.
For development of the soundscape for a wind farm much more work needs to be undertaken to establish the actual impacts that are related to the relatively low A-weighted noise levels emitted by turbines (when compared to road traffic noise). The presence of inaudible infrasound signatures, modulation and tonal characteristics that do not get considered or picked up in general environmental acoustics needs to be measured, and studied in greater detail.
For road traffic, rail traffic or aircraft noise assessments, generally the noise events being considered generate audible noise well above the background level and are easily measured.
In considering the impacts of wind turbines one has a noise contribution similar to or below the ambient background level. As such there needs to be an acknowledgement that the limited dose response data is very much out of date and if restricted to predicted A-weighted levels, not the actual sound level contribution, is therefore not appropriate for assessing the impact. The presence of special frequency characteristics (including infrasound and low frequency) are not the normal everyday concept in environmental acoustics, but are issues that require a different approach and a more detailed investigation into determining the soundscape of a wind farm.
Acousticians working in noise control for industry are well aware a small number of complainers who persistently complain require a different approach to that from the general acoustical planning concepts for industry because there is a need to investigate the noise signature the relates to the disturbance. Unique situations/operation of compressors, exhaust ducts, stacks etc., that generate an intermittent noise that may have special frequency characteristics (including low frequency and infrasound) are not the normal everyday concept in environmental acoustics, but are issues that have required a different approach in resolving the complaints.
If one considers the impact of wind farms on an acoustic environment from the perspective of complaints, rather than from computer-generated predicted levels, undertaking acoustic assessments from that perspective will provide a more meaningful appraisal of the soundscape for such areas.
For the benefit of soundscape researchers, the methodology employed in Cape Bridgewater study to work from the complainant end of an acoustic investigation should be considered as an appropriate method for investigating complaints where there is a heightened level of disturbance.
For determination of a wind farm soundscape the social survey method used in the Cape Bridgewater study (that added the concept of vibration and sensation) should be considered as sensation was the major disturbance reported by the residents, rather than noise or vibration.