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Resource Documents: Noise (528 items)


Also see NWW press release on noise

Documents presented here are not the product of nor are they necessarily endorsed by National Wind Watch. These resource documents are provided to assist anyone wishing to research the issue of industrial wind power and the impacts of its development. The information should be evaluated by each reader to come to their own conclusions about the many areas of debate.

Date added:  February 13, 2015
Australia, Health, NoisePrint storyE-mail story

Review of the Cape Bridgewater acoustic testing program and where it is leading

Author:  Schomer, Paul; and Hessler, George

Recently [Steve Cooper, The Acoustic Group,] has completed a first of its kind test regarding the acoustical emissions of wind turbines. His is the first study of effects on people that includes a cooperating windfarm operator in conjunction with a researcher that does not work exclusively for windfarms. This study makes three very simple points:

  1. There is at least one non-visual, non-audible pathway for wind turbine emissions to reach, enter, and affect some people
  2. This is a longitudinal study wherein the subjects record in a diary regularly as a function of time the level of the effects they are experiencing at that time
  3. This periodic recording allows for responses as the wind-turbine power changes up and down, changes not known by the subject

The results are presented in a 218 page report augmented by 22 appendices spread over 6 volumes so that every single detail in the study has been documented for all to see and examine. The methods and results are totally transparent. The 22 appendices and the main text exhaustively document everything involved with this study.

Six subjects, 3 couples from different homes are the participants in this study. They do not represent the average resident in the vicinity of a wind farm. Rather, they are self-selected as being particularly sensitive and susceptible to wind farm acoustic emissions, so much so that one couple has abandoned their house. Cooper finds that these six subjects are able to sense attributes of the wind turbine emissions without there being an audible or visual stimulus present. More specifically, he finds that the subject responses correlate with the wind turbine power being generated but not with either the sound or vibration.

Although the very nature of a longitudinal study provides for a finding of cause and effect, some will undoubtedly argue that a correlation does not show cause and effect. In this case they must postulate some other thing like an unknown “force” that simultaneously causes the wind turbine power being generated and symptoms such as nausea, vertigo, and headaches to change up and down together. But that is the kind of “creative” logic it takes to say that this correlation does not represent cause-and-effect. So, rather than making such groundless arguments, perhaps something like an “expert statistical analysis” can be expected “proving” this is not a “valid sample” of the public at large, or proving the study does not do something else it was never intended to do.

So it is important to sort out what, by design, this study was intended to do and does do, and what, by design, it was not intended to do and does not do. This study is not in any way a sample of the general population nor is it in any way a sample of the general population in the vicinity of windfarms. According to Cooper’s report, this study was intended to address the issue of complaints from residents in the vicinity of Pacific Hydro’s Cape Bridgewater Wind Farm. Pacific Hydro requested the conduct of an acoustic study at 3 residential properties to ascertain any identifiable noise impacts of the wind farm operations or certain wind conditions that could relate to the complaints that had been received. The study was to incorporate three houses that are located between 650 m and 1600 m from the nearest turbine. This research represents a case study at 3 houses, each with one couple, 6 people. This is one sample, and only one sample, of a small group of people who are all self-selected as being very or extremely sensitive to wind turbine acoustic emissions. A similar group could be assembled elsewhere such as in Shirley Wisconsin, USA or Ontario Canada.

This study finds that these 6 people sense the operation of the turbine(s) via other pathways than hearing or seeing, and that the adverse reactions to the operations of the wind turbine(s) correlates directly with the power output of the wind turbine(s) and fairly large changes in power output.

Attempts may be made to obfuscate these simple points with such arguments as it cannot be proved that infrasound is the cause of the discomfort. But that again is a specious argument. The important point here is that something is coming from the wind turbines to affect these people and that something increases or decreases as the power output of the turbine increases or decreases. Denying infra-sound as the agent accomplishes nothing. It really does not matter what the pathway is, whether it is infra-sound or some new form of rays or electromagnetic field coming off the turbine blades. If the turbines are the cause, then the windfarm is responsible and needs to fix it. Anyone who truly doubts the results should want to replicate this study using independent acoustical consultants[1] at some other wind farm, such as Shirley Wisconsin, USA, where there are residents who are self-selected as being very or extremely sensitive to wind turbine acoustic emissions.[2]

Some may ask, this is only 6 people, why is it so important? The answer is that up until now windfarm operators have said there are no known cause and effect relations between windfarm emissions and the response of people living in the vicinity of the windfarm other than those related to visual and/or audible stimuli, and these lead to some flicker which is treated, and “some annoyance with noise.” This study proves that there are other pathways that affect some people, at least 6. The windfarm operator simply cannot say there are no known effects and no known people affected. One person affected is a lot more than none; the existence of just one cause-and-effect pathway is a lot more than none. It only takes one example to prove that a broad assertion is not true, and that is the case here. Windfarms will be in the position where they must say: “We may affect some people.” And regulators charged with protecting the health and welfare of the citizenry will not be able to say they know of no adverse effects. Rather, if they choose to support the windfarm, they will do so knowing that they may not be protecting the health and welfare of all the citizenry.

[1] Independent Consultants are those who have worked for both industry and communities, and or have espoused the need for research to sort out the issues of people reacting to non-audible non-visual stimuli.

[2] Cooper’s test shows cause and effect for at least one non-visual, no-audible pathway to affect people. If one only wanted to test for the ability to sense the turning on of wind turbines, and not replicate the cause and effect portion of Cooper’s study, this reduced test could be accomplished in one to two months with a cooperative windfarm where there are residents who are self-selected as being very or extremely sensitive to wind turbine acoustic emissions and who also assert that they have this sensing ability. This study, a subset of the full Cooper tests, would only prove, again, that non-visual, non-auditory pathways exist by which wind turbine emissions may affect the body and “signal” the brain.

Paul D. Schomer, Ph.D., P.E.
Schomer and Associates, Inc.
Standards Director, Acoustical Society of America

George Hessler
Hessler Associates, Inc.

10 February 2015

Download original document: “Review of the Cape Bridgewater acoustic testing program and where it is leading”

20 February 2015:  Download “Further comments on the Cape Bridgewater Wind Farm Study — Muddying the waters”

9 March 2015:  Download “Comments on the Cooper Study at the Cape Bridgewater Wind Farm and Wind Turbine Infrasound by George Hessler”

15 March 2015:  Download “Reply from Sarah Laurie, Waubra Foundation, to comments by George Hessler”

Go to: “Results of an Acoustic Testing Program – Cape Bridgewater Wind Farm”

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Date added:  January 31, 2015
Canada, Noise, TechnologyPrint storyE-mail story

Wind turbine noise propagation below 100 Hz

Author:  MG Acoustics

The measurement of sound at very low frequencies, below 100 Hz, is difficult and requires the use of special instruments. In the Wind Turbine Noise & Health Study conducted by Health Canada, microbarometers were used because they are capable of measuring frequencies between 0.1 Hz and 100 Hz. Microbarometers were installed at distances of 125 m, 2.5 km, 5 km and 10 km from the nearest wind turbine in […] wind turbines.

The noise levels from a wind turbine can and has been predicted using mathematical models. However, commercially available software that is in general use today cannot be used for very low frequencies and long distances. For example, the calculation procedure published by the International Standards Organization (ISO) is not intended to be used for frequencies below 63 Hz. Further, the ISO procedure was not originally intended to be used for distances greater than 1 km or for sources as high as modern day wind turbines […]. …

One key source of noise from wind turbines is the periodic passage of the wind turbine blades in front of the main supporting mast. There must be sufficient wind for the operation of a wind turbine. However, for wind speeds greater than approximately 8 metres per second (m/s), the wind turbine rotors in the current study were restricted to rotational speeds no greater than 16 revolutions per minute (RPM). With three blades spinning at 16 RPM, a blade passes the mast 48 times per minute, or once every 0.8 second (i.e. 0.8 Hz). Measurements of wind turbine noise showed a peak in the spectrum at 0.8 Hz with additional peaks at 1.6 Hz, 2.4 Hz, 3.2 Hz, 4.0 Hz, 4.8 Hz, 5.6 Hz, 6.4 Hz, 7.2 Hz, and 8.0 Hz. The presence of a peak at 0.8Hz and the associated harmonics of this frequency in field measurements confirms that the measured sound is (at least partially) resulting from wind turbine operations. These frequencies below 20 Hz are generally called infrasound. At frequencies measured between 20 and 100 Hz, there are other sources of noise, such as the gearbox or generator.

Like all sound, when wind turbine noise propagates over flat ground, levels generally decrease as one moves away from the source. Estimating what the levels will be at any given receptor requires an understanding of how fast the levels decrease over distance. The main factor affecting the decrease in levels for frequencies below 100 Hz is the prevailing weather conditions. [The prevailing weather conditions refer to the wind speed, wind direction, the amount of cloud cover, and daytime or nighttime.]

During the day, the sun heats the ground resulting in an air temperature that is highest close to the ground. Sound travels faster in warm air than cold air. As a consequence, when traveling along the ground, sound will bend from the warmer air towards the colder air above causing the sound rays to curve upwards away from the ground as illustrated in Figure 1. Sound rays also curve upwards when the sound is propagating upwind. The upward-curving rays cause the sound at ground level to decrease in level very rapidly as one moves away from the turbines.

Figure 1

Figure 1

During the night, the ground cools faster than the air causing a temperature inversion. During an inversion, sound rays curve downward towards the ground as illustrated in Figure 2. Sound rays also curve downward when the sound is propagating downwind. The downward-curving rays cause the sound to decrease in level relatively slowly as one moves away from the turbines.

The difference between daytime and nighttime conditions influenced the ability to measure wind turbine noise in this study, Thus, noise from wind turbines was more often measured during the night or downwind from the turbines at distances beyond 2.5 km.

Figure 2

Figure 2

The topology [hills and valleys of non-flat ground] of the surrounding area is another import ant factor that affects how sound levels will decrease over distance. If the wind turbine is hidden by a hill, the sound levels will be reduced. On the other hand if the wind turbine is situated on the top of a hill, the sound levels are often apparent at larger distances .

Other factors that normally affect how sound levels decrease as the distance to a source of noise increases include the type of ground cover (snow, grass, pine needles, asphalt, etc.) and air absorption. [Air absorption is caused by the collision of the air molecules. The collisions produce a rapid decrease in sound level at high frequencies as sound propagates, but has only a negligible impact on frequencies below 100 Hz.] Sound frequencies above 100 Hz do not propagate far when there is a fresh layer of snow, for example. Instabilities in the air called turbulence can also affect how sound levels drop under certain weather conditions. However, for wind turbine frequencies below 100 Hz, these other factors are not significant.

In addition to the prevailing weather conditions, topography and ground coverage, the number of wind turbines in operation will determine the levels measured by the microbarometers. In the current study, measurements were based on the collective contribution from only four wind turbines. As the number of turbines increases, the noise levels are expected to increase. Similarly, if the number of turbines decreases, the noise levels are expected to decrease.

One of the main challenges found in this study related to measurement of wind turbine noise below 100 Hz was separating the wind turbine noise from the ambient background noise. The ambient background noise is comprised of noise generated by man-made sources, such as highway traffic, trains, aircraft, and industry, and by naturally-occurring sources including surf-generated infrasound noise. The separation was only possible when the measured spectra showed the characteristic peaks related to the blade passage frequency as discussed above.

Figure 3 shows noise generated by a wind turbine clearly evident above the ambient background. The red peaks are the infrasonic frequencies between 0.8 Hz and 8.0 Hz produced by the […] windturbines. They are an example measured by the microbarometer at a distance of 2.5 km during one night. The blue line is the ambient background noise. Observe that the blade passage frequency of 0.8 Hz is just evident above the ambient background noise around this frequency.

Figure 3

Figure 3

By comparison, Figure 4 shows what the microbarometer typically measures at the same distance during a sunny day (note that Figure 4 and Figure 3 are plotted on the same scale). The peaks characteristic of the wind turbines have dropped in level so much that they cannot be seen above the ambient background noise even though the turbines were in operation. Finally, we note that as the wind speed increases, both the ambient background noise levels and the wind turbine frequencies (when present) will change.

Figure 4

Figure 4

Levels measured by the microbarometers can be compared to theoretical predictions at different distances from the wind turbines by focusing on the characteristic wind turbine peaks. A comparison for the infrasonic frequency of 2.4 Hz is shown in Figure 5. The red circles are the average levels measured during several nights when there was an inversion and the microbarometers were downwind from the wind turbines. Using the temperature measured at heights of 2 m and 10 m, wind speed and wind direction measured at a height of I0 m at a distance of 2.5 km from the wind turbines, the computational models predict levels shown by the red line. The measured levels (circles) and the predicted levels (line) are in agreement at 2.5 km, 5 km, and 10 km.

Figure 5

Figure 5

The magenta circles in Figure 5 are the levels at 2.4 Hz measured during an afternoon in July. For this daytime condition, the levels decrease much more rapidly as one moves away from the wind turbines, but can still be measured at 2.5 km and 5 km during this July afternoon. However, at 10 km, the microbarometer cannot measure the contribution from the wind turbines at this infrasonic frequency. The magenta line represents the predicted levels obtained using the weather station measurements. The measured levels and the predicted levels are in agreement at 2.5 km and 5 km.

The drop in levels seen in Figure 5 at 2.4 Hz was also seen at the other frequencies during the same weather conditions. For example, the red circles in Figure 6 are the average levels at a frequency of 30 Hz also measured during several nights when there was an inversion and the microbarometers were downwind from the turbines. Note that at 10 km, the microbarometer was not able to measure this frequency. The red line shows the predicted levels which agree with the measured levels at 2.5 km and 5 km. It was further found that it was not possible to separate wind turbine noise from the ambient background noise at frequencies above 30 Hz and at distances of 2.5 km and beyond.

Figure 6

Figure 6

Observe in Figures 5 and 6 that the levels are dropping relatively slowly as one moves from 2.5 km to 10 km during the nighttime inversions (red lines). In fact the levels are dropping by 3 dB for each doubling of distance. For example, the level at 2.4 Hz is 49 dB at 5 km and drops to 46 dB at 10 km.

At distances over 1 km the 3 dB decrease in level for each doubling of distance is typical o flow frequency sound below 100 Hz during an inversion and/or downwind propagation and has been observed in other wind turbine noise studies. [At distances less than 1 km, the decrease in level when only one turbine is in operation will not be the same as when all four are in operation, for example.] For comparison, at frequencies above 100 Hz, the type of ground cover and air absorption become much more significant at affecting how sound levels decrease with distance. For example, traffic noise contains frequencies above 100 Hz. According to the calculation procedure of the ISO, traffic noise is expected to decrease by 7 to 8 dB for eve1y doubling of distance between 0.5 km and 2 km during a temperature inversion or downwind from the traffic.

One outcome of this study is a general idea of how often wind turbine noise below 100 Hz is likely to be measured up to a distance of 10 km. Weather observations published by Environment Canada were monitored for an entire year. The weather observations were classified in terms of wind speed and daytime and nighttime cloud cover. For example, in 2013 at one weather station, nighttime inversion conditions occurred about 45% of the time (or 3942 hours out of 8760 hours. Thus, taking into account wind speed and direction, infrasonic frequencies from wind turbines are likely be measured fairly often at distances up to10 km, and even beyond.

In conclusion, measuring wind turbine noise below 100 Hz requires special equipment. The ability to measure wind turbine noise depends strongly on the prevailing weather conditions and the ambient background noise, especially at large distances. Further, it was found that wind turbine noise below 100 Hz can be predicted with accuracy down to a frequency of 1.6 Hz and up to distances of 10 km using on-site weather station measurements.

Download original document: “Summary of wind turbine noise propagation below 100 Hz”

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Date added:  January 31, 2015
Noise, Ontario, Prince Edward Island, TechnologyPrint storyE-mail story

Analysis, modeling, and prediction of infrasound and low frequency noise from wind turbine installations

Author:  MG Acoustics

MG Acoustics has carried out the analysis, modeling, and prediction of infrasound and low frequency noise from wind turbines at two different sites, as part of the Health Canada study. This work has been divided into two parts, Phase 1 and Phase 2, associated with a Prince Edward Island site and a Southern Ontario site, respectively. There are several overall objectives:

Infrasound and low frequency noise from two wind turbine sites (PEI and Southern Ontario locations) has been addressed. This work allows Health Canada to evaluate whether or not infrasound and/or low frequency noise (from wind turbines in the locations specified) can be detected at different distances; and secondly to determine whether the Parabolic Equation method of calculation gives an adequate explanation of the experimental values with regards to infrasound and/low frequency and distances at which it can be detected. Thirdly, the results should allow Health Canada to reliably make infrasound and low frequency noise predictions (using Harmonoise) at southern Ontario sites.

The work has been completed in two phases:

1st Phase – Analysis of infrasound and low frequency noise measurements and analysis of meteorological data will be completed including the generation of theoretical predictions at the PEI site. This phase of the project has been described in the report “Analysis, Modeling, and Prediction of Infrasound and Low Frequency Noise from Wind Turbine Installation. Phase 1: PEI Site. Final Report”, submitted in February 2014.

2nd Phase – Modeling has been carried out and applied to wind turbines sites in southern Ontario. This phase of the project has been described in the report “Analysis, Modeling, and Prediction of Infrasound and Low Frequency Noise from Wind Turbine Installation. Phase 2: Southern Ontario Site. Final Report”, submitted in February 2014.

Wind turbine noise calculation results

This file presents results from the calculation of wind turbine noise levels for 1238 homes in the study. Noise results are presented according to the distance from the closest wind turbine to the participant’s home.

dBA calculations were based on wind turbine sound power levels from the manufacturers, which were verified for consistency with field measurements, and were derived according to international standards (ISO 9613-1 and ISO 9613-2), which were incorporated into a sound propagation modelling package (Cadna A version 4.4). The model also took into account geographical features which can influence sound propagation around the dwellings in the study, such as topography, vegetation and water features.

dBC noise levels were also derived from manufacturer supplied sound spectra and were supplemented by field measurements to extend the wind turbine sound power levels to lower frequencies (down to 16Hz). Following the same methodology and parameters that were used to determine A-weighted levels, the C-weighted sound levels were derived using the Cadna A version 4.4 software package.

The standard uncertainties in these results are +/- 30m for the distances to the nearset wind turbine and +/-5dB for the dBA and dBC noise levels for residences that are situated up to 1.6 km to the closest wind turbine. After 1.6 km, the uncertainties, evaluated according to the ISO 1996-2 standard, are derived according to the following formula: 1 + d/0.4, where d represents the distance to the nearest turbine (in km). As such, the uncertainty for a dwelling that is situated 10km away would be +/- 26 dB.

When examining these results, it is important to keep in mind that although some dwellings may be situated at approximately the same distance to the nearest wind turbine, they can receive different noise levels. This can be explained by the fact that each residence can be exposed to different numbers and models of wind turbines, which can generate more or less noise depending on their power output and physical characteristics, as well as the different geographical features that surround each residence, which can have an impact on noise propagation.

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Date added:  January 24, 2015
Australia, Noise, TechnologyPrint storyE-mail story

Results of an Acoustic Testing Program – Cape Bridgewater Wind Farm

Author:  Acoustic Group

A number of findings arise from the study and have been grouped as follows.

11.1 Non-acoustic findings

The following non-acoustic findings of the study are considered to be significant:

11.2 Acoustic findings

There are a significant number of acoustic based findings obtained from the study. Due to the complex interaction of various components of the study the findings have been grouped as follows.


Other acoustic parameters, including infrasound





Turbine operation

11.3 Subjects of further investigation

As the basis of the study was to start from the complaints end of the equation, rather from a noise end, material/advice and comments are provided to assist others in further studies or an extension to this study in relation to wind farm operations.

11.4 Suggestions

During the course of the study there were significant issues in terms of instrumentation that requires for other researchers in this area identification of problems and the essential need for persons involved in the measurements of noise, and particular infrasound, in proximity to wind farm affected environments to utilise calibrated instrumentation covering the entire signal chain from the microphone (or pressure sensor) through to the read out. Reliance upon manufacturer’s data does not always cover the entire spectrum of concern, with an entire section of this study report addressing instrumentation issues that have been established during this study.

From the resident’s subjective observations a wind turbine signature has been derived that indicates the averaged unacceptable presence of sensation inside a dwelling (for those 6 residents) occurs at an level of 51 dB(WTS) – when assessed as rms values 400 lines for analysis range of 25 Hz. Utilising PSD values (400 line 25 Hz range) the unacceptable level for the 6 residents occurs at 61 dB(WTS).

Being the first study to document or to identify “sensation” associated with the wind farm and the wind turbine signature, it is noted that the sample data is small and has persons already affected by the “noise”. The findings must be considered as preliminary and warrants further detailed studies of the scientific rigour necessary for the purpose of confirming/verifying the suggestions for the use of the nominated dB(WTS) thresholds.

On the basis of a limited number of affected residents for the study, it is suggested that:

On the basis of a limited number of affected residents for the study, it is suggested that:

Download the original documents from the following links:
The Results of an Acoustic Testing Program – Cape Bridgewater Wind Farm
Appendices A to H
Appendices I to J
Appendices K to M
Appendices N to P
Appendices Q to S
Appendices T to V

Letters of endorsement:
Bob Thorne, Noise Measurement Services
Stephen Ambrose, S.E. Ambrose & Associates
Robert Rand, Rand Acoustics
Carmen Krogh, independent researcher
Richard Mann, University of Waterloo
Les Huson, L Huson & Associates — with corroborating data

Review of the Cape Bridgewater acoustic testing program and where it is leading
Further comments on the Cape Bridgewater Wind Farm Study — Muddying the waters
—Paul Schomer, Schomer and Associates, Standards Director, Acoustical Society of America; and
George Hessler, Hessler Associates

Radio Victoria interviews:

Steven Cooper (principal, Acoustic Group)

Andrew Richards (executive manager, external affairs, Pacific Hydro)

Annie Gardner (resident)

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