Wisconsin Study Finds Wind Turbine Noise at Sub-audible Levels

Infrasound and low frequency noise (ILFN) were studied in a cooperative arrangement by firms who have worked for wind development companies and for wind turbine opponents. The study, released in late December, found that ILFN is a serious concern and that there are inadequate standards for measuring emissions at lower frequencies.

The study (“A Cooperative Measurement Survey and Analysis of Low Frequency and Infrasound at the Shirley Wind Farm in Brown County, Wisconsin”) was initiated by Clean Wisconsin with partial funding by the Wisconsin Public Service Commission.

The four investigating firms are of the opinion that enough evidence and hypotheses have been given herein to classify LFN and infrasound as a serious issue, possibly affecting the future of the industry. It should be addressed beyond the present practice of showing that wind turbine levels are magnitudes below the threshold of hearing at low frequencies.

The report has four appendices – one written by each acoustical firm.

One technical finding pointed to the inadequacies of using only sound waves measured by A-weighting.

Currently the wind turbine industry presents only A‐weighted octave band data down to 31 Hz. They have stated that the wind turbines do not produce low frequency sound energies. The measurements at Shirley have clearly shown that low frequency infrasound is clearly present and relevant.

…Thus, the International Electro‐technical Commission (IEC) standard needs to include both infrasonic measurements and a standard for the instrument by which they are measured.

Measurements were collected from three unoccupied homes (abandoned) in the Shirley Wind Park, a complex of eight Nordex100 (2.5 MW) wind turbines.

Duke Energy runs the wind park and refused to cooperate with the researchers’ needs for turbines to be turned ON and OFF. The study would have been more definitive in establishing impacts if the wind turbine data were collected under nearly identical wind and power conditions.

Here are some important quotes.

Dr. Bruce Walker, Channel Island Acoustics

The apparent and tentative result indicates that at the second residence, located approximately 1,280 ft from the nearest turbine, blade-passage induced infrasound was correlated between outdoor and indoor locations and peak amplitudes of periodic waves composed of blade harmonics 0.7 to 5.6 Hz on the order 76 dB were detected both indoors and outdoors. Well correlated broadband low frequency noise at this nearest residence was also detected, with one-third octave band sound pressure levels approximately 50 dB in the frequency range 16-25 Hz. Both of these sounds are below normal hearing threshold; residents report being intensely affected without audibility.

At the other two residences, located approximately 3,300 and 7,100 ft from the nearest turbine, respectively, high levels of infrasound were detected indoors but the correlation with outdoor acoustic signals was not clear except at the 3,300 ft residence, where the broadband noise in the 20 Hz range was moderately correlated and produce one-third octave band level approximately 40 dB, which is well below normal hearing threshold. At the 7,100 ft residence, outdoor-to-indoor correlation was low except during motor vehicle passages or in particular a helicopter overflight. Again, residents report being intensely affected despite inaudibility and to be aware of turbine operation when the turbines are not visible.

Hessler Associates:

Walker [Dr. Bruce Walker, Channel Island Acoustics] showed unequivocally that low level infrasonic sound emissions from the wind turbines were detectable during near full load operation with specialized instrumentation inside of residence R2 as a series of peaks associated with harmonics of the blade passing frequency. The long-term response of the inhabitants at R2 has been severely adverse for the wife and child while the husband has experienced no ill effects, which illustrates the complexity of the issue. The family moved out of the area to solve the problem.

The study also showed that a wind turbine is indeed a unique source with ultra low frequency energy.
The next figure plots the same R2 data above compared to a more commonly recognized low frequency noise source, an open cycle industrial gas turbine complex sited too close to homes.

Rand Acoustics

Nauseogenicity is a factor at Shirley. Acceleration of the inner ear is suggested due to extremely low-frequency pulsations at the rotation and blade pass rates that occur in or near the frequencies of highest potential for nauseogenicity and, are coupled strongly into the homes now abandoned. More research at Shirley is recommended to understand nauseogenicity from wind turbine operations, to properly design and site large industrial wind turbines (over 1 MW) near residential areas to prevent the severe health effects. More work is needed to establish what infrasonic levels are consistent with relief for the neighbors.

Medical research and measurement is urgently needed to be field coordinated along with infrasonic acoustic and vibration testing. The correlations to nauseogenicity at the 2.5MW power rating and size suggest worsening effects as larger, slower-rotating wind turbines are sited near people.

Neighbors do not always hear the turbines. The neighbors indicated there is no real difference in wind compass direction on the negative health effects. The house could be upwind, downwind or crosswind to the turbine; no difference.

The team agreed prior to testing that neighbor reports would be useful. They also agreed that neighbor reports are sincere and truthful, not “claims” as often alleged by the wind industry.

Schomer and Associates, Inc.

Four of the five researchers; George Hessler, David Hessler, Bruce Walker, and Paul Schomer met with affected residents of Shirley and discussed the problems they had that were precipitated by the wind turbines. This discussion produced several notable points not previously known by this researcher.
1. At most locations where these health problems occurred, the wind turbines were generally not audible. That is, these health problems are devoid of noise problems and concomitant noise annoyance issues. The wind turbines could only be heard distinctly a one of the 3 residences examined, and they could not even be heard indoors at this one residence during high wind conditions.
2. The residents could sense when the turbines turned on and off; this was independent of hearing the turbines.
3. The residents reported “bad spots” in their homes but pointed out that these locations were as likely to be “bad” because of the time they spent at those locations, as because of the “acoustic” (inaudible) environment. The residents certainly did not report large changes from one part of their residences to another.
4. The residents reported little or no change to the effects based on any directional factors. Effects were unchanged by the orientation of the rotor with respect to the house; the house could be upwind, downwind, or crosswind of the source.
5. Residents of the nearest house reported that their baby son, now 2 years old, would wake up 4 times a night screaming. This totally stopped upon their leaving the vicinity of the wind turbines, and he now sleeps 8 hours and awakens happy.

Paul Schomer lists implications of these observations.

1. The fact that these residents largely report wind turbines as inaudible, and the reported effects on a baby seem to rule out the illness being caused by extreme annoyance as some have suggested. 2. The lack of change with orientation of the turbine with respect to the house and the lack of change with position in the house suggest that we are dealing with very low frequencies; frequencies where the wind turbine size is a fraction of the wavelength‐‐about 3 Hz or lower.

In addition Paul Schomer did some preliminary literature review.

We consider a 1987 paper entitled: Motion Sickness Symptoms and Postural Changes Following Flights in Motion‐Based Flight Trainers. This paper was motivated by Navy pilots becoming ill from using flight simulators. The problems encountered by the Navy pilots appear to be somewhat similar to those reported by the Shirley residents.

The researchers concluded and added.

The completed testing was extremely helpful and a good start to uncover the cause of such severe adverse impact reported at this site. The issue is complex and relatively new. Such reported adverse response is sparse or non-existent in the peer-reviewed literature.

Virginia Irvine developed this summary of the report. Mark J. Cool also produced a summary for his Firetower Wind blog. National Wind Watch‘s post regarding the study offers the commentary of acoustician Rick James.