by Willem Post
(Editor’s note: the following is an excerpt from “Wind Turbine Noise and Air Pressure Pulses“. It was excerpted because of the information showing that “The Vermont code is hopelessly out of date and does not protect the public health, safety and welfare; it is a wind turbine vendor’s dream come true.”)
Traditionally, state and local government codes dealt mostly with measured sound values that are weighed (adjusted) using the A scale which covers most of the audible frequencies. The A scale corrects dB measurements according to the sensitivity of human hearing. It should not be used for frequencies less than 200 HZ, as the low frequency noise (LFN) and infrasound would be “weighed” out.
The following scales should be used to properly weigh all frequencies, especially those less than 20 Hz that are emitted by wind turbines:
Most audible noises in the range of 200 – 20,000 Hz; dB weighed with the A scale, dB(A).
LFN, in the range of 20 – 200 Hz; dB weighed with the C scale, dB(C).
Infrasound less than 20 Hz; dB weighed with the G scale, dB(G)*.
*The instrumentation to quantify infrasound frequencies and amplitudes is expensive and the values obtained vary with the method and instruments used. Applying the G scale to such values may not be meaningful.
The human ear can hear LFN at 95 dB(G) levels, the inner ear is sensitive to LFN at 65 dB(G) levels. Audible thresholds for perception, ToP, of 95 dB(G) represent the median response to a steady pure tone in a laboratory environment.
If a person is more sensitive to LFN and infrasound, say at the 10% boundary, the ToP may be as low as 85 dB(G) for a steady pure tone. The ToP will also be lower with multiple tones between 0 and 100 Hz that rapidly modulate in amplitude and frequency, as with wind turbine noise.
Professional acoustical engineers know the government codes, the outcome government regulators are expected to hear and conduct their tests according to standard procedures using mostly the A scale. Wind turbine vendors report sound levels adjusted to the A scale and everyone is satisfied. The LFN and infrasound are usually not covered by government codes.
According to the US EPA, noise levels above 45 dB(A) disturb sleep and most people cannot sleep at noise levels above 70 dB(A).
In Massachusetts, noise is considered pollution if it exceeds the ambient noise level by 10 dB(A). The Department of Environmental Protection, MassDEP, measures noise levels at the complainant’s location and at other nearby locations that may be affected, such as residences and/or buildings with other sensitive receptors. If the noise level at a sensitive receptor’s location is more than 10 dB(A) above ambient, MassDEP requires the noise source to mitigate its impact. The LFN and infrasound are not covered.
In Michigan, the Centerville Township, after 4 years of study, developed and approved a 19-page zoning ordinance for commercial wind energy systems. It is strict and comprehensive and should serve as a model for other government entities. Here are some excerpts:
Audible Noise Standard:
From 6:00 A.M. until 10:00 P.M., for wind speeds from cut-in to rated-output of the wind turbine facility, the noise level due to the wind turbine facility at the property line closest and at locations within 1 mile of the wind turbine facility shall not exceed the greater of 35 dB(A), or the established outdoor background sound level by more than 5 dB(A).
From 10:00 P.M. until 6:00 A.M., the noise level due to the wind turbine facility at the property line closest and at locations within 1 mile of the wind turbine facility shall not exceed the established outdoor background sound level by more than 3 dB(A). Background sound level shall be established separately for daytime (6:00 A.M.-10:00 P.M.) and for nighttime (10:00 P.M.-6:00 A.M.) values.
LFN or Infrasound: No LFN or infrasound from wind turbine facility operations shall be created which causes the noise level both within the project boundary and a 1 mile radius beyond the project boundary to exceed the following limits:
Octave Band Center Frequency, Hz Sound Pressure Level (dB-SPL)
Tonality and/or Repetitive, Impulsive Tone Penalty:
In the event the audible noise due to wind turbine facility operations exhibits tonality, contains a pure tone and/or repetitive, impulsive noise, the Audible Noise Standard shall be reduced by a total of 5 dB(A).
In Maine, codes require noise levels not to exceed the one-hour average daytime limit (between 7 a.m. and 7 p.m.) of 55 dB(A), and one-hour nighttime limit (between 7 p.m. and 7 a.m.) of 42 dB(A), as measured within 500 feet from a residence, seasonal camp or business at “protected locations”, and 55 dB(A) 24 hours of the day at greater than 500 feet from a residence, seasonal camp or business at “protected locations”, and 75 dB(A) at the wind turbine project boundary.
Maine Department of Environmental Protection 06-096 CMR c. 375.10.
Till now, 32 Maine towns have passed their own wind facility ordinances that are stricter than the state ordinance, because they do not trust the state to protect the public safety, health, property values and welfare of the people. This site provides the URLs of the text of the wind ordinances of 12 Maine towns. Vermont towns should get copies of them and use them as a guide to write their own ordinances before it is too late.
In Vermont, codes require nighttime noise levels not to exceed 40 dB(A) as measured at the exterior of a dwelling facade and averaged over a 12-month exposure, the same as the recommendations of the 2009 World Health Organization report that mostly cover road noise, air traffic, and community noise and do not mention wind turbine noise. LFN and infrasound are not covered. The Vermont code is hopelessly out of date and does not protect the public health, safety and welfare; it is a wind turbine vendor’s dream come true.
dB values should be measured “at the property line” to ensure people can enjoy their entire property and should not be “averaged over a 12-month period” which would average higher noise levels at higher wind speeds occurring mostly during nighttimes with lower noise levels at lower wind speeds occurring mostly during daytimes.
Wind Turbine Noise Annoyance:
On an annoyance scale that is based on interviews of people who live near wind turbines, airports, railroads and highways, wind turbine noise is much more annoying at less than 40 dB(A), than the noise from aircraft, highway and rail traffic at less than 70 dB(A).
This additional annoyance is due to the LFN and infrasound emitted by wind turbines. The measured wind turbine noise appears to be benign and within code, but the annoying/unhealthy LFN and infrasound were filtered out by the A scale weighing.
Willem Post: BSME New Jersey Institute of Technology, MSME Rensselaer Polytechnic Institute, MBA, University of Connecticut. P.E. Connecticut. Consulting Engineer and Project Manager.