I have been reading about the anticipated noise impact of wind farms on the North Country. One of the largest problems the public can have is in understanding the issue, particularly a complex, technical issue. Often understanding a problem is in itself a very big part of the problem. The purpose of my letter is to help you better understand noise as it relates to evaluating impacts of wind farms.
Background noise levels at your home represent the typical noise level when all of the transient intruding noises produced by vehicle traffic, helicopters, airplanes, snowmobiles, dogs barking and so forth, are not present. Most commonly it is the minimum noise level reading that you would read on a hand held sound level meter at that time. It is commonly approximated by the noise level that is exceeded 90% of the time and is called L90.
North Country background noise levels are extremely low. Typical levels are in the range of 25dBA to 35 dBA and are significantly lower than the noise in towns and cities.
If levels of 50 to 55dBA average noise levels from Wind Turbine Farms are permitted to intrude on your property, you will be allowing noise levels that are typical of very commercial, urban areas.
Unfortunately the loudness, color, fluctuation, and low frequency noise produced by an operating wind farm and those now present at your home are grossly different and cannot be compared and evaluated by a single sound measure such as the dBA.
So my point is simply this: the noise impacts must be evaluated on a level playing field and you cannot permit an over simplification of the problem by setting compliance levels and comparing background dBA readings with predicted dBA levels for a proposed wind farm. We need to do more!
Extremely quiet noise levels are not easily measured using automated sound data collection systems. These instruments are typically unable to measure very low background noise levels, because the wind induced noise on the microphone is often much larger than the actual background noise.
All of us are familiar with hearing the whooshing and whistling wind affect the microphones of outdoor weather broadcasts. Unless the low level background noises are recorded with special microphone wind screens and monitored to verify that wind induced measurement errors are not present the measurements become garbage in garbage out.
I am a retired Acoustical Engineer and have spent most of my adult life working for Carrier Air Conditioning in the area of product noise control, and the development and
application of noise criteria and measurement standards. I’ve had to deal with how and what to measure to provide meaningful noise ratings and how best to communicate this information to our customers. After retirement n 1999 I taught acoustics, noise control and the use of noise criteria in design at Syracuse University and RPI.
I have reviewed the Environmental Impact Statements of several local communities who are considering approval of large wind farms in their localities. The only acoustical
quantity that was considered was the average dBA which only take into account the higher frequencies. Average values can be very dangerous. Many people have drowned in creeks that have an average depth of a few feet but have many holes in which to drown.
The reality is this: these draft Environmental Impact Statements do not evaluate the annoying noises that rapidly change in noise levels, such as the cyclic whooshing noises produced when the turbine blades pass by the support pedestal or the transient noises produced at turbine cut in or cut out or those loud periodic noises produced by the turbine gearboxes.
Nor do they document the presence or absence of significant annoyance due to low frequency noises or vibrations. It is well known that low frequencies can be a very significant problem if the noise levels are high enough. The air conditioning industry has been dealing with this problem since 1978.
Low frequency noise problems are not imaginary; I have had to deal with large buildings in which the occupants were refusing to come to work if the low frequency problems were not fixed. So the bottom line is that meeting reasonable dBA target levels in a community will not insure that the low frequency noise (below 125 Hz) will not cause an unreasonable annoyance.
Much of our area affected considered for wind farm development is located on very shallow limestone shelves which will be directly connected to the massive concrete bases of the wind turbine. This means that the rotor vibrations, blade slap induced vibration on the turbine pedestal and the gearbox vibrations can be telegraphed to many houses whose foundations rest on the same slab.
I would hope that the planning boards in our area who are reviewing wind farm noise studies would insist on nothing less than comprehensive, accurate assessments. Assessments that consider all the possible noise impacts associated with wind turbines.
I would hope we decide to pay and study now rather than complain and pay later.
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