To the Clerk of the Vermont Public Service Board:
I support the proposed wind turbine sound rules as a first step to protect the aural environment of our mountains.
As you know, a quiet rural night in Vermont is likely to have a sound level of only 25 dBA or even less. An increase in ambient noise of 5 dB is recognized as a cause of widespread complaints. So limiting the sound level at night to 35 dBA is not severe but actually rather lenient.
The proposed rule does not address low-frequency noise, which Denmark (the world's leader in wind energy technology and implementation) since 2011 has limited to 20 dBA indoors (10-160 Hz).
Infrasound (which is not heard but instead felt) is also a concern, with many acoustic engineers determining that a C-weighted indoor limit of 50 dB is necessary to protect health.
Nor does the proposed rule address amplitude modulation, the distinct "swish" or "thump" of large wind turbines. In the UK, planning permission for the Den Brook project included a rule to limit amplitude modulation: A 125-ms pulse of 3 dBA or greater (3 dB being the difference in noise level detectable by the human ear) can not occur in any 2-second period five or more times in six or more minutes of any hour, when those minute-long average noise levels are 28 dBA or more.
While these limits, as well as the proposed setback of 10 times the total height from residences (which should be at least 15 times the height, and from property lines, so that people can enjoy all of their property), begin to protect human neighbors, they do nothing to protect the wildlife of the mountains, who in most cases are much more sensitive to sound than humans.
[See also: Proposal and comments for implementing a rule regarding sound from wind generation projects, by Stephen Ambrose]