In this by-law:
“A-weighting” means the frequency weighting characteristic as specified in the International Electrotechnical Commission (IEC) Standard 61672, and intended to approximate the relative sensitivity of the normal human ear to different frequencies (pitches) of sound. It is denoted as “A”. …
“Infra-sound” means sound with energy in the frequency range of 0-20 Hz. It is often considered to be inaudible for most people unless at a relatively high amplitude but has been shown to be perceived via other senses at lower amplitudes especially for complex non-steady, non-sinusoidal pressure waves. Modern utility scale wind turbines produce an infrasonic blade passage tone typically centered at a frequency of 1 Hz or lower. [E.G. a wind turbine with hub rotation of 10 rpm would have a blade pass frequency of 0.5 Hz.] The most significant noise inside of a dwelling occurs from tones produced by the rotating blades of the wind turbine in the frequency range between 0.25 Hz and 10 Hz. Most of this energy is below 3 Hz with sound pressures increasing as frequency decreased down to the blade pass frequency. …
“Low Frequency Noise” (LFN) refers to sounds with energy in the lower frequency range of 20 to 200 Hz. …
“Sound” is an oscillation in pressure, stress, particle displacement or particle velocity, in a medium with internal forces (e.g. elastic, viscous), or the superposition of such propagated oscillations, which may or may not cause an auditory sensation.
“Sound Level” means the A-weighted Sound Pressure Level.
“Sound Level Limit” is the limiting value described in terms of the one hour A-weighted Equivalent Sound Level.
“Sound Pressure” means the instantaneous difference between the actual pressure and the average or barometric pressure at a given location. The unit of measurement is the micro pascal (μPa).
“Sound Pressure Level” means twenty times the logarithm to the base 10 of the ratio of the effective pressure (μPa) of a sound to the reference pressure of 20 μPa. …
(d) No Wind Facility shall make, cause or permit the emission of Infra-sound in the frequency between 0 Hz and 20 Hz. Infra-sound is deemed to be excessive when measurements inside of dwellings, using instruments suitable for sound pressure measurements at frequencies of 0.1 Hz to 20 Hz identify blade pass tones and their harmonics, present during wind turbine operation, when the following conditions are present and limits are exceeded:
ii) If the blade pass tone, or any of its harmonics, produce a sound pressure level of 50 dB or more when energy averaged over a period of one to several minutes or more, and,
iii) The crests (peaks) of the sound pressure exceed this average by 10 dB or more.
(e) No Wind Facility shall make, cause or permit the emission of LFN, where the difference between a C-weighted sound level and an A-weighted sound level is greater than 15 decibels at any measurement point inside or outside an occupied structure, or the difference between an un-weighted sound level (including Infra-sound from 0.1 Hz and above, using an instrument rated to measure Infra-sound down to 0.1 Hz) and an A-weighted sound level is greater than 20 decibels at any such measurement point.
(f) No Wind Facility shall make, cause or permit infrasonic barometric pressure disturbance (IBPD), where any cyclic pressure disturbance having amplitude at any given frequency in the stated frequency range exceeds 2 milli-Pascals RMS (0.002 Pascal RMS) for a repeatable duration of 10 seconds or more in any 40-second period.
Read a third time and finally passed this 8th day of October, 2014.
Download original document: “By-law to provide for the regulation of wind turbine noise within the Town of Plympton-Wyoming ”