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People often ask, “What is a responsible noise level from industrial wind turbines, to protect the health of nearby residents?” And, “How should background noise levels be properly measured prior to turbine construction, and how should noise levels be measured after the turbines are up and running?”

George W. Kamperman and Richard R. James, two American noise control engineers with formidable credentials and reputations, provide answers in their “How To” Guide to Siting Wind Turbines to Prevent Health Risks from Sound. (If you can’t open this link, click here and try this one.) It’s well worth reading, although admittedly pretty heavy going for non-engineers. Both men have many years experience in industrial noise control, and both have studied wind turbine noise intensively and given papers on the subject at professional meetings (meetings that were not wind-industry influenced, by the way). Furthermore, neither Kamperman nor James has worked as a consultant for a wind developer, hence neither has a financial stake in wind energy. Hence, no conflict of interest. This bears emphasizing.

For the short answer to the above questions, click here for a 9-page summary of their recommendations. If 9 pages are too much for you, click here for a 1-pager, or read the same single page, “Proposed Wind Turbine Siting Sound Limits,” 10/24/08, below.

But before you read anything, be sure you’re conversant in the specialized language of noise engineers and acousticians. Read this list of definitions.

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Proposed Wind Turbine Siting Sound Limits

October 24, 2008

1. Establishing Long-Term Background Noise Level

a. Instrumentation: ANSI or IEC Type 1 Precision Integrating Sound Level Meter plus meteorological instruments to measure wind velocity, temperature and humidity near the sound measuring microphone. Measurement procedures must meet ANSI S12.9, Part 3.

b. Measurement location(s): Nearest property line(s) from proposed wind turbines representative of all non-participating residential property within 2.0 miles.

c. Time of measurements and prevailing weather: The atmosphere must be classified as stable with no vertical heat flow to cause air mixing. Stable conditions occur in the evening and middle of the night with a clear sky and very little wind near the surface. Sound measurements are only valid when the measured wind speed at the microphone does not exceed 2 m/s (4.5 mph).

d. Long-Term Background sound measurements: All data recording shall be a series of contiguous ten (10) minute measurements. The measurement objective is to determine the quietest ten minute period at each location of interest. Nighttime test periods are preferred unless daytime conditions are quieter. The following data shall be recorded simultaneously for each ten (10) minute measurement period: dBA data includes L_A90, L_A10, L_Aeq and dBC data includes L_C90, L_C10, L_Ceq, plus maximum wind speed at the microphone during the ten minutes and a single measurement of temperature and humidity at the microphone for each new location or each hour whichever is oftener. A ten minute measurement contains valid data provided: Both L_A10 minus L_A90 and L_C10 minus L_C90 are not greater than 10 dB and the maximum wind speed at the microphone did not exceed 2 m/s during the same ten minute period as the acoustic data.

2. Wind Turbine Sound Immission Limits

No wind turbine or group of turbines shall be located so as to cause wind turbine sound immission at any location on non-participating property containing a residence in excess of the limits in the following table:

3. Wind Farm Noise Compliance Testing

All of the measurements outlined above in 1. Establishing Long-Term Background Noise Level must be repeated to determine compliance with 2. Wind Turbine Sound Immission Limits. The compliance test location is to be the pre-turbine background noise measurement location nearest to the home of the complainant in line with the wind farm and nearer to the wind farm. The time of day for the testing and the wind farm operating conditions plus wind speed and direction must replicate the conditions that generated the complaint. Procedures of ANSI S12.9- Part 3 apply. The effect of instrumentation limits for wind and other factors must be recognized and followed.

—George W. Kamperman & Richard R. James

George Kamperman

Richard James

Take a look at the following graphs. They illustrate the difference between dBA and dBC noise measurements.

» dB = decibel
» “A” refers to A-filtering (also known as A-weighting)
» “C” refers to C-filtering (C-weighting)
» therefore dBA = noise measurement with an A-filter (or A-weighted filter)
» and dBC = noise measurement with a C-filter (or C-weighted filter)

It’s clear that C-filtering is preferable to A-filtering as you shift into lower Hertz (the yellow zone on both graphs)—that is, as you encounter low frequency noise and infrasound. It’s obvious from the graphs that a C-weighted filter picks up vastly more noise (literally, exponentially more) in the low frequency and infrasound range. It’s equally obvious that an A-weighted filter picks up exponentially less and less low frequency noise and infrasound, as the frequency drops. (Notice that the noise data are plotted on logarithmic graph paper. This explains why the intervals between levels of frequency are unequal. Frequency is measured in Hz = Hertz.)

With appreciation to diracdelta.co.uk

With appreciation to diracdelta.co.uk

If you’re still confused after reading all this, here’s what to do. Begin by watching this short movie. (Fix yourself a bowl of hot, buttery popcorn.)

Now read the commentary by George Kamperman, Letter 2, below, dated November 19, 2008. As you read, keep in mind Mr. Kamperman’s credentials: he’s Board Certified in Noise Control Engineering by the American Institute of Noise Control Engineering. That’s a big deal. Mr. Kamperman wrote Letter 2 (11/19/08) after listening to this video.