Considerations Regarding an Acoustic Criterion for Wind Turbines
“What the audio files show is that even though the sounds might all be the same value of dBA, the levels of annoyance can be very different.”
By William K.G. (Bill) Palmer
Currently, in Ontario and generally worldwide, the criterion used to determine if sounds are acceptable use A-Weighted calculations of the “equivalent” sound level. A-weighting is used as it represents the audible acuity of the human ear, taking particular interest in frequencies between about 300 Hz and 3400 Hz, which represent the most important for speech intelligibility and speech recognition and are the frequency band of the usual telephone system. For interest, Middle C on a piano is 261.6 Hz, the lowest key is 27.5 Hz, and the highest key on a piano is 4186 Hz. Actually, A-Weighting gives the most emphasis on sound between about 500 Hz and 8000 Hz, slightly higher than used for speech recognition, and low frequency sounds in particular, are much reduced in emphasis.
This paper raises considerations that suggest other criteria should also be used when determining if sound from wind turbines is acceptable. The large number of complaints filed with the Ministry of the Environment and Climate Change “Spills Line” suggests citizens are not happy with the current situation.
- The paper first reviews some of the common complaints, and gives reasons for why they actually do make sense.
- Measurements show that the sound levels at a home with wind turbines in the environment rises more as frequency falls, than does the sound at a similar location distant from wind turbines.
- The sound from wind turbines is cyclical, typically rising and falling about 5 to 6 dB during each blade passage that can influence annoyance.
- Finally, the paper demonstrates how 40 dBA of different sounds can result in very different levels of annoyance.
The examples used in the demonstration include:
- “white noise” which has the same amplitude at all frequencies
- “pink noise” which rises at 3 dB per octave as frequency falls like the sound at a home distant from wind turbines. (An octave represents halving the sound frequency, as from 1000 to 500 Hz or from 500 to 250 Hz.)
- “Brownian noise” which rises at 6 dB per octave as frequency falls (as recorded at a home adjacent to wind turbines.)
- “Modulated Brownian noise” which varies about 5 dB per cycle at a frequency typical of blade passage for wind turbines, of 1 Hz.
What the audio files show is that even though the sounds might all be the same value of dBA, the levels of annoyance can be very different.
The intent of the paper is to help regulators and legislators understand the reasons why the existing system presents problems.
Presentation with embedded audio files
In the presentation, you can listen to five audio files when clicking on a speaker symbol. These audio files are also available below.Presentation to Acoustics Week in Canada – 2016 36.6 dBA White Noise File 36.6 dBA Pink Noise File 36.5 dBA Brownian Noise File 36.7 dBA Modulated Brownian Noise File 30.6 dBA Pink Noise File