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Niall McMahon © 2013
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These notes contain an overview of noise and wind turbines. See Wind Energy Explained for expanded descriptions of some of the material.
Noise is any unwanted sound. For our purposes, there are three levels of noise:
The speed of sound in air varies with air density; at sea-level it is about 340 m/s. The speed of a sound wave (c) is equal to the frequency (the pitch) of the sound in Hertz (f) multiplied by the wavelength of the sound in metres (λ), i.e. c = f λ.
Two quanties are important for describing sound and noise:
where w and w0 are the power of the sound at the source and some reference power, respectively; p is the instantaneous sound pressure at the location of the receiver (the listener) and p0 is a reference pressure. The reference power, w0, can be taken as 10-12 W and the reference pressure, p0 can be taken as 20 x 10-5 Pa. This reference pressure corresponds to the lowest audible pressure level of a young healthy ear. Speakers for home entertainment are rated by power level.
A logarithmic scale is used as sound pressure levels vary from 20 micro Pa (the threshold of hearing) to 20 Pa: this spans seven orders of magnitude and a linear scale would be unworkable.
Sounds of frequency 20 Hz and below can be classed as infrasound. Infrasound lies below the lower threshold of human perception. Very low frequency sounds, of this order, can be perceived in a tactile way, depending on source power levels and distance from the source. High power, low frequency sound-waves can be felt. Sonic weapons, much in the news a few years ago, use high power, low frequency sounds.
In general, people cannot detect a 1 dB change in sound pressure level. A 5 dB change is generally sensed. A 10 dB step is perceived by people as a doubling of the loudness of the sound.
When assessing the environmetal impact of noise, three filters are commonly used. For environmental sound measurement, some frequencies that have little impact on humans are weighted differently in terms of importance. Even though sounds may have the same pressure level, the human ear perceives different frequencies differently. Very low and very high frequencies are considered less important than audible frequencies. The A-filter, which ranks sound waves in one particular order of importance, is commonly used; the measured sound pressure is modified by the measurement equipment to "amplify" the effect of important frequencies. When a measurement includes A-filter data, the units are often denoted dBA or dB(A) to make this clear. The modified sound pressure measurement is then used in the equation above to calculate the A-weighted sound pressure level, Lp.
Some other derived quantities that are important include:
Wind turbine noise, then, consists of:
Models of varying complexity exist to describe turbine noise. The simplest models use empirical equations to estimate power and pressure levels. For example,
LwA = 10 log10(PWT) + 50
Where LwA is the overall A-weighted power level (i.e. at the wind turbine) and PWT is the rated power of the machine.
LwA = 22 log10(D) + 72
Where D is the rotor diameter of the machine.
LwA = 50 log10(Vtip) + 10 log10(D) - 4
Where Vtip is the normal operating tip speed of the machine.
The sound pressure level at a distance R from a wind turbine can then be estimated using,
Lp = LwA - 10 log10(2 π R2) - αR
Where Lp is the sound pressure level at a distance R from the source, i.e. the turbine, Lw is the power level at the source, and the constant α is called the absorption coefficient. α often takes the value 0.005 dBA m-1.
This last formula captures the decrease in sound pressure level with distance from the sound power source in the last two terms.
Complete noise assessments are required by planning authorities. Local authorities can have different requirements. IEC 61400-11 details the recommended acoustic noise measurement techniques.
Please see here.
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