Wind Turbine Acoustic Noise  

Author:  Rogers, Anthony; Manwell, James; and Wright, Sally

The human response to sounds measured in decibels has the following characteristics:

• Except under laboratory conditions, a change in sound level of 1 dB cannot be perceived.
• Doubling the energy of a sound source corresponds to a 3 dB increase.
• Outside of the laboratory, a 3 dB change in sound level is considered a barely discernible difference.
• A change in sound level of 5 dB will typically result in a noticeable community response.
• A 6 dB increase is equivalent to moving half the distance towards a sound source.
• A 10 dB increase is subjectively heard as an approximate doubling in loudness.

For the determination of the human ear’s response to changes in sound, sound level meters are generally equipped with filters that give less weight to the lower frequencies …

• A-Weighting: This is the most common scale for assessing environmental and occupational noise. It approximates the response of the human ear to sounds of medium intensity.
• B-Weighting: this weighting is not commonly used. It approximates the ear for medium-loud sounds, around 70 dB.
• C-Weighting: Approximates response of human ear to loud sounds. It can be used for low-frequency sound.
• G-Weighting: Designed for infrasound.

Once the A-weighted sound pressure is measured over a period of time, it is possible to determine a number of statistical descriptions of time-varying sound and to account for the greater community sensitivity to nighttime sound levels. Terms commonly used in describing environmental sound include:

• L₁₀, L₅₀, and L₉₀: The A-weighted sound levels that are exceeded 10%, 50%, and 90% of the time, respectively. During the measurement period L₉₀ is generally taken as the background sound level.
• L_eq: Equivalent Sound Level: The average A-weighted sound pressure level which gives the same total energy as the varying sound level during the measurement period of time. Also referred to as L_{A eq}.
• L_dn: Day-Night Level: The average A-weighted sound level during a 24 hour day, obtained after addition of 10 dB to levels measured in the night between 10 p.m. and 7 a.m. …

If a wind turbine is proposed within a distance equivalent to three times the blade-tip height of residences or other noise-sensitive receptors, a noise study should be performed and publicized.

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