Outdoor to indoor reduction of wind farm noise for rural residences

• A-weighted residential noise reductions depend on the incident noise spectrum shape.
• Allowable A-weighted external levels for traffic noise are inappropriate for wind farm noise.
• Low ambient noise levels make wind farm noise intrusive in rural areas.
• Room and structural resonances result in negative noise reductions at some frequencies.
• Means for estimating room and structural resonance frequencies are provided.

Compliance limits for wind farm noise are usually based on allowable outdoor levels. Since these limits are designed to protect the amenity of the people residing indoors, the outdoor to indoor noise reduction is an important consideration. World Health Organisation recommendations for outdoor noise are based on outdoor to indoor noise reductions for traffic noise. However, traffic noise is dominated by mid-frequency energy, whereas wind farm noise is dominated by low-frequency energy for which expected noise reductions are much less. This paper investigates typical noise reductions for residences near wind farms that are located in rural areas in Australia. It is found that during the night, when the wind farm is operating and the local wind speed is low, the A-weighted outdoor to indoor noise reductions with closed windows are less than 20 dB, which is at least 10 dB lower than the value generally assumed for traffic noise in urban areas. Furthermore, the C-weighted, G-weighted and low-frequency A-weighted (10 Hz – 160 Hz) noise reductions are lower still, indicating that A-weighted noise reduction values are not representative for noise dominated by low-frequencies. Outdoor to indoor noise reduction generally decreases with frequency, however, there are some variations to this trend which are related to housing construction. Structural resonances, room modes and coupling between the air volume inside the residence and the stiffness of the walls, roof and ceiling can contribute to reducing the noise reduction, sometimes to negative values. Below 2.5 Hz, the outdoor to indoor noise reduction is zero.