Planning Wind Turbine Installation in Regard to Sound
To make matters easy, it is quite simple to predict what noise wind turbines will generate in advance. This is quite standard and it is only in exceptional circumstances that the noise would be considered unacceptable.
Background Noise: Masking Noise Drowns out Turbine Noise
No landscape will ever be completely silent. Noise will be generated by wildlife, especially birds and human activities. Then there is the noise produced by wind agitating foliage (grass, leaves, trees, etc) and man made structures (especially metal building materials and masts etc.). These background noises will be louder than any potential sound from a wind turbine, especially as the noise produced by wind movement over the landscape will increase at a greater rate with wind speed than the noise produced by a modern turbine.
Because of this variable, it can make it very difficult to measure the sound impact on a location from a wind turbine with any accuracy. With wind speeds of around eight metres per second and above, the issue becomes moot as any background noise will effectively drown out the noise a turbine can make completely.
How the surroundings effect sound propagation
Sound, as you may remember from your school science lessons, is a physical force which is reflected by hard surfaces, and absorbed by soft surfaces. This means that sound generated by a turbine will radiate out unevenly as it is effected terrain and building surfaces. This can have a large impact on the sound levels at different locations, despite being the same distance from the source. On top of the effects of the physical environment there is also the wind direction to consider. Sound does not travel well against the flow of air, meaning that upwind of a wind turbine is sheltered to a greater extent than downwind.
Human perceptions of sound
Noise is a very subjective thing, ask anyone who has lived next to a railway line for a few years. It isn't just how used to a noise you are, but how pleasant you find it. The sound of waves on the beach, or birds in the trees can be quite enjoyable and relaxing, yet the neighbour's radio can grate on all but the most tranquil nerves. This is despite the fact that the neighbours stereo may be much quieter than the waves and the birds.
This generally comes down to how the brain processes sound inputs. Although it may simply be that you and your neighbour do not see eye to eye on the definition of good music. What really defiens the difference is information content. Waves emit an entirely random "white" noise, bird song is also (as far as most people are concerned) random, whereas a radio has human created content, something which your brain will automatically try to process. Like speech or music, your brain picks out the meaning entirely unconsciously. It doesn't help if you can barely hear the radio, in fact this can make it worse, your brain may not be able to determine the information contained in the sound and you find yourself straining to hear what is being said or sung.
Irritating sound is often referred to as noise (or more specifically 'unwanted noise'). Since the difference between unwanted noise and other sounds is a personal and highly psychological phenomenon, it is never a simple task to make a satisfactory model of sound phenomena.
A study carried out by the research institute DK Teknik, in Denmark indicates that perception of the noise generated by a wind turbine has more to do with the individuals attitude to the win turbines, than the actual noise itself.