In the real world you also have to take into account sound reflection and absoption (from hard and soft surfaces). These can play a role depending on the site, to the point of increasing or decreasing the sound level at different distances.
Sounds from several sources
Although the decibel count follows the addition of multiple sound sources, so two wind turbines located at the same distance us would double the sound energy reaching us. So a pair of turbines will mean that the measured sound level will increase by 3 dB(A). Four turbines at that distance means a sound level increased by 6 dB(A). But the vagaries of human hearing mean that you will actually have to install ten turbines, all at the same distance from the target area to perceive a doubling of the subjective loudness (i.e. the dB level has increased by 10).
The human ear (and brain) are able to discern pure tones far more easily than white, or random, noise. This means that many authorities take this into account when calculating sound estimates. This penalises any planned structure which produces noise at pure tones.
In compliance with the relevant international standards, turbine manufacturers will specify a theoretical dB(A) level for their products when in use. This will assume the sound produced when the turbine is operating all originates from a single point. However, in fact the noise originates from across the entire surface of the machine and rotor.
This single figure is useful, however, as it allows us to carry out the required calculations. The sound level produced by a modern turbine is typically in the range of 96-101 dB(A). This figure is itself rather pointless for all other purposes, as there will never be a point at which you could experience that level of sound. It is purely used to calculate the sound pressure produced during use to calculate the dB(A) at distance.
Pure tones, with their penalties have by and large been eradicated from all modern wind turbine designs from major manufacturers.
This various from jurisdiction to jurisdiction, but generally speaking it is 40 to 45 dB(A) at a distance of 300m. This generally sets a distance of 300 for any wind farm of more than ten turbines, but as with many aspects of wind energy, it really does vary from place to place.
Calculating what the potential sound emission will be from wind turbines is an important factor in obtaining planning permission from the local authorities. Especially when installing wind turbines near any populated areas. Even in the most complex terrian, it is usually much simpler to calculate what the potential sound emissions will be than to go in and measure them once the structure is built.
This is simply due to the fact that the background sound will usually be over 30dB(A). The difference in sound levels between the background and 'target' noise has to be over 10dB(A) in order to be accurately measured. This makes it almost impossible to measure the noise of the turbine with any sort of accuracy. Which is what makes the calculation far more important.