If there's any organization that is (a)competent, (b)disinterested and (c)experienced in matters relating to health, it would be the World Health Organization, or WHO. Their current noise recommendations have been around for quite some time, predating modern wind turbines, but they still are useful in comparing what the world's experts are recommending with what, as an example, Ontario requires. Anything italicized is from their publication, the WHO Guidelines for Community Noise, published in 1995, updated in 1999.

Section 4, Guideline Values

Part 4.3, Specific Environments

Noise measures based solely on LAeq values [LAeq is a logrithmic average, using the A weighting] do not adequately characterize most noise environments and do not adequately assess the health impacts of noise on human well-being. It is also important to measure the maximum noise level and the number of noise events when deriving guideline values. If the noise includes a large proportion of low-frequency components, values even lower than the guideline values will be needed, because low-frequency components in noise may increase the adverse effects considerably.

In other words, if the noise in question has either a large low-frequency component or a maximum level that is above the LAeq average - and the noise from wind farms has both - then you should use lower limits. How much lower is not specified. Since just about everyone, including Ontario, uses LAeq (even the WHO in the following sections) the numbers below should be even smaller than they are.

Part 4.3.1, Dwellings

In dwellings, the critical effects of noise are on sleep, annoyance and speech interference. To avoid sleep disturbance, indoor guideline values for bedrooms are 30 dB LAeq for continuous noise and 45 dB LAmax for single sound events. Lower levels may be annoying, depending on the nature of the noise source. The maximum sound pressure level should be measured with the instrument set at "Fast".

To protect the majority of people from being seriously annoyed during the daytime, the sound pressure level on balconies, terraces and outdoor living areas should not exceed 55 dB LAeq for a steady, continuous noise. To protect the majority of people from being moderately annoyed during the daytime, the outdoor sound pressure level should not exceed 50 dB LAeq. These values are based on annoyance studies, but most countries in Europe have adopted 40 dB LAeq as the maximum allowable level for new developments (Gottlob 1995). Indeed, the lower value should be considered the maximum allowable sound pressure level for all new developments whenever feasible.

At night, sound pressure levels at the outside façades of the living spaces should not exceed 45 dB LAeq and 60 dB LAmax, so that people may sleep with bedroom windows open. These values have been obtained by assuming that the noise reduction from outside to inside with the window partly open is 15 dB.

For outdoors, the important numbers (all in LAeq) are 55 causes serious annoyance, 50 is moderate, and most European countries have adopted 40, which is also what this Guideline recommends. At night the levels must be consistent with an ability to sleep, which translates to an outside LAeq of 45 and an LAmax of 60, both of which assume a house will produce a reduction of 15. The reduction of 15 may be optimistic - in other words, many houses have walls that will not provide this much reduction. This is especially true if windows are open and with low-frequency sounds. In my actual case with the outside noise measuring at 40dbA and the window closed my bedroom measures 25dbA, which is very quiet. But then, I have very-well-insulated 6" walls. With my one smallish window open, the noise goes up to 27dbA, which is still very quiet.

LAmax is generally not available, so the next best measurement is LA10, which would be the level that is exceeded 10% of the time. This is probably the best measurement available for the "swooshes" that windmills produce. But Ontario still uses LAeq.

There are many ways in addition to what I've covered here in which the Ontario regulations are looser than the WHO guidelines. John Harrison has written a report, 0.2mb that details these ways.

The WHO Guidelines for Community Noise, section 4 "Guideline Values" is here.