(1) Low-Frequency Sound Generation by Wind-Turbines
The opinions of the two UK acousticians relating to wind-turbine noise differ. Professor Ffowcs-Williams has stated "It is known that modern, very tall turbines, do cause problems, and many think the current guidelines fail adequately to protect the public."
while Dr Geoff Leventhall has commented "I can state quite categorically that there is no significant infrasound from current designs of wind turbines. • Infrasound is not a problem, • Low frequency noise may be audible under certain conditions, • The regular 'swish' is not low frequency noise."
In practice, the transition from infrasound to low-frequency sound may be blurred. Based on my own experience, the consistent reports of physical discomfort resulting from wind-turbine noise reinforce my perception that low-frequency noise can indeed be a
problem. The reported effects are entirely consistent with those that I have experienced at first hand, 20-30 years ago.
Low frequency noise can induce feelings of discomfort and nausea, not unlike seasickness. Like seasickness, the sensitivity of different individuals varies enormously, some being immediately sensitive, while others can barely detect anything. I have stood
beside two people on a site where low-frequency noise was present. One person said "I can't really hear anything". The other said "I feel ill - I should like to leave". Both were reporting accurately; there can often be more than 12dB difference ( a factor of 4) in the sensitivity of individuals to low-frequency noise. Given that for very low frequencies, 12dB represents the difference between just audible, and uncomfortably loud, it is clear that very real problems are experienced by some individuals, while others remain largely unaffected.
It is important to emphasize that there does not yet appear to be a full understanding of how to assess low-frequency wind-turbine noise. As recently as April 2008, A Danish researcher, T.H.Pedersen demonstrates clearly in  how different conventions for measuring the noise field of a turbine can lead to diametrically opposite conclusions. He summarizes by writing "The above mentioned issue has been discussed with a number of researchers (Henrik Moller, Aaborg University, Torsten Dau, Ranish Technical University, Hugo Fastl and Geoff Leventhall) and solutions have been sought for without result." He goes on to describe a procedure involving weighting the spectra with the inverse hearing-threshold (HT-weighting) but while clarifying the problem, this does nothing to resolve the issue.
So it is difficult to understand how it can be argued emphatically that there is no problem, when it is clearly reported that significant ambiguity still remains in assessing these effects.
The present author has considered this aspect, and believes that the misunderstanding may lie in a failure to take into account correctly the impulsive nature of the turbine noise, as each blade passes the tower, and interaction takes place between the blade, the wake, and the tower. Although it is now widely recognized that this can give rise to lowfrequency modulation of higher frequency aerodynamic noise, resulting in a "swishing sound" (aerodynamic modulation), it remains the case that the low-frequency effects of the impulse are often incorrectly analyzed. This latter effect has been described as a distinct repetitive "thumping sound" audible at distances of 500 to 1000 meters (~ 1600 to 3300 ft.)
The feature of impulsive noise is that there is a large signal present for a short period of time. Consequently, the mean, or root-mean-square (rms) level of the signal may be very low, apparently well below the threshold of hearing, but the peak level is much higher and can be perceived. This ratio of peak-to- mean level is the Crest-Factor.
The present convention of combining frequency-weighted spectral or octave levels only measures the rms level - it does not take any account of the crest-factor.
The hearing threshold has been determined experimentally using individual sinusoidal sound waves. But sinusoidal waves have the lowest of all crest factors. C.S.Pedersen  has reported that band-limited 2Hz-20Hz, and 2Hz-40Hz white noise is audible 7-10dB below the threshold defined for sinusoidal signals. This observation is consistent with the increased crest-factor of such noise. But low-frequency, repetitive impulsive sounds possessing a multiplicity of harmonic components, have an even more recognizable characteristic, and are likely to be audible at even lower levels. Preliminary calculations indicate that periodic 1Hz impulses may be audible even when the individual components of spectral lines lie 25dB below the threshold of hearing. So simply examining low-frequency spectra and observing that individual spectral lines lie well below the threshold of hearing does not begin to summarize this situation accurately.
A further comment relates to this impulsive component of noise. If an observer stands near to the wind-turbine, the distance from him to different portions of the tower and blade varies significantly. Consequently, the time taken for sound to propagate to this observer differs for each portion of the blade segment. As a result, the arrival times of the impulsive effects are "smeared-out", and much less audible, despite the close-up distance. But for an observer positioned several hundred feet away, along the line of the axis of the turbine, the impulsive components all tend to arrive at the same time, giving a much enhanced effect.