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2. Literature and websites from the wind industry often contained strong statements that wind turbine infrasound was of no significance. This view was largely based on publications by Leventhall (2006; 2007). Wind turbine noise was de- scribed as comparable to rustling leaves, flowing streams, air- conditioned offices or refrigerators heard from the next room. If wind turbine noise really was comparable to such sources then complaints would not be expected. But the turbines sounds are only comparable to these sources if the ultra-low frequencies emitted by the turbines are ignored through A- weighting. Stations that monitor infrasound or low frequency seismic (vibrational) noise for other purposes (for the detec- tion of explosions, meteors, volcanic activity, atmospheric activity, etc.) are well-aware that low frequency sounds ema- nating from distant wind farms, or coupling to the ground
as vibrations, can influence their measurements. The UK, Ministry of Defense has opposed wind turbines cited within 50 km of the Eskdalemuir Seismic Array. We have seen no reports of the Ministry opposing the presence of refrigerators in the region, suggesting they appreciate that sounds emitted from wind turbines and refrigerators are quite different. It was thus quite astounding to see the vast majority of wind tur- bine noise measurements excluding the low frequency noise content. Given the knowledge that the ear responds to low frequency sounds and infrasound, we knew that comparisons with benign sources were invalid and the logic to A-weight sound measurements was deeply flawed scientifically.
The Ear’s Response to Infrasound
Experimental measurements show robust electrical responses from the cochlea in response to infrasound (Salt and DeMott, 1999; Salt and Lichtenhan 2013). This finding was initially difficult to reconcile with measures showing that hearing
was notably insensitive to such sounds but the explanation became clear from now-classic physiological studies of the ear showing that the two types of sensory cell in the cochlea had very different mechanical properties (Cheatham and Dallos 2001).
Figure 1 : The sensory organ of the cochlea, showing inner and outer hair cell and neural anatomy.
The auditory portion of the inner ear, the cochlea, has two types of sensory cell. The inner hair cells (IHC; shown green in Figure 1) are innervated by type I afferent nerve fibers that mediate hearing. The stereocilia (sensory hairs) of the IHCs are free-floating and do not contact the overlying gelatinous tectorial membrane (shown gray). They are mechanically dis- placed by fluid movements in the space below the membrane. As their input is fluid-coupled to the vibrations of the sensory organ they exhibit “velocity sensitive” responses. As the veloc- ity of motions decreases for lower-frequency sounds, their fluid-coupled input renders the IHC insensitive to very low- frequency sounds. The other type of sensory cell, the outer hair cells (OHC; shown red in Figure 1) are innervated by type II afferent nerve fibers that are not as well understood as type I fibers and probably do not mediate conscious hearing per se. In contrast to the IHC, the stereocilia of the OHCs are inserted into the tectorial membrane. This direct mechani- cal coupling gives them “displacement sensitive” properties, meaning they respond well to low–frequency sounds and infrasound. The electrical responses of the ear we had been recording and studying originate from the sensitive OHCs. From this understanding we conclude that very low frequency sounds and infrasound, at levels well below those that are heard, readily stimulate the cochlea. Low frequency sounds and infrasound from wind turbines can therefore stimulate the ear at levels well below those that are heard.
The million-dollar question is whether the effects of wind turbine infrasound stimulation stay confined to the ear and have no other influence on the person or animal. At present, the stance of wind industry and its acoustician advisors is that there are no consequences to long-term low-frequency and in- frasonic stimulation. This is not based on studies showing that long-term stimulation to low-level infrasound has no influ-
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