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COMMENTS ON RECENTLY PUBLISHED ARTICLE, “CONCERNS ABOUT INFRASOUND FROM WIND TURBINES”
Paul D. Schomer
Schomer and Associates, Inc. Champaign, IL 61821
\\\[Editor’s note: Acoustics Today welcomes articles that comment on previously published articles. This is with the firm belief that serious controversy on scientific topics is healthy. The opinions expressed in articles are not necessarily those of the Editor or the membership of the Acoustical Society of America. Articles in Acoustics Today are frequently edited by the Editor, but should not be regarded as having undergone a rigorous peer review.\\\]
The July, 2013, paper in Acoustics Today entitled “Concerns about Infrasound from Wind Turbine Noise” by Geoff Leventhall \\\[Volume 9, Issue 3, pp. 30-38\\\] seems to be centered on proof by assertion. The argument appears to be an assertion that infra- sound from wind turbines cannot possi- bly affect people because (1) infrasonic body-generated sound is 20 dB or more greater than the measured levels from wind turbines and (2) that atmospheric noise in the wind turbine infrasonic fre- quency range is on the order of or greater than wind turbine noise.
Regarding the first assertion, it is true that environmen- tal infrasound is of the same order or greater, or sometimes less, than the infrasonic emissions of wind turbines. The sec- ond assertion is a half-truth. The signals impinging on the inner ear are 20 dB greater but this energy does not readily couple to the inner ear because its entry point is right next to the round window. Sound, including wind turbine sound, enters through the outer ear, and travels to and through the middle ear which matches the impedance of an acoustic wave in air to the impedance of an acoustic wave in the fluid-filled inner ear, preventing what would otherwise be a 29 dB loss. The wave travels through the inner ear from the oval window to the round window, which provides a compliance that per- mits the fluid in the inner ear to oscillate up and back in sync with the infrasonic acoustic pressures. This process can be thought of as something like how heated air will travel from a supply duct on the floor of a room to a ceiling return air duct and thereby fill the room with heat. In contrast, the body-generated sound entry point is right next to the round window which is something like having an HVAC supply duct entry point right next to the return-air grill; the in-room flow is short-circuited and the room is difficult to heat.
The point is that the acoustic pressures (applied via the stapes) from the wind turbine may be as great, or greater than the body-generated pressures that reach the whole of the inner ear. Professor Leventhall then goes on to assert that because the noise levels are greater than the signal, the signal won't be registered in the inner ear. These assertions, of course, are not necessarily true.
Seasickness symptoms are one set of symptoms exhibit-
This process can be thought of as something like how heated air will travel from a supply duct on the floor of a room to a ceiling return air duct and thereby fill the room with heat.
ed by persons affected by wind turbine noise, and there is evidence that symp- toms occur more strongly when there is just one or two nearby turbines so that one or two infrasonic “tones” are pres- ent. In contrast, if there are several near- by asynchronous turbines, then there are typically no clear tones—just ran- dom noise, and the problems with the seasickness symptoms near to wind farms appear to be less. This finding relates directly to Leventhall’s assertion that atmosphere infrasound, which is random, masks wind turbine-generated infrasound. A tonal signal with an
amplitude that is of the same order as the noise can easily be detected in the presence of that random noise via autocorre- lation methods, or via a tuned circuit, etc.
There is evidence that the seasickness response of the body to the sensing of accelerations is tuned to the funda- mental frequencies generated by large contemporary wind turbines. Figure 1 shows what has been termed the nau- seogenic region, developed by the Navy to better understand and deal with seasickness (Kennedy et al., 1987). It relates linear acceleration levels (amplitudes) and frequencies to the time it takes for 50% of test subjects (upper curve) or 10 % of test subjects (lower curve) to vomit. For example in Figure 1, at 0.63 Hz it takes about 3.5 m/s2 acceleration to be on the “8 hour” curve. Newer wind turbines have fundamental rota- tional speeds of around 0.2 Hz and, being three-bladed, their blade passage frequencies are three times the fundamentals, so their frequencies are well into the nauseogenic region shown in Figure 1. Taking Figure 1 to be a tuned circuit, it does not represent a high Q but it provides at least a little selectivity; certainly enough to not reject the possibility or probability of detecting a tone in this frequency range when the levels of the tone(s) and the noise are by the same order.
But the Navy’s seasickness study is for acceleration; the wind farm emissions are pressures. It is the otoliths in the inner ear that sense horizontal and vertical acceleration, so a natural question is whether or not there is a relation between the seasickness symptoms generated by acceleration at say 0.63 Hz and the symptoms generated by acoustic pressures emitted at 0.63 Hz. Part of the answer relates to whether the otoliths directly measure acceleration or if they measure
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