Page 55 - Volume 12, Issue 2 - Spring 2012

unlike the road traffic noise which abated at night. Additionally, the inherent, high levels of infrasound in wind turbine noise may also modulate brain activity and increase annoyance levels.
In his famous “ghost-buster” study, Tandy recorded a continuous infrasound emission in a 14th century cellar near Coventry University, England.2 The cellar has been rumored to be haunted since 1997. Various local visitors reported “very strong feeling of presence,” “cold chill,” and apparitions upon entering the cellar. Moreover, tourists who have never heard of the rumors also reported paranormal experiences. Tandy’s previous study in a supposedly haunted laboratory
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Since the 19 Hz was significantly below its audible threshold, visitors’ paranormal experiences could be due to changes in brain activities, despite the absence of tonal per- ceptions. It is known that temporal lobe epilepsy patients suf- fer from high risks of depression, anxiety, irritability, insom-
25,26
drome and paranormal experiences.
Conclusions and future directions for infrasound research
Based on CM and DPOAE modulation studies, infrason- ic frequencies can have clear effects on human cochlear state and function. Contrary to the belief that the inner ear does not register infrasound, it was found that infrasound can actually be detected by the OHCs. As OHC slow motility controls hearing sensitivity, the responsiveness of these sen- sory cells to infrasound could potentially enhance one’s abil- ity to perceive infrasound’s higher harmonics. Whether OHC-generated CMs can trigger spike generation in IHCs’ type I auditory nerve fibers, resulting in direct perception of infrasonic frequencies, is a major research focus today. Infrasound induced OHC activation of auditory nerves pres- ents an alternative pathway of focus, as about 5% of all type I
26
afferent fibers synapse with OHCs. High levels of infra-
sound have been shown to induce shifts in the basilar mem-
brane position, modulating DPOAE patterns. The shift in
basilar membrane parallels the function of OHC slow motil-
ity by altering subtectorial space. As changes in subtectorial
space affect IHC sensitivity, Hensel et al. concluded that
infrasound itself can affect the overall gain of the cochlear
vivo electrophysiology of human auditory afferent fibers is ethically unacceptable, self-reported sound perceptions and fMRI scans dominate current experimental efforts. While Dommes et al. showed significant auditory cortical activity in response to infrasound,9 additional studies are needed to cor- roborate their findings. For example, activity in primary somatosensory cortex (Brodmann’s Area 2, 3) should be examined and compared to that in the auditory cortex. This would reveal whether the auditory or vestibular pathway plays the more important role in human infrasound detec- tion. In addition, subjects’ hearing perceptions during fMRI- infrasound scans should be reported, as done by Hensel et al.8 Since auditory cortical activity increased significantly in response to a 12 Hz tone compared to its lower-level 36 Hz harmonic, infrasound detection in humans may be more common than previously thought. In future experiments, should the subjects report tonal or humming perceptions, along with pronounced auditory cortical activities, then it may be that infrasound itself triggers the perception, as opposed to its harmonics. If the subjects do not report any perceptions, auditory cortical activity could be considered unrelated to the stimulus.
Psychosomatic health risks have been proposed to be the result of infrasound exposure, as changes in temporal lobe activity have been linked to several psychiatric disorders. With nearby communities reporting annoyance toward wind turbine noise, further studies are needed to examine the effects of wind farms on the quality of life in sensitive indi- viduals. Long-term studies on wind turbine noise exposure are also needed. As wind energy is widely accepted for its promising role in clean energy production, putting a hold on wind farm development is highly unlikely. For now, engi- neering efforts and isolated geographical placements of wind farms serve as the best methods for minimizing community exposure to substantial and potentially harmful levels of wind turbine noise.AT
References
1 American Wind Energy Association (AWEA), “Wind Turbines and Health,” http:// www.awea.org/learnabout/publications/ upload/Wind-Turbines-and-Health-Factsheet_WP11.pdf.
2 V. Tandy, “Something in the cellar,” J. Soc. Psychical Res. 64.3(860), 129–140 (2000).
3 E. Pedersen, F. van den Berg, R. Bakker, and J. Bouma, “Response to noise from modern wind farms in the Netherlands,” J. Acoust. Soc. Am. 126(2), 634–643 (2009).
4 ISO: 226, 2003. Acoustics—normal equal-loudness contours (International Organization for Standardization, Geneva, 2003). 5 G. Leventhall, “What is infrasound?” Progress in Biophys. and
Molecular Biol. 93, 130–137 (2007).
6 T. Watanabe and H. Møller, “Low frequency hearing thresholds
in pressure field and free field,” J. Low Freq. Noise and Vib. 9(3),
106–115 (1990).
7 N. S. Yeowart, M. E. Bryan, and W. Tempest, “The monaural
M.A.P. threshold of hearing at frequencies from 1.5 to 100 c/s.,”
J. Sound and Vib. 6(3), 335–342 (1967).
8 J. Hensel, G. Scholz, U. Hurttig, D. Mrowinski, and T. Janssen,
“Impact of infrasound on the human cochlea,” Hearing Res. 223, 67–76 (2007).
revealed a steady 18.9 Hz emission by a laboratory machine. Once the machine was turned off, reports of paranormal sen- sations and sightings also ceased. Assuming a similar phe- nomenon in the cellar, Tandy used broadband sound level meters and recorded a distinct 19 Hz spectral peak in the ambient noise at 38 dB SPL. Other background infrasound signals were also recorded at very low levels between 7-30 dB SPL. Given the variable sensitivities to ultra-low frequencies demonstrated by Dommes et al.,9 the 19 Hz may have had an effect on sensitive visitors and evoked abnormal experiences.
This suggests that hyper or abnormal activity patterns in the temporal lobe, which includes the pri- mary and secondary auditory cortex, could be linked to the psychiatric symptoms observed in the wind turbine syn-
nia, and psychosis.
8
Knowledge gaps between changes in cochlear function,
system.
auditory cortical activity, and sound perception remain. As in
54 Acoustics Today, April 2012