Page 54 - Volume 12, Issue 2 - Spring 2012

ing wave interactions as the result of basilar membrane dis- placement.
Although the SPLs used for the low-frequency biasing tones approached the pain threshold for human hearing at 1 kHz, the biasing tones did not damage the subjects’ cochlear integrity, as shown by consistent primaries-generated DPOAEs before and after biasing tone presentations. None of the subjects reported painful pressure at the eardrum during the experiment. While the biasing tones’ high SPLs create large pressure differences in the ear, the sensation of pain may have been reduced by the tones’ low vibrational velocity. It was also reported that some subjects perceived a “weak but clearly audible sound sensation, described as humming” but not a “tonal audible stimulus.”7,8,19 The absence of a clear pure- tone percept suggests that infrasonic frequencies do not ade- quately stimulate the IHCs and hence may not be the sources of the humming. Rather, the source of this percept is likely to be the harmonics of the biasing tone.20
Infrasound processing by the auditory pathway
An fMRI study by Dommes et al. offers additional insight to infrasound responses in humans.9 When presented with tones of 12 Hz at 110 and 120 dB SPL, the subjects showed bilateral activation in the primary and secondary auditory cortices (superior temporal gyrus, Brodmann’s Area 41, 42, 22). The subjects were also exposed to tones in the human audible frequency range, 500 Hz at 105 dB SPL and 48 Hz at 100 dB SPL. The cortical sites activated for all these fre- quencies were similar, suggesting that infrasound can have a major impact on brain activation via the auditory pathway. When the 12 Hz tone was reduced to 90 dB SPL, the audito- ry cortex showed no significant activity, except in one sub- ject. This observation supports the idea of inter-individual differences in low-frequency sensitivity.
Intrinsic noise of fMRI machines can present severe experimental constraints. The scanner noise spectra showed frequencies from 3-10 Hz and 50-900 Hz at levels between 60-75 dB SPL and 60-80 dB SPL, respectively. While infra- sound noise remained estimated below threshold,19 noise between 50-900 Hz was audible and may have affected brain activities. However, Dommes et al. believe that the auditory cortex can distinguish and dismiss such background noise.9 Infrasonic tones must also be presented at high levels in order to overcome fMRI machine background noise. At high levels, the tones produce increased harmonic distortion resulting in high level and more easily detectable harmonics that can potentially alter fMRI results. To evaluate the effects of harmonics, a 36 Hz tone (third harmonic) at 70 dB SPL was presented as a fundamental frequency to the subjects. Auditory cortical activation was observed, though noticeably less than that evoked by a 12 Hz tone at 120 dB SPL. Dommes et al. concluded that infrasonic frequencies themselves play significant roles in activating the auditory cortex.9
Infrasound exposure on physical and psychological health
Although current research provides no conclusive evi- dence for infrasound hearing perception by humans, it is
nevertheless a worthy exercise to investigate infrasound sources in the immediate environment, as they may contain detectable harmonics. Typical infrasound sources include ocean waves, thunder, wind, machinery engines, slow speed fans, and driving a car with open windows.5,19 As pure tones are rarely generated in nature, these infrasonic sources typi- cally generate multiple harmonic components and other background noise. It is not unlikely for humans to be exposed to high levels of infrasound. For example, a child on a swing
5
However, these claims are based on A- weighted sound analysis, which removes all infrasound com- ponents from wind turbine broadband noise. A-weighted fil- ters are inadequate evaluations because they assume human insensitivity to infrasound. Wind turbine spectral analysis by Jung and Cheung has revealed substantial noise levels
22
may experience infrasound around 0.5 Hz at 110 dB SPL. One of the most heavily studied infrasound sources is wind farms. Many wind turbine companies claim that an operating wind farm produces negligible “whooshing” sounds that are comparable only to a kitchen refrigerator
1,21
As demonstrated by CMs, DPOAE modulations, and fMRI stud- ies, high levels of infrasound can alter cochlear function and activate the auditory cortex. Potential long term changes in brain activity by nearby wind farms have raised serious con- cerns. Some physical and psychological health risks from infrasound exposures include the “wind turbine syndrome”
disturbance, headache, annoyance, irritability, and chronic
fatigue. The symptoms often surface when one is close to
wind turbines, or an infrasound source, and disappear when
the person moves away. As reported, a family exposed con-
tinuously to 10 Hz at 35 dB SPL produced by a boiler house
complained of bodily pains, increased annoyance, and diffi-
culties sleeping.5 This family’s high sensitivity to a supposed-
ly subthreshold stimulus supports the notion that inter-indi-
vidual differences are real and that some individuals are more
sensitive and susceptible to the effects of low level infrasound
around 45 dB SPL.
between 60 to 100 dB SPL for frequencies below 20 Hz.
2,10, 23, 24
and paranormal experiences.
Symptoms of the wind turbine syndrome include sleep
than others. In another study, Pedersen et al. interviewed 3
70,000 adults living within 2.5 km of wind farms. They found that adults exposed to levels of A-weighted noise of 40- 50 dB SPL reported higher levels of annoyance than those exposed to levels below 40 dB SPL. Moreover, 12% of the sub- jects exposed to noise at 40-45 dB SPL reported feeling “very annoyed” versus only 6% from subjects exposed to 35-40 dB SPL; in these cases, individual psychological distress due to wind turbine noise is evident. As audible noise levels increase with increasing proximity to wind turbines, the levels of the infrasonic components also increase. Most subjects described the noise as “swishing/lashing,” rather than a pure tone sensation. The discontinuity in sound perception can be attributed the inner ear’s increased sensitivity to the infra-
8
sonic harmonics, as suggested by Hensel et al.’s study. When
compared to road traffic noise of similar levels, the subjects reported higher annoyance levels from wind turbines. The high annoyance levels are in part due to the ubiquitous pres- ence of wind turbine sounds throughout the day and night,
Wind Turbines and Ghost Stories 53