Infrasound, human health, and adaptation: an integrative overview of recondite hazards in a complex environment
Abstract: Infrasound displays a special capacity to affect human health and adaptation because its frequencies and amplitudes converge with those generated by the human body. Muscle sounds and whole-body vibrations are predominately within the 5- to 40-Hz range. The typical amplitudes of the oscillations are within 1–50 μm, which is equivalent to the pressures of about 1 Pa and energies in the order of 10−11 W m−2. Infrasound sources from the natural environment originate from winds, microbaroms, geomagnetic activity, and microseisms and can propagate for millions of meters. Cultural sources originate from air moving through duct systems within buildings, large machinery, and more recently, wind turbines. There are also unknown sources of infrasound. It is important to differentiate the effects of infrasound from the awareness or experience of its presence. Moderate strength correlations occur between the incidences of infrasound and reports of nausea, malaise, fatigue, aversion to the area, non-specific pain, and sleep disturbances when pressure levels exceed about 50 db for protracted periods. Experimental studies have verified these effects. Their validity is supported by convergent quantitative biophysical solutions. Because cells interact through the exchange of minute quanta of energy that corresponds with remarkably low levels of sound pressure produced by natural phenomena and wind turbines upon the body and its cavities, traditional standards for safety and quality of living might not be optimal.
9.3 Modern wind turbines
Modern wind turbines, because of their size (height) and rotational velocity, have the capacity to generate significant intensities of infrasound with complex waveforms and harmonics. Assuming a rotation speed (that can change with the driving wind velocity) of 14 rotations per minute (0.24 Hz multiplied three blades = 0.72 Hz), the relative pressure level at about 100 m distance is about 50 db outside a typical brick house and about 45 db inside the dwelling. At certain frequencies, such as around 7–8 and 40–45 Hz, the magnitude of the vibration components inside and outside the house may not differ. Significant proportions of acoustic pressure are contained within the harmonic frequencies for this rotation, for example, at increments of about 1 Hz. Some expert acoustic engineers have detected specific acoustic “fingerprints” from specific wind turbines tens of kilometers from the source. Stabilizing the structures in subsurface bedrock increases the probabilities of longer-distance transmission.
The frequency between 1 and 4 Hz, the delta range for brain waves (electroencephalographic measures), is the one involved with slow wave (deep sleep). Disruptions in this sleep, particularly during the first approximately 5 h of the sleep cycle, can affect the remarkably synchronized release of hormones and proteins that facilitate tissue repair andnormal homeostasis. The energy available from the pressure fluctuations within a human cerebral volume of 10−3 m3 would be equivalent to 10−6 J. Assuming each neuron utilized about a pJ of glucose per second, the energy from the fluctuating pressure wave would be sufficient to compensate for or augment the activity of approximately a million neurons that were most active at the time. The numbers of neurons involved with the core nuclei of the brain stem controlling slow-wave sleep are within this range. Maintained, diminished brain stem concentrations of serotonin (and noradrenalin) are associated with increased incidences of depression.
A recent review by Salt and Kaltenbach (2011) described values recorded by several researchers for infrasound levels between ~150 and 750 m from turbines with hub heights of 62 and 36 m blade lengths. The sound spectrum was dominated by frequencies below 10 Hz. Within the range of 1 Hz, the sound pressure levels were over 90 db when unweighted measurements were taken. These magnitudes were associated with the inaudible infrasound rather than the intermittent “swooshing” sounds that are more conspicuous. The authors emphasized the fact that people living near these turbines could be exposed 24 h per day for weeks that extend into years.
Michael A. Persinger
Behavioural Neuroscience and Biomolecular Sciences Programs, Laurentian University, Sudbury, Ontario
Natural Hazards, September 2013, DOI 10.1007/s11069-013-0827-3
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