Review of Published Research on Low Frequency Noise and Its Effects
Author:
Report for Defra (U.K. Department for Environment, Food and Rural Affairs), May 2003.
Geoff Leventhall is a “consultant in noise, vibration, and acoustics” and organizer of the biannual Wind Turbine Noise Conference.
He is also a favorite of the wind industry for insisting, with scant data to back up the claim, and as data to the contrary continue to be collected, that low-frequency noise and/or infrasound is not a concern from wind turbines.
This paper, nonetheless, is very informative about low-frequency noise/infrasound and its effects on people. It notes that only a small fraction of people may be affected, but the effect is severe and must be considered. It also shows that many complaints of noise occur when the more audible (A-weighted dB) noise level is quite low. Excerpts:
8. Annoyance
8.2.4 Annoyance and the dBA. A comparison of a band of noise peaking at 250Hz with a band peaking at 100Hz, whilst both were adjusted to the same A-weighted level, showed that the annoyance from the low frequency noise was greater than that from the higher frequency noise at the same A-weighted level (Persson et al., 1985). This work was subsequently extended (Persson and Bjorkman, 1988; Persson et al., 1990) using a wider range of noises, for example, peaking at 80Hz, 250Hz. 500Hz and 1000Hz, leading to the following conclusions:
There is a large variability between subjects.
The dBA underestimates annoyance for frequencies below about 200Hz.
10. Low frequency noise and stress
10.1 Low frequency noise and cortisol secretion. It is difficult to measure stress directly, but cortisol secretion has been used as a stress indicator (Ising and Ising, 2002; Persson-Waye et al., 2002; Persson-Waye et al., 2003). Under normal circumstances, cortisol levels follow a distinct circadian pattern in which the diurnal variation of cortisol is to drop to very low levels during the early morning sleep period, rising towards the awakening time. The rise continues until about 30 minutes after awakening, followed by a fall until midday and further fluctuations. Stress disrupts the normal cortisol pattern.
Ising and Ising (2002) discuss how noise, perceived as a threat , stimulates release of cortisol. This also occurs during sleep, thus increasing the level of night cortisol, which may interrupt recreative and other qualities of sleep. Measurements were made of the effect on children who, because of traffic changes, had become exposed to a high level of night lorry noise. There were two groups of subjects, exposed to high and low noise levels. The indoor noise spectrum for high levels typically peaked at around 60Hz, at 65dB, with a difference of maximum LC and LA of 26dB. The difference of average levels was 25dB, thus indicating a low frequency noise problem. Children exposed to the higher noise levels in the sample had significantly more problems with concentration, memory and sleep and also had higher cortisol secretions. Conclusions of the work were that the A-weighting is inadequate and that safer limits are needed for low frequency noise at night.
Perrson Waye et al (2003), studied the effect on sleep quality and wakening of traffic noise ( 35dB LAeq, 50dB LAmax) and low frequency noise (40dB LAeq). The low frequency noise peaked at 50Hz with a level of 70dB. In addition to cortisol determinations from saliva samples, the subjects completed questionnaires on their quality of sleep, relaxation and social inclinations. The main findings of the study were that levels of the cortisol awakening response were depressed after exposure to low frequency noise and that this was associated with tiredness and a negative mood.
In a laboratory study of noise sensitive subjects performing work tasks, it was found that enhanced salivary cortisol levels were produced by exposure to low frequency noise (Persson-Waye et al., 2002). A finding was that subjects who were sensitive to low frequency noise generally maintained higher cortisol levels and also had impaired performance. A hypothesis from the study is that changes in cortisol levels, such as produced by low frequency noise, may have a negative influence on health, heightened by chronic noise exposure. The three studies reviewed above show how low frequency noise disturbs the normal cortisol pattern during night, awakening and daytime exposure. The disturbances are associated with stress related effects.
13. General Review of Effects of Low Frequency Noise on Health
13.2 Effects on humans. Infrasound exposure is ubiquitous in modern life. It is generated by natural sources such as earthquakes and wind. It is common in urban environments, and as an emission from many artificial sources: automobiles, rail traffic, aircraft, industrial machinery, artillery and mining explosions, air movement machinery including wind turbines [emphasis added], compressors, and ventilation or air-conditioning units, household appliances such as washing machines, and some therapeutic devices. The effects of infrasound or low frequency noise are of particular concern because of its pervasiveness due to numerous sources, efficient propagation, and reduced efficiency of many structures (dwellings, walls, and hearing protection) in attenuating low-frequency noise compared with other noise.
13.6 Conclusion. There is no doubt [emphasis added] that some humans exposed to infrasound experience abnormal ear, CNS, and resonance induced symptoms that are real and stressful. If this is not recognised by investigators or their treating physicians, and properly addressed with understanding and sympathy, a psychological reaction will follow and the patientís problems will be compounded. Most subjects may be reassured that there will be no serious consequences to their health from infrasound exposure and if further exposure is avoided they may expect to become symptom free.
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